The immense importance of the forest, as a reservoir of this stock of moisture, becomes apparent, when we consider that a large proportion of the summer rain either flows into the valleys and the rivers, because it falls faster than the ground can imbibe it; or, if absorbed by the warm superficial strata, is evaporated from them without sinking deep enough to reach wells and springs, which, of course, depend much on winter rains and snows for their entire supply. This observation, though specially true of cleared and cultivated grounds, is not wholly inapplicable to the forest, particularly when, as is too often the case in Europe, the underwood and the decaying leaves are removed.

The quantity of snow that falls in extensive forests, far from the open country, has seldom been ascertained by direct observation, because there are few meteorological stations in or near the forest. According to Thompson, [Footnote: Thompson's Vermont, Appendix, p. 8.] the proportion of water which falls in snow in the Northern States does not exceed one-fifth of the total precipitation, but the moisture derived from it is doubtless considerably increased by the atmospheric vapor absorbed by it, or condensed and frozen on its surface. I think I can say from experience—and I am confirmed in this opinion by the testimony of competent observers whose attention has been directed specially to the point—that though much snow is intercepted by the trees, and the quantity on the ground in the woods is consequently less than in open land in the first part of the winter, yet most of what reaches the ground at that season remains under the protection of the wood until melted, and as it occasionally receives new supplies the depth of snow in the forest in the latter half of winter is considerably greater than in the cleared fields. Careful measurements in a snowy region in New England, in the month of February, gave a mean of 38 inches in the open ground and 44 inches in the woods. [Footnote: As the loss of snow by evaporation has been probably exaggerated by popular opinion, an observation or two on the subject may not be amiss in this place. It is true that in the open grounds, in clear weather and with a dry atmosphere, snow and ice are evaporated with great rapidity even when the thermometer is much below the freezing-point; and Darwin informs us that the snow on the summit of Aconcagua, 23,000 feet high, and of course in a temperature of perpetual frost, is sometimes carried off by evaporation. The surface of the snow in our woods, however, does not indicate much loss in this way. Very small deposits of snow-flakes remain unevaporated in the forest, for many days after snow which fell at the same time in the cleared field has disappeared without either a thaw to melt it or a wind powerful enough to drift it away. Even when bared of their leaven, the trees of a wood obstruct, in an important degree, both the direct action of the sun's rays on the snow and the movement of drying and thawing winds.

Dr. Piper (Trees of America, p. 48) records the following observations: "A body of snow, one foot in depth and sixteen feet square, was protected from the wind by a tight board fence about five feet high, while another body of snow, much more sheltered from the sun than the first, six feet in depth, and about sixteen feet square, was fully exposed to the wind. When the thaw came on, which lasted about a fortnight, the larger body of snow was entirely dissolved in less than a week, while the smaller body was not wholly gone at the end of the second week. "Equal quantities of snow were placed in vessels of the samekind and capacity, the temperature of the air being seventy degrees. In the one case, a constant current of air was kept passing over the open vessel, while the other was protected by a cover. The snow in the first was dissolved in sixteen minutes, while the latter had a small unthawed proportion remaining at the end of eighty-five minutes." The snow in the woods is protected in the same way, though not literally to the same extent, as by the fence in one of these cases and the cover in the other.]

The general effect of the forest in cold climates is to assimilate the winter state of the ground to that of wooded regions under softer skies; and it is a circumstance well worth noting, that in Southern Europe, where Nature has denied to the earth a warm winter-garment of flocculent snow, she has, by one of those compensations in which her empire is so rich, clothed the hillsides with umbrella and other pines, ilexes, cork-oaks, bays and other trees of persistent foliage, whose evergreen leaves afford to the soil a protection analogous to that which it derives from snow in more northern climates.

The water imbibed by the soil in winter sinks until it meets a more or less impermeable or a saturated stratum, and then, by unseen conduits, slowly finds its way to the channels springs, or oozes out of the ground in drops which unite in rills, and so all is conveyed to the larger streams, and by them finally to the sea. The water, in percolating through the vegetable and mineral layers, acquires their temperature, and is chemically affected by their action, but it carries very little matter in mechanical suspension.

The process I have described is a slow one, and the supply of moisture derived from the snow, augmented by the rains of the following seasons, keeps the forest-ground, where the surface is level or but moderately inclined, in a state of approximate saturation throughout almost the whole year. [Footnote: The statements I have made, here and elsewhere, respecting the humidity of the soil in natural forests, have been, I understand, denied by Mr. T. Meehan, a distinguished American naturalist, in a paper which I have not seen He is quoted as maintaining, among other highly questionable propositions that no ground is "so dry in its subsoil as that which sustains a forest on its surface." In open, artificially planted woods, with a smooth and regular surface, and especially in forests where the fallen leaves and branches are annually burnt or carried off, both the superficial and the subjacent strata may under certain circumstances, become dry, but this rarely, if ever, happens in a wood of spontaneous growth, undeprived of the protection afforded by its own droppings, and of the natural accidents of surface which tend to the retention of water. See, on this point, a very able article by Mr. Henry Stewart, in the New York Tribune of November 23, 1873.] It may be proper to observe here that in Italy, and in many parts of Spain and France, the Alps, the Apennines, and the Pyrenees, not to speak of less important mountains, perform the functions which provident nature has in other regions assigned to the forest, that is, they act as reservoirs wherein is accumulated in winter a supply of moisture to nourish the parched plains during the droughts of summer. Hence, however enormous may be the evils which have accrued to the above-mentioned countries from the destruction of the woods, the absolute desolation which would otherwise have smitten them through the folly of man, has been partially prevented by those natural dispositions, by means of which there are stored up in the glaciers, in the snow-fields, and in the basins of mountains and valleys, vast deposits of condensed moisture which are afterwards distributed in a liquid form during the season in which the atmosphere furnishes a slender supply of the beneficent fluid so indispensable to vegetable and animal life. [Footnote: The accumulation of snow and ice upon the Alps and other mountains—which often fills up valleys to the height of hundreds of feet—is due not only to the fall or congealed and crystallized vapor in the form of snow, to the condensation of atmospheric vapor on the surface of snow-fields and glaciers, and to a temperature which prevents the rapid melting of snow, but also to the well-known fact that, at least up to the height of 10,000 feet, rain and snow are more abundant on the mountains than at lower levels.

But another reason may be suggested for the increase of atmospheric humidity, and consequently of the precipitation of aqueous vapor on mountain chains. In discussing the influence of mountains on precipitation, meteorologists have generally treated the popular belief, that mountains "attract" to them clouds floating within a certain distance from them, as an ignorant prejudice, and they ascribe the appearance of clouds about high peaks solely to the condensation of the humidity of the air carried by atmospheric currents up the slopes of the mountain to a colder temperature. But if mountains do not really draw clouds and invisible vapors to them, they are an exception to the universal law of attraction. The attraction of the small Mount Shehallien was found sufficient to deflect from the perpendicular, by a measurable quantity, a plummet weighing but a few ounces. Why, then, should not greater masses attract to them volumes of vapor weighing many tons, and floating freely in the atmosphere within moderate distances of the mountains ]

Summer Rains, Importance of.

Babinet quotes a French proverb: "Summer rain wets nothing," and explains it by saying that at that season the rainwater is "almost entirely carried off by evaporation." "The rains of summer," he adds, "however abundant they may be, do not penetrate the soil beyond the depth of six or eight inches. In summer the evaporating power of the heat is five or six times greater than in winter, and this force is exerted by an atmosphere capable of containing five or six times as much vapor as in winter." "A stratum of snow which prevents evaporation [from the ground], causes almost all the water that composes it to filter into the earth, and forms a provision for fountains, wells, and streams which could not be furnished by any quantity whatever of summer rain. This latter, useful to vegetation like the dew, neither penetrates the soil nor accumulates a store to supply the springs and to be given out again into the open air." [Footnote: Etudes et Lectures, vol. vi., p. 118. The experiments or Johnstrup in the vicinity of Copenhagen, where the mean annual precipitation is 23 1/2 inches, and where the evaporation must be less than in the warmer and drier atmosphere of France, form the most careful series of observations on this subject which I have met with. Johnstrup found that at the depth at a metre and a half (50 inches) the effects of rain and evaporation were almost imperceptible, and became completely so at a depth of from two to three metres (6 1/2 to 10 feet). During the summer half of the year the evaporation rather exceeded the rainfall; during the winter half the entire precipitation was absorbed by the soil and transmitted to lower strata by infiltration. The stratum between one metre and a half (50 inches) and three metres (10 feet) from the surface was then permanently in the condition of a saturated sponge, neither receiving nor losing humidity during the summer half of the year, but receiving from superior, and giving off to lower, strata an equal amount of moisture during the winter half.—Johnstrup, Om Fugtighedens Bezagelse i den naturlige Jordbund. Kjobenhavn, 1866.]

This conclusion, however applicable to the climate and to the soil of France, is too broadly stated to be received as a general truth; and in countries like the United States, where rain is comparatively rare during the winter and abundant during the summer half of the year, common observation shows that the quantity of water furnished by deep wells and by natural springs depends almost as much upon the rains of summer as upon those of the rest of the year, and consequently that a large portion of the rain of that season must find its way into strata too deep for the water to be wasted by evaporation.

[Footnote: According to observations at one hundred military stations in the United States, the precipitation ranges from three and a quarter inches at Fort Yuma in California to about seventy-two inches at Fort Pike, Louisiana, the mean for the entire territory, not including Alaska, being thirty-six inches. In the different sections of the Union it is as follows:

North-eastern States.................. 41 inches, New York.............................. 36 " Middle States......................... 40 1/2 " Ohio.................................. 40 " Southern States....................... 51 " S. W. States and Indian Territories... 39 1/2 " Western States and Territories........ 30 " Texas and New Mexico.................. 24 1/2 " California............................ 18 1/2 " Oregon and Washington Territory....... 50 "

The mountainous regions, it appears, do not recieve the greatest amount of precipitation. The avenge downfall of the Southern States bordering on the Atlantic and the Gulf of Mexico exceeds the mean of the whole United States, being no less than fifty-one inches, while on the Pacific coast it ranges from fifty to fifty-six inches.

As a general rule, it may be stated that at the stations on or near the sea-coast the precipitation is greatest in the spring months, though there are several exceptions to this remark, and at a large majority of the stations the downfall is considerably greater in the summer months than at any other season.]

Dalton's experiments in the years 1796, 1797, and 1798 appeared to show that the mean absorption of the downfall by the earth in those years was twenty-nine per cent.

Dickinson, employing the same apparatus for eight years, found the absorption to vary widely in different years, the mean being forty-seven per cent.

Charnock's experiments in two years show an absorption of from seventeen to twenty-seven per cent.] Besides, even admitting that the water from summer rains is so completely evaporated as to contribute nothing directly to the supply of springs, it at least tends indirectly to maintain their flow, because it saturates in part the atmosphere, and at the same time it prevents the heat of the sun from drying the earth to still greater depths, and bringing within the reach of evaporation the moisture of strata which ordinarily do not feel the effects of solar irradiation.

Influence of the Forest on the Flow of Springs.

It is an almost universal and, I believe, well-founded opinion, that the protection afforded by the forest against the escape of moisture from its soil by superficial flow and evaporation insures the permanence and regularity of natural springs, not only within the limits of the wood, but at some distance beyond its borders, and thus contributes to the supply of an element essential to both vegetable and animal life. As the forests are destroyed, the springs which flowed from the woods, and, consequently, the greater water-courses fed by them, diminish both in number and in volume. This fact is so familiar throughout the American States and the British Provinces, that there are few old residents of the interior of those districts who are not able to testify to its truth as a matter of personal observation. My own recollection suggests to me many instances of this sort, and I remember one case where a small mountain spring, which disappeared soon after the clearing of the ground where it rose, was recovered about twenty years ago, by simply allowing the bushes and young trees to grow up on a rocky knoll, not more than half an acre in extent, immediately above the spring. The ground was hardly shaded before the water reappeared, and it has ever since continued to flow without interruption. The hills in the Atlantic States formerly abounded in springs and brooks, but in many parts of these States which were cleared a generation or two ago, the hill-pastures now suffer severely from drought, and in dry seasons furnish to cattle neither grass nor water.

Almost every treatise on the economy of the forest adduces facts in support of the doctrine that the clearing of the woods tends to diminish the flow of springs and the humidity of the soil, and it might seem unnecessary to bring forward further evidence on this point. [Footnote: "Why go so far for the proof of a phenomenon that is repeated every day under our own eyes, and of which every Parisian may convince himself, without venturing beyond the Bois de Boulogne or the forest of Meudon Let him, after a few rainy days, pass alone the Chevreuse road, which is bordered on the right by the wood, on the left by cultivated fields. The fall of water and the continuance of the rain have been the same on both sides; but the ditch on the side of the forest will remain filled with water proceeding from the infiltration through the wooded soil, long after the other, contiguous to the open ground, has performed its office of drainage and become dry. The ditch on the left will have discharged in a few hours a quantity of water, which the ditch on the right requires several days to receive and carry down to the valley."—Clave, Etudes, etc., pp. 53, 54.] But the subject is of too much practical importance and of too great philosophical interest to be summarily disposed of; and it ought to be noticed that there is at least one case—that of some loose sandy soils which, as observed by Valles, [Footnote: Valles, Etudes sur les Inondations, p. 472.] when bared of wood very rapidly absorb and transmit to lower strata the water they receive from the atmosphere—where the removal of the forest may increase the flow of springs at levels below it, by exposing to the rain and melted snow a surface more bibulous, and at the same time less retentive, than its original covering. Under such circumstances, the water of precipitation, which had formerly been absorbed by the vegetable mould and retained until it was evaporated, might descend through porous earth until it meets an impermeable stratum, and then be conducted along it, until, finally, at the outcropping of this stratum, it bursts from a hillside as a running spring. But such instances are doubtless too rare to form a frequent or an important exception to the general law, because it is very seldom the case that such a soil as has just been supposed is covered by a layer of vegetable earth thick enough to retain, until it is evaporated, all the rain that falls upon it, without imparting any water to the strata below it.

If we look at the point under discussion as purely a question of fact, to be determined by positive evidence and not by argument, the observations of Boussingault are, both in the circumstances they detail and in the weight to be attached to the testimony, among the most important yet recorded. The interest of the question will justify me in giving, nearly in Boussingault's own words, the facts and some of the remarks with which he accompanies the detail of them. "In many localities," he observes, [Footnote: Economie Rurale t. ii, p. 780.] "it has been thought that, within a certain number of years, a sensible diminution has been perceived in the volume of water of streams utilized as a motive-power; at other points, there are grounds for believing that rivers have become shallower, and the increasing breadth of the belt of pebbles along their banks seems to prove the loss of a part of their water; and, finally, abundant springs have almost dried up. These observations have been principally made in valleys bounded by high mountains, and it has been noticed that this diminution of the waters has immediately followed the epoch when the inhabitants have begun to destroy, unsparingly, the woods which were spread over the face of the land. "And here lies the practical point of the question; for if it is once established that clearing diminishes the volume of streams, it is less important to know to what special cause this effect is due. The rivers which rise within the valley of Aragua, having no outlet to the ocean, form, by their union, the Lake of Tacarigua or Valencia, having a length of about two leagues and a half [= 7 English miles].

At the time of Humboldt's visit to the valley of Aragua, the inhabitants were struck by the gradual diminution which the lake had been undergoing for thirty years. In fact, by comparing the descriptions given by historians with its actual condition, even making large allowance for exaggeration, it was easy to see that the level was considerably depressed. The facts spoke for themselves. Oviedo, who, toward the close of the sixteenth century, had often traversed the valley of Aragua, says positively that New Valencia was founded, in 1555, at half a league from the Lake of Tacarigua; in 1800, Humboldt found this city 5,260 metres [= 3 1/2 English miles] from the shore.

"The aspect of the soil furnished new proofs. Many hillocks on the plain retain the name of islands, which they more justly bore when they were surrounded by water. The ground laid bare by the retreat of the lake was converted into admirable plantations; and buildings erected near the lake showed the sinking of the water from year to year. In 1796, new islands made their appearance. A fortress built in 1740 on the island of Cabrera, was now on a peninsula; and, finally, on two granitic islands, those of Cura and Cabo Blanco, Humboldt observed among the shrubs, somo metres above the water, fine sand filled with helicites.

"These clear and positive facts suggested numerous explanations, all assuming a subterranean outlet, which permitted the discharge of the water to the ocean. Humboldt disposed of these hypotheses, and did not hesitate to ascribe the diminution of the waters of the lake to the numerous clearings which had been made in the valley of Aragua within half a century."

Twenty-two years later, Boussingault explored the valley of Aragua. For some years previous, the inhabitants had observed that the waters of the lake were no longer retiring, but, on the contrary, were sensibly rising. Grounds, not long before occupied by plantations, were submerged. The islands of Nuevas Aparecidas, which appeared above the surface in 1796, had again become shoals dangerous to navigation. Cabrera, a tongue of land on the north side of the valley, was so narrow that the least rise of the water completely inundated it. A protracted north wind sufficed to flood the road between Maracay and New Valencia. The fears which the inhabitants of the shores had so long entertained were reversed. Those who had explained the diminution of the lake by the supposition of subterranean channels were suspected of blocking them up, to prove themselves in the right.

During the twenty-two years which had elapsed, the valley of Aragua had been the theatre of bloody struggles, and war had desolated these smiling lands and decimated their population. At the first cry of independence a great number of slaves found their liberty by enlisting under the banners of the new republic; the great plantations were abandoned, and the forest, which in the tropics so rapidly encroaches, had soon recovered a large proportion of the soil which man had wrested from it by more than a century of constant and painful labor.

Boussingault proceeds to state that two lakes near Ubate, in New Granada, had formed but one, a century before his visit; that the waters were gradually retiring, and the plantations extending over the abandoned bed; that, by inquiry of old hunters and by examination of parish records, he found that extensive clearings had been made and were still going on.

He found, also, that the length of the Lake of Fuquene, in the same valley, had, within two centuries, been reduced from ten leagues to one and a half, its breadth from three leagues to one. At the former period, the neighboring mountains were well wooded, but at the time of his visit the mountains had been almost entirely stripped of their wood. Our author adds that other cases, similar to those already detailed, might be cited, and he proceeds to show, by several examples, that the waters of other lakes in the same regions, where the valleys had always been bare of wood, or where the forests had not been disturbed, had undergone no change of level.

Boussingault further states that the lakes of Switzerland have sustained a depression of level since the too prevalent destruction of the woods, and arrives at the general conclusion that, "in countries where great clearings have been made, there has most probably been a diminution in the living waters which flow upon the surface of theground." This conclusion he further supports by two examples: one, where a fine spring, at the foot of a wooded mountain in the Island of Ascension, dried up when the mountain was cleared, but reappeared when the wood was replanted; the other at Marmato, in the province of Popayan, where the streams employed to drive machinery were much diminished in volume, within two years after the clearing of the heights from which they derived their supplies. This latter is an interesting case, because, although the rain-gauges, established as soon as the decrease of water began to excite alarm, showed a greater fall of rain for the second year of observation than the first, yet there was no appreciable increase in the flow of the mill-streams. From these cases, the distinguished physicist infers that very restricted local clearings may diminish and even suppress springs and brooks, without any reduction in the total quantity of rain.

It will have been noticed that these observations, with the exception of the last two cases, do not bear directly upon the question of the diminution of springs by clearings, but they logically infer it from the subsidence of the natural reservoirs which springs once filled. There is, however, no want of positive evidence on this subject. Marchand cites the following instances: "Before the felling of the woods, within the last few years, in the valley of the Soulce, the Combe-es-Monnin and the Little Valley, the Sorne furnished a regular and sufficient supply of water for the ironworks of Unterwyl, which was almost unaffected by drought or by heavy rains. The Sorne has now become a torrent, every shower occasions a flood, and after a few days of fine weather, the current falls so low that it has been necessary to change the water-wheels, because those of the old construction are no longer able to drive the machinery, and at last to introduce a steam-engine to prevent the stoppage of the works for want of water.

"When the factory of St. Ursanne was established, the river that furnished its power was abundant, and had, from time immemorial, sufficed for the machinery of a previous factory. Afterwards, the woods near its sources were cut. The supply of water fell off in consequence, the factory wanted water for half the year, and was at last obliged to stop altogether.

"The spring of Combefoulat, in the commune of Seleate, was well known as one of the best in the country; it was remarkably abundant, and sufficient, in the severest droughts, to supply all the fountains of the town; but as soon as considerable forests were felled in Combe-de-pre Martin and in the valley of Combefoulat, the famous spring, which lies below these woods, has become a mere thread of water, and disappears altogether in times of drought.

"The spring of Varieux, which formerly supplied the castle of Pruntrut, lost more than half its water after the clearing of Varieux and Rougeoles. These woods have been replanted, the young trees are growing well, and, with the woods, the waters of the spring are increasing.

"The Dog Spring between Pruntrut and Bressancourt has entirely vanished since the surrounding forest-grounds were brought under cultivation.

"The Wolf Spring, in the commune of Soubey, furnishes a remarkable example of the influence of the woods upon fountains. A few years ago this spring did not exist. At the place where it now rises, a small thread of water was observed after very long rains, but the stream disappeared with the rain. The spot is in the middle of a very steep pasture inclining to the south. Eighty years ago, the owner of the land, perceiving that young firs were shooting up in the upper part of it, determined to let them grow, and they soon formed a flourishing grove. As soon as they were well grown, a fine spring appeared in place of the occasional rill, and furnished abundant water in the longest droughts. For forty or fifty years this spring was considered the best in the Clos du Doubs. A few years since, the grove was felled, and the ground turned again to a pasture. The spring disappeared with the wood, and is now as dry as it was ninety years ago." [Footnote: Ueber Die Entwaldung Der Gebirge, pp. 20 et seqq.]

Siemoni gives the following remarkable facts from his own personal observation:

"In a rocky nook near the crest of a mountain in the Tuscan Apennines, there flowed a clear, cool, and perennial fountain, uniting three distinct springs in a single current. The ancient beeches around and particularly above the springs were felled. On the disappearance of the wood, the springs ceased to flow, except in a thread of water in rainy weather, greatly inferior in quality to that of the old fountain. The beeches were succeeded by firs, and as soon as they had grown sufficiently to shade the soil, the springs begun again to flow, and they gradually returned to their former abundance and quality." [Footnote: Manuale D'arte Forestale. 2me editione, p. 492.]

This and the next preceding case are of great importance both as to the action of the wood in maintaining springs, and particularly as tending to prove that evergreens do not exercise the desiccative influence ascribed to them in France. The latter instance shows, too, that the protective influence of the wood extends far below the surface, for the quality of the water was determined, no doubt, by the depth from which it was drawn. The slender occasional supply after the beeches were cut was rain-water which soaked through the superficial humus and oozed out at the old orifices, carrying the taste and temperature of the vegetable soil with it; the more abundant and grateful water which flowed before the beeches were cut, and after the firs were well grown, came from a deeper source and had been purified, and cooled to the mean temperature of the locality, by filtering through strata of mineral earth. "The influence of the forest on springs," says Hummel, "is strikingly shown by an instance at Heilbronn. The woods on the hills surrounding the town are cut in regular succession every twentieth year. As the annual cuttings approach a certain point, the springs yield less water, some of them none at all; but as the young growth shoots up, they flow more and more freely, and at length bubble up again in all their original abundance." [Footnote: Physische Geographie, p. 32.] Dr. Piper states the following case: "Within about half a mile of my residence there is a pond upon which mills have been standing for a long time, dating back, I believe, to the first settlement of the town. These have been kept in constant operation until within some twenty or thirty years, when the supply of water began to fail. The pond owes its existence to a stream that has its source in the hills which stretch some miles to the south. Within the time mentioned, these hills, which were clothed with a dense forest, have been almost entirely stripped of trees; and to the wonder and loss of the mill-owners, the water in the pond has failed, except in the season of freshets; and, what was never heard of before, the stream itself has been entirely dry. Within the last ten years a new growth of wood has sprung up on most of the land formerly occupied by the old forest; and now the water runs through the year, notwithstanding the great droughts of the last few years, going back from 1856."

Dr. Piper quotes from a letter of William C. Bryant the following remarks: "It is a common observation that our summers are becoming drier and our streams smaller. Take the Cuyahoga as an illustration. Fifty years ago large barges loaded with goods went up and down that river, and one of the vessels engaged in the battle of Lake Erie, in which the gallant Perry was victorious, was built at Old Portage, six miles north of Albion, and floated down to the lake. Now, in an ordinary stage of the water, a canoe or skiff can hardly pass down the stream. Many a boat of fifty tons burden has been built and loaded in the Tuscarawas, at New Portage, and sailed to New Orleans without breaking bulk. Now, the river hardly affords a supply of water at New Portage for the canal. The same may be said of other streams—they are drying up. And from the same cause—the destruction of our forests—our summers are growing drier and our winters colder." [Footnote: The Trees of America, pp. 50, 51.]

No observer has more carefully studied the influence of the forest upon the flow of the waters, or reasoned more ably on the ascertained phenomena, than Cantegril. The facts presented in the following case, communicated by him to the Ami des Sciences for December, 1859, are as nearly conclusive as any single instance well can be:

"In the territory of the commune of Labruguiere there is a forest of 1,834 hectares [4,530 acres], known by the name of the Forest of Montaut, and belonging to that commune. It extends along thenorthern slope of the Black Mountains. The soil is granitic, the maximum altitude 1,243 metres [4,140 feet], and the inclination ranges between 15 and 60 to 100.

"A small current of water, the brook of Caunan, takes its rise in this forest, and receives the waters of two-thirds of its surface. At the lower extremity of the wood and on the stream are several fulleries, each requiring a force of eight horse-power to drive the water-wheels which work the stampers. The commune of Labruguiere had been for a long time famous for its opposition to forest laws. Trespasses and abuses of the right of pasturage had converted the wood into an immense waste, so that this vast property now scarcely sufficed to pay the expense of protecting it, and to furnish the inhabitants with a meagre supply of fuel. While the forest was thus ruined, and the soil thus bared, the water, after every abundant rain, made an eruption into the valley, bringing down a great quantity of pebbles which still clog the current of the Caunan. The violence of the floods was sometimes such that they were obliged to stop the machinery for some time. During the summer another inconvenience was felt. If the dry weather continued a little longer than usual, the delivery of water became insignificant. Each fullery could for the most part only employ a single set of stampers, and it was not unusual to see the work entirely suspended.

"After 1840, the municipal authority succeeded in enlightening the population as to their true interests. Protected by a more watchful supervision, aided by well-managed replantation, the forest has continued to improve to the present day. In proportion to the restoration of the forest, the condition of the manufactories has become less and less precarious, and the action of the water is completely modified. For example, sudden and violent floods, which formerly made it necessary to stop the machinery, no longer occur. There is no increase in the delivery until six or eight hours after the beginning of the rain; the floods follow a regular progression till they reach their maximum, and decrease in the same manner. Finally, the fulleries are no longer forced to suspend work in summer; the water is always sufficiently abundant to allow the employment of two sets of stampere at least, and often even of three.

"This example is remarkable in this respect, that, all other circumstances having remained the same, the changes in the action of the stream can be attributed only to the restoration of the forest—changes which may be thus summed up: diminution of flood-water during rains—increase of delivery at other seasons."

Becquerel and other European writers adduce numerous other cases where the destruction of forests has caused the disappearance of springs, a diminution in the volume of rivers, and a lowering of the level of lakes, and in fact, the evidence in support of the doctrine I have been maintaining on this subject seems to be as conclusive as the nature of the case admits. [Footnote: See, in the Revue des Eaux et Forets for April, 1867, an article entitled De l'influence des Forets sur le Regime des Eaux, and the papers in previous numbers of the same journal therein referred to.] We cannot, it is true, arrive at the same certainty and precision of result in these inquiries as in those branches of physical research where exact quantitative appreciation is possible, and we must content ourselves with probabilities and approximations. We cannot positively affirm that the precipitation in a given locality is increased by the presence, or lessened by the destruction, of the forest, and from our ignorance of the subterranean circulation of the waters, we cannot predict, with certainty, the drying up of a particular spring as a consequence of the felling of the wood which shelters it; but the general truth, that the flow of springs and the normal volume of rivers rise and fall with the extension and the diminution of the woods where they originate and through which they run, is as well established as any proposition in the science of physical geography. [Footnote: Some years ago it was popularly believed that the volume of the Mississippi, like that of the Volga and other rivers of the Eastern Hemisphere, was diminished by the increased evaporation from its basin and the drying up of the springs in consequence of the felling of the forests in the vicinity of the source of its eastern affluents. The boatmen of this great river and other intelligent observers now assure us, however, that the mean and normal level of the Mississippi has risen within a few years, and that in consequence the river is navigable at low water for boats of greater draught and at higher points in its course than was the case twenty-five years ago. This supposed increase of volume has been attributed by some to the recent re-wooding of the prairies, but the plantations thus far made are not yet sufficiently extensive to produce an appreciable effect of this nature; and besides, while young trees have covered some of the prairies, the destruction of the forest has been continued perhaps in a greater proportion in other parts of the basin of the river. A more plausible opinion is that the substitution of ground that is cultivated, and consequently spongy and absorbent, for the natural soil of the prairies, has furnished a reservoir for the rains which are absorbed by the earth and carried gradually to the river by subterranean flow, instead of running off rapidly from the surface, or, as is more probable, instead of evaporating or being taken up by the vigorous herbaceous vegetation which covers the natural prairie.

A phenomenon so contrary to common experience, as would be a permanent increase in the waters of a great river, will not be accepted without the most convincing proofs. The present greater facility of navigation may be attributed to improvements in the model of the boats, to the removing of sand-banks and other impediments to the flow of the waters, or to the confining of these waters in a narrower channel, by extending the embankments of the river, or to yet other causes. So remarkable a change could not have escaped the notice of Humphreys and Abbot, whose most able labors comprise the years 1850-1861, had it occurred during that period or at any former time within the knowledge of the many observers they consulted; but no such fact is noticed in their exhaustive report. However, even if an increase in the volume of the Mississippi, for a period of ten or twenty years, were certain, it would still be premature to consider this increase as normal and constant, since it might very well be produced by causes yet unknown and analogous to those which influence the mysterious advance and retreat of those Alpine ice-rivers, the glaciers. Among such causes we may suppose a long series of rainy seasons in regions where important tributaries have their far-off and almost unknown sources; and with no less probability, we may conceive of the opening of communications with great subterranean reservoirs, which may from year to year empty large quantities of water into the bed of the stream; or the closing up of orifices through which a considerable portion of the water of the river once made its way for the supply of such reservoirs.—See upon this point, Chap. IV., Of Subterranean Waters; post.]

Of the converse proposition, namely, that the planting of new forests gives rise to new springs and restores the regular flow of rivers, I find less of positive proof, however probable it may be that such effects would follow. [Footnote: According to the Report of the Department of Agriculture for February, 1872, it is thought in the Far West that the young plantations have already influenced the water-courses in that region, and it is alleged that ancient river-beds, never known to contain water since the settlement of the country, have begun to flow since these plantations were commenced. See also Hayden, Report on Geological Survey of Wyoming, 1870, p. 104, and Bryant. Forest Trees, 1871, chap. iv.

In the Voyage autour du Monde of the Comte da Beauvoir, chap. x., this passage occurs: Dr. Muller, Director of the Botanic Garden at Melbourne, "has distributed through the interior of Australia millions of seedling trees from his nursuries. Small rivulets are soon formed under the young wood; the results are superb, and the observation of every successive year confirms them. On bare soils he has created, at more than a hundred points, forests and water-courses."] A reason for the want of evidence on the subject may be, that, under ordinary circumstances, the process of conversion of bare ground to soil with a well-wooded surface is so gradual and slow, and the time required for a fair experiment is consequently so long, that many changes produced by the action of the new geographical element escape the notice and the memory of ordinary observers. The growth of a forest, including the formation of a thick stratum of vegetable mould beneath it, is the work of a generation, its destruction may be accomplished in a day; and hence, while the results of the one process may, for a considerable time, be doubtful if not imperceptible, those of the other are immediate and readily appreciable. Fortunately, the plantation of a wood produces other beneficial consequences which are both sooner realized and more easily estimated; and though he who drops the seed is sowing for a future generation as well as for his own, the planter of a grove may hope himself to reap a fair return for his expenditure and his labor.

Influence of the Forest on Inundations and Torrents.

Inasmuch as it is not yet proved that the forests augment or diminish the precipitation in the regions they principally cover, we cannot positively affirm that their presence or absence increases or lessens the total volume of the water annually delivered by great rivers or by mountain torrents. It is nevertheless certain that they exercise an action on the discharge of the water of rain and snow into the valleys, ravines, and other depressions of the surface, where it is gathered into brooks and finally larger currents, and consequently influence the character of floods, both in rivers and in torrents. For this reason, river inundations and the devastations of torrents, and the geographical effects resulting from them, so far as they are occasioned or modified by the action of forests or of the destruction of the woods, may properly be discussed in this chapter, though they might seem otherwise to belong more appropriately to another division of this work.

Besides the climatic question, which I have already sufficiently discussed, and the obvious inconveniences of a scanty supply of charcoal, of fuel, and of timber for architectural and naval construction and for the thousand other uses to which wood is applied in rural and domestic economy, and in the various industrial processes of civilized life, the attention of European foresters and public economists has been specially drawn to three points, namely: the influence of the forests on the permanence and regular flow of springs or natural fountains; on inundations by the overflow of rivers; and on the abrasion of soil and the transportation of earth, gravel, pebbles, and even of considerable masses of rock, from higher to lower levels, by torrents. There are, however, connected with this general subject, several other topics of minor or strictly local interest, or of more uncertain character, which I shall have occasion more fully to speak of hereafter.

The first of these three principal subjects—the influence of the woods on springs and other living waters—has been already considered; and if the facts stated in that discussion are well established, and the conclusions I have drawn from them are logically sound, it would seem to follow, as a necessary corollary, that the action of the forest is as important in diminishing the frequency and violence of river-floods as in securing the permanence and equability of natural fountains; for any cause which promotes the absorption and accumulation of the water of precipitation by the superficial strata of the soil, to be slowly given out by infiltration and percolation, must, by preventing the rapid flow of surface-water into the natural channels of drainage, tend to check the sudden rise of rivers, and, consequently, the overflow of their banks, which constitutes what is called inundation.

The surface of a forest, in its natural condition, can never pour forth such deluges of water as flow from cultivated soil. Humus, or vegetable mould, is capable of absorbing almost twice its own weight of water. The soil in a forest of deciduous foliage is composed of humus, more or less unmixed, to the depth of several inches, sometimes even of feet, and this stratum is usually able to imbibe all the water possibly resulting from the snow which at any one time covers, or the rain which in any one shower falls upon it. But the vegetable mould does not cease to absorb water when it becomes saturated, for it then gives off a portion of its moisture to the mineral earth below, and thus is ready to receive a new supply; and, besides, the bed of leaves not yet converted to mould takes up and retains a very considerable proportion of snow-water, as well as of rain.

The stems of trees, too, and of underwood, the trunks and stumps and roots of fallen timber, the mosses and fungi and the numerous inequalities of the ground observed in all forests, oppose a mechanical resistance to the flow of water over the surface, which sensibly retards the rapidity of its descent down declivities, and diverts and divides streams which may have already accumulated from smaller threads of water. [Footnote: In a letter addressed to the Minister of Public Works, after the terrible inundations of 1857, the late Emperor of France thus happily expressed himself: "Before we seek the remedy for an evil, we inquire into its cause. Whence come the sudden floods of our rivers From the water which falls on the mountains, not from that which falls on the plains. The waters which fall on our fields produce but few rivulets, but these which fall on our roofs and are collected in the gutters, form small streams at once. Now, the roofs are mountains—the gutters are valleys."

"To continue the comparison," observes D'Hericourt, "roofs are smooth and impermeable, and the rain-water pours rapidly off from their surfaces; but this rapidity of flow would be greatly diminished if the roofs were carpeted with mosses and grasses; more still, if they were covered with dry leaves, little shrubs, strewn branches, and other impediments—in short, if they were wooded."—Annales Forestieres, Dec. 1857, p. 311.

The mosses and fungi play a more important part in regulating the humidity of the air and of the soil than writers on the forest have usually assigned to them. They perish with the trees they grow on; but, in many situations, nature provides a compensation for the tree-mosses and fungi in ground species, which, on cold soils, especially those with a northern exposure, spring up abundantly both before the woods are felled, and when the land is cleared and employed for pasturage, or deserted. These humble plants discharge a portion of the functions appropriated to the wood, and while they render the soil of improved lands much less fit for agricultural use, they, at the same time, prepare it for the growth of a new harvest of trees, when the infertility they produce shall have driven man to abandon it and suffer it to relapse into the hands of nature.

In primitive forests, when the ground is not too moist to admit of a dense growth of trees, the soil is generally so thickly covered with leaves that there is little room for ground mosses and mushrooms. In the more open artificial woods of Europe these forms of vegetation, as well as many more attractive plants, are more frequent than in the native groves of America. See, on cryptogamic and other wood plants, Rossmassler, Der Wald, pp. 82 et seqq., and on the importance of such vegetables in checking the flow of water, Mengotti, Idraulica Fisica e Sperimentale, chapters xvi. and xvii. No writer known to me has so well illustrated this function of forest vegetation as Mengotti, though both he and Rossmassler ascribe to plants a power of absorbing water from the atmosphere which they do not possess, or rather can only rarely exercise.]

The value of the forest as a mechanical check to a too rapid discharge of rain-water was exemplified in numerous instances in the great floods of 1866 and 1868, in France and Switzerland, and I refer to the observations made on those occasions as of special importance because no previous inundations in those countries had been so carefully watched and so well described by competent investigators. In the French Department of Lozere, which was among those most severely injured by the inundation of 1866—an inundation caused by diluvial rains, not by melted snow—it was everywhere remarked that "grounds covered with wood sustained no damage even on the steepest slopes, while in cleared and cultivated fields the very soil was washed away and the rocks laid bare by the pouring rain." [Footnote: See, for other like observations, an article entitled Le Reboisement et les Inondations, in the Revue des Eaux et Forets of September, 1868]

The Italian journals of the day state that the province of Brescia and a part of that of Bergamo, which have heretofore been exposed to enormous injury, after every heavy rain, from floods of the four principal streams which traverse them, in a great degree escaped damage in the terrible inundation of October, 1872, and their immunity is ascribed to the forestal improvements executed by the former province, within ten or twelve years, in the Val Camonica and in the upper basins of the other rivers which drain that territory. Similar facts were noticed in the extraordinary floods of September and October, 1868, in the valley of the Upper Rhine, and Coaz makes the interesting observation that not even dense greensward was so efficient a protection to the earth as trees, because the water soaked through the sod and burst it up by hydrostatic pressure. [Footnote: Die Hochwasser in 1868 im Bandnerischen Rheingebiet, pp. 12, 68.

Observations of Forster, cited by Cezanne from the Annales Forestieres for 1859, p. 358, are not less important than those adduced in the text. The field of these observations was a slope of 45 degrees divided into three sections, one luxuriantly wooded from summit to base with oak and beech, one completely cleared through its whole extent, and one cleared in its upper portion, but retaining a wooded belt for a quarter of the height of the slope, which was from 1,360 to 1,800 feet above the brook at its foot.

In the first section, comprising six-sevenths of the whole surface, the rains had not produced a single ravine; in the second, occupying about a tenth of the ground, were three ravines, increasing in width from the summit to the valley beneath, where they had, all together, a cross-section of 600 square feet; in the third section, of about the same extent as the second, four ravines had been formed, widening from the crest of the slope to the belt of wood, where they gradually narrowed and finally disappeared.

For important observations to the same purpose, see Marchand, Les Torrents des Alpes, in Revue des Eaux et Forets for September, 1871.]

The importance of the mechanical resistance of the wood to the flow of water OVER THE SURFACE has, however, been exaggerated by some writers. Rain-water is generally absorbed by the forest-soil as fast as it falls, and it is only in extreme cases that it gathers itself into a superficial sheet or current overflowing the ground. There is, nevertheless, besides the absorbent power of the soil, a very considerable mechanical resistance to the transmission of water BENEATH the surface through and along the superior strata of the ground. This resistance is exerted by the roots, which both convey the water along their surface downwards, and oppose a closely wattled barrier to its descent along the slope of the permeable strata which have absorbed it. [Footnote: In a valuable report on a bill for compelling the sale of waste communal lands, now pending in the Parliament of Italy, Senator Torelli, an eminent man of science, calculates that four-fifths of the precipitation in the forest are absorbed by the soil, or detained by the obstructions of the surface, only one-fifth being delivered to the rivers rapidly enough to create danger of floods, while in open grounds, in heavy rains, the proportions are reversed. Supposing a rain-fall of four inches, an area measuring 100,000 acres, or a little more than four American townships, would receive 53,777,777 cubic yards of water. Of this quantity it would retain, or rather detain, if wooded, 41,000,000 yards, if bare, only 11,000,000. The difference of discharge from wooded and unwooded soils is perhaps exaggerated in Col. Torelli's report, but there is no doubt that in very many cases it is great enough to prevent, or to cause, destructive inundations.] Rivers fed by springs and shaded by woods are comparatively uniform in volume, in temperature, and in chemical composition. [Footnote: Dumont gives an interesting extract from the Misopogon of the Emperor Julian, showing that, in the fourth century, the Seine—the level of which now varies to the extent of thirty feet between extreme high and extreme low water mark—was almost wholly exempt from inundations, and flowed with a uniform current through the whole year. "Ego olim eram in hibernis apud curam Lutetiam, [sic] enim Galli Parisiorum oppidum appellant, quae insula est non magna, in fluvio sita, qui eam omni ex parte cingit. Pontes sublicii utrinque ad eam ferunt, raroque fluvius minuitur ac crescit; sed qualis aestate talis esse solet hyeme."—Des Travaux Publics dans leur Rapports avec l'Agriculture, p. 361, note.

As Julian was six years in Gaul, and his principal residence was at Paris, his testimony as to the habitual condition of the Seine, at a period when the provinces where its sources originate were well wooded, is very valuable.] Their banks are little abraded, nor are their courses much obstructed by fallen timber, or by earth and gravel washed down from the highlands. Their channels are subject only to slow and gradual changes, and they carry down to the lakes and the sea no accumulation of sand or silt to fill up their outlets, and, by raising their beds, to force them to spread over the low grounds near their mouth. [Footnote: Forest rivers seldom if ever form large sedimentary deposits at their points of discharge into lakes or larger streams, such accumulations beginning or at least advancing far more rapidly, after the valleys are cleared.]

Causes of Inundations.

The immediate cause of river inundations is the flow of superficial and subterranean waters into the beds of rivers faster than those channels can discharge them. The insufficiency of the channels is occasioned partly by their narrowness and partly by obstructions to their currents, the most frequent of which is the deposit of sand, gravel, and pebbles in their beds by torrential tributaries during the floods. [Footnote: The extent of the overflow and the violence of the current in river- floods are much affected by the amount of sedimentary matter let fall in their channels by their affluents, which have usually a swifter flow than the main stream, and consequently deposit more or less of their transported material when they join its more slowly-moving waters. Such deposits constitute barriers which at first check the current and raise its level, and of course its violence at lower points is augmented, both by increased volume and by the solid material it carries with it, when it acquires force enough to sweep away the obstruction.—Risler, Sur L influence des Forets sur les Cours d eau, in Revue des Eaux et Forets, 10th January, 1870.

In the flood of 1868 the torrent Illgraben, which had formerly spread its water and its sediment over the surface of a vast cone of dejection, having been forced, by the injudicious confinement of its current to a single channel, to discharge itself more directly into the Rhone, carried down a quantity of gravel, sand, and mud, sufficient to dam that river for a whole hour, and in the same great inundation the flow of the Rhine at Thusis was completely arrested for twenty minutes by a similar discharge from the Nolla. Of course, when the dam yielded to the pressure of the accumulated water, the damage to the country below was far greater than it would have ben had the currents of the rivers not been thus obstructed.—Marchand, Les Torrents des Alpes, in Revue des Eaux et Forets, Sept., 1871.]

In accordance with the usual economy of nature, we should presume that she had everywhere provided the means of discharging, without disturbance of her general arrangements or abnormal destruction of her products, the precipitation which she sheds upon the face of the earth. Observation confirms this presumption, at least in the countries to which I confine my inquiries; for, so far as we know the primitive conditions of the regions brought under human occupation within the historical period, it appears that the overflow of river-banks was much less frequent and destructive than at the present day, or, at least, that rivers rose and fell less suddenly, before man had removed the natural checks to the too rapid drainage of the basins in which their tributaries originate. The affluents of rivers draining wooded basins generally transport, and of course let fall, little or no sediment, and hence in such regions the special obstruction to the currents of water-courses to which I have just alluded does not occur. The banks of the rivers and smaller streams in the North American colonies were formerly little abraded by the currents. [Footnote: In primitive countries, running streams are very generally fringed by groves, for almost every river is, as Pliny, Nat. Hist., v. 10, says of the Upper Nile, an opifex silvarum, or, to use the quaint and picturesque language of Holland's translation, "makes shade of woods as he goeth."] Even now the trees come down almost to the water's edge along the rivers, in the larger forests of the United States, and the surface of the streams seems liable to no great change in level or in rapidity of current. [Footnote: A valuable memoir by G. Doni, in the Rivista Forestale for October, 1863, p. 438, is one of the best illustrations I can cite of the influence of forests in regulating and equalizing the flow of running water, and of the comparative action of water-courses which drain wooded valleys and valleys bared of trees, with regard to the erosion of their banks and the transportation of sediment.

"The Sestajone," remarks this writer, "and the Lima, are two considerable torrents which collect the waters of two great valleys of the Tuscan Apennines, and empty them into the Serchio. At the junction of these two torrents, from which point the combined current takes the name of Lima, a curious phenomenon is observed, which is in part easily explained. In rainy weather the waters of the Sestajone are in volume only about one-half those of the Lima, and while the current of the Lima is turbid and muddy, that of the Sestajone appears limpid and I might almost say drinkable. In clear weather, on the contrary, the waters of the Sestajone are abundant and about double those of the Lima. Now the extent of the two valleys is nearly equal, but the Sestajone winds down between banks clothed with firs and beeches, while the Lima flows through a valley that has been stripped of trees, and in great part brought under cultivation."

The Sestajone and the Lima are neither of them what is technically termed a torrent—a name strictly applicable only to streams whose current is not derived from springs and perennial, but is the temporary effect of a sudden accumulation of water from heavy rains or from a rapid melting of the snows, while their beds are dry, or nearly so, at other times. The Lima, however, in a large proportion of its course, has the erosive character of a torrent, for the amount of sediment which it carries down, even when it is only moderately swollen by rains, surpasses almost everything of the kind which I have observed, under analogous circumstances, in Italy.

Still more striking is the contrast in the regime of the Saint-Phalez and the Combe-d'Yeuse in the Department of Vancluse, the latter of which became subject to the most violent torrential floods after the destruction of the woods of its basin between 1823 and 1833, but has now been completely subdued, and its waters brought to a peaceful flow, by replanting its valley. See Labussiere, Revue Agric. et Forestiere de Provence, 1866, and Revue des Eaux et Forets, 1866.]

Inundations in Winter.

In the Northern United States, although inundations are not very unfrequently produced by heavy rains in the height of summer, it will be found generally true that the most rapid rise of the waters, and, of course, the most destructive "freshets," as they are called in America, are occasioned by the sudden dissolution of the snow before the open ground is thawed in the spring. It frequently happens that a powerful thaw sets in after a long period of frost, and the snow which had been months in accumulating is dissolved and carried off in a few hours. When the snow is deep, it, to use a popular expression, "takes the frost out of the ground" in the woods, and, if it lies long enough, in the fields also. But the heaviest snows usually fall after midwinter, and are succeeded by warm rains or sunshine, which dissolve the snow on the cleared land before it has had time to act upon the frost-bound soil beneath it. In this case, the snow in the woods is absorbed as fast as it melts, by the soil it has protected from freezing, and does not materially contribute to swell the current of the rivers. If the mild weather, in which great snow-storms usually occur, does not continue and become a regular thaw, it is almost sure to be followed by drifting winds, and the inequality with which they distribute the snow over the cleared ground leaves the ridges of the surface-soil comparatively bare, while the depressions are often filled with drifts to the height of many feet. The knolls become frozen to a great depth; succeeding partial thaws melt the surface-snow, and the water runs down into the furrows of ploughed fields, and other artificial and natural hollows, and then often freezes to solid ice. In this state of things, almost the entire surface of the cleared land is impervious to water, and from the absence of trees and the general smoothness of the ground, it offers little mechanical resistance to superficial currents. If, under these circumstances, warm weather accompanied by rain occurs, the rain and melted snow are swiftly hurried to the bottom of the valleys and gathered to raging torrents. It ought further to be considered that, though the lighter ploughed soils readily imbibe a great deal of water, yet grass-lands, and all the heavy and tenacious earths, absorb it in much smaller quantities, and less rapidly than the vegetable mould of the forest. Pasture, meadow, and clayey soils, taken together, greatly predominate over sandy ploughed fields, in all large agricultural districts, and hence, even if, in the case we are supposing, the open ground chance to have boon thawed before the melting of the snow which covers it, it is already saturated with moisture, or very soon becomes so, and, of course, cannot relieve the pressure by absorbing more water. The consequence is that the face of the country is suddenly flooded with a quantity of melted snow and rain equivalent to a fall of six or eight inches of the latter, or even more. This runs unobstructed to rivers often still-bound with thick ice, and thus inundations of a fearfully devastating character are produced. The ice bursts, from the hydrostatic pressure from below, or is violently torn up by the current, and is swept by the impetuous stream, in large masses and with resistless fury, against banks, bridges, dams, and mills erected near them. The bark of the trees along the rivers is often abraded, at a height of many feet above the ordinary water-level, by cakes of floating ice, which are at last stranded by the receding flood on meadow or ploughland, to delay, by their chilling influence, the advent of the tardy spring.

Another important effect of the removal of the forest shelter in cold climates may be noticed here. We have observed that the ground in the woods either does not freeze at all, or that if frozen it is thawed by the first considerable snow-fall. On the contrary, the open ground is usually frozen when the first spring freshet occurs, but is soon thawed by the warm rain and melting snow. Nothing more effectually disintegrates a cohesive soil than freezing and thawing, and the surface of earth which has just undergone those processes is more subject to erosion by running water than under any other circumstances. Hence more vegetable mould is washed away from cultivated grounds in such climates by the spring floods than by the heaviest rain at other seasons.

In the warm climates of Southern Europe, as I have already said, the functions of the forest, so far as the disposal of the water of precipitation is concerned, are essentially the same at all seasons, and are analogous to those which it performs in the Northern United States in summer. Hence, in the former countries, the winter floods have not the characteristics which mark them in the latter, nor is the conservative influence of the woods in winter relatively so important, though it is equally unquestionable.

If the summer floods in the United States are attended with less pecuniary damage than those of the Loire and other rivers of France, the Po and its tributaries in Italy, the Emme and her sister torrents which devastate the valleys of Switzerland, it is partly because the banks of American rivers are not yet lined with towns, their shores and the bottoms which skirt them not yet covered with improvements whose cost is counted by millions, and, consequently, a smaller amount of property is exposed to injury by inundation. But the comparative exemption of the American people from the terrible calamities which the overflow of rivers has brought on some of the fairest portions of the Old World, is, in a still greater degree, to be ascribed to the fact that, with all our thoughtless improvidence, we have not yet bared all the sources of our streams, not yet overthrown all the barriers which nature has erected to restrain her own destructive energies. Let us be wise in time, and profit by the errors of our older brethren!

The influence of the forest in preventing inundations has been very generally recognized, both as a theoretical inference and as a fact of observation; but the eminent engineer Belgrand and his commentator Valles have deduced an opposite result from various facts of experience and from scientific considerations. They contend that the superficial drainage is more regular from cleared than from wooded ground, and that clearing diminishes rather than augments the intensity of inundations. Neither of these conclusions appears to be warranted by their data or their reasoning, and they rest partly upon facts, which, truly interpreted, are not inconsistent with the received opinions on these subjects, partly upon assumptions which are contradicted by experience. Two of these latter are, first, that the fallen leaves in the forest constitute an impermeable covering of the soil over, not through, which the water of rains and of melting snows flows off, and secondly, that the roots of trees penetrate and choke up the fissures in the rocks, so as to impede the passage of water through channels which nature has provided for its descent to lower strata.

As to the first of those, we may appeal to familiar facts within the personal knowledge of every man acquainted with the operations of sylvan nature. Rain-water never, except in very trifling quantities, flows over the leaves in the woods in summer or autumn. Water runs over them only in the spring, in the rare cases when they have been pressed down smoothly and compactly by the weight of the snow—a state in which they remain only until they are dry, when shrinkage and the action of the wind soon roughen the surface so as effectually to stop, by absorption, all flow of water. I have observed that when a sudden frost succeeds a thaw at the close of the winter, after the snow has principally disappeared, the water in and between the layers of leaves sometimes freezes into a solid crust, which allows the flow of water over it. But this occurs only in depressions and on a very small scale; and the ice thus formed is so soon dissolved that no sensible effect is produced on the escape of water from the general surface.

As to the influence of roots upon drainage, we have seen that there is no doubt that they, independently of their action as absorbents, mechanically promote it. Not only does the water of the soil follow them downwards, but their swelling growth powerfully tends to enlarge, not to obstruct, the crevices of rock into which they enter; and as the fissures in rocks are longitudinal, not mere circular orifices, every line of additional width gained by the growth of roots within them increases the area of the crevice in proportion to its length. Consequently, the widening of a fissure to the extent of one inch might give an additional drainage equal to a square foot of open tubing.

The observations and reasonings of Belgrand and Valles, though their conclusions have not been accepted by many, are very important in one point of view. There writers insist much on the necessity of taking into account, in estimating the relations between precipitation and evaporation, the abstraction of water from the surface and surface-currents, by absorption and infiltration—an element unquestionably of great value, but hitherto much neglected by meteorological inquirers, who have very often reasoned as if the surface-earth were either impermeable to water or already saturated with it; whereas, in fact, it is a sponge, always imbibing humidity and always giving it off, not by evaporation only, but by infiltration and percolation.

The remarkable historical notices of inundations in France in the Middle Ages collected by Champion [Footnote: Les Inondations en France depuis le VIe siecle jusqu'a nos jours, 6 vols, 8vo. Paris, 1858-64. See a very able review of this learned and important work by Prof. Messedaglia, read before the Academy of Agriculture at Verona in 1864.] are considered by many as furnishing proof, that when that country was much more generally covered with wood than it now is, destructive inundations of the French rivers were not less frequent than they are in modern days. But this evidence is subject to this among other objections: we know, it is true, that the forests of certain departments of France were anciently much more extensive than at the present day; but we know also that in many portions of that country the soil has been bared of its forests, and then, in consequence of the depopulation of great provinces, left to reclothe itself spontaneously with trees, many times during the historic period; and our acquaintance with the forest topography of ancient Gaul or of mediaeval France is neither sufficiently extensive nor sufficiently minute to permit us to say, with certainty, that the sources of this or that particular river were more or less sheltered by wood at any given time, ancient or mediaeval, than at present. [Footnote: Alfred Maury has, nevertheless, collected, in his erudite and able work, Les Forets de la Gaule et de l'ancienne France, Paris, 1867, an immense amount of statistical detail on the extent, the distribution, and the destruction of the forests of France, but it still remains true that we can very seldom pronounce on the forestal condition of the upper valley of a particular river at the time of a given inundation in the ancient or the mediaeval period.] I say the sources of the rivers, because the floods of great rivers are occasioned by heavy rains and snows which fall in the more elevated regions around the primal springs, and not by precipitation in the main valleys or on the plains bordering on the lower course.

The destructive effects of inundations, considered simply as a mechanical power by which life is endangered, crops destroyed, and the artificial constructions of man overthrown, are very terrible. Thus far, however, the flood is a temporary and by no means an irreparable evil, for if its ravages end here, the prolific powers of nature and the industry of man soon restore what had been lost, and the face of the earth no longer shows traces of the deluge that had overwhelmed it. Inundations have even their compensations. The structures they destroy are replaced by better and more secure erections, and if they sweep off a crop of corn, they not unfrequently leave behind them, as they subside, a fertilizing deposit which enriches the exhausted field for a succession of seasons. [Footnote: The productiveness of Egypt has been attributed too exclusively to the fertilizing effects of the slime deposited by the inundations of the Nile; for in that climate a liberal supply of water would produce good crops on almost any ordinary sand, while, without water, the richest soil would yield nothing. The sediment deposited annually is but a very small fraction of an inch in thickness. It is alleged that in quantity it would be hardly sufficient for a good top-dressing, and that in quality it is not chemically distinguishable from the soil inches or feet below the surface. But to deny, as some writers have done, that the slime has any fertilizing properties at all, is as great a error as the opposite one of ascribing all the agricultural wealth of Egypt to that single cause of productiveness. Fine soils deposited by water are almost uniformly rich in all climates; those brought down by rivers, carried out into salt-water, and then returned again by the tide, seem to be more permanently fertile than any others. The polders of the Netherland coast are of this character, and the meadows in Lincolnshire, which have been covered with slime by warping, as it is called, or admitting water over them at high tide, are remarkably productive.

Recent analysis is said to have detected in the water of the Nile a quantity of organic matter—derived mainly, no doubt, from the decayed vegetation it bears down from its tropical course—sufficiently large to furnish an important supply of fertilizing ingredients to the soil.

It is computed that the Durance—a river fed chiefly by torrents, of great erosive power—carries down annually solid material enough to cover 272,000 acres of soil with a deposit of two-fifths of an inch in thickness, and that this deposit contains, in the combination most favorable to vegetation, more azote than 110,000 tons of guano, and more carbon than 121,000 acres of woodland would assimilate in a year. Elisee Reclus, La Terre, vol. i., p. 467. On the chemical composition, quantity, and value of the solid matter transported by river, see Herve Magnon, Sur l'Emploi des Eaux dans les Irrigations, 8vo. Paris, 1869, pp. 132 et seqq. Duponchel, Traite d'Hydraulique et de Geologie Agricoles. Paris, 1868, chap. i., xii., and xiii.]

If, then, the too rapid flow of the surface-waters occasioned no other evil than to produce, once in ten years upon the average, an inundation which should destroy the harvest of the low grounds along the rivers, the damage would be too inconsiderable, and of too transitory a character, to warrant the inconveniences and the expense involved in the measures which the most competent judges in many parts of Europe believe the respective governments ought to take to obviate it.

Destructive Action of Torrents.

But the great, the irreparable, the appalling mischiefs which have already resulted, and which threaten to ensue on a still more extensive scale hereafter, from too rapid superficial drainage, are of a properly geographical, we may almost say geological, character, and consist primarily in erosion, displacement, and transportation of the superficial strata, vegetable and mineral—of the integuments, so to speak, with which nature has clothed the skeleton frame-work of the globe. It is difficult to convey by description an idea of the desolation of the regions most exposed to the ravages of torrent and of flood; and the thousands who, in these days of swift travel, are whirled by steam near or even through the theatres of these calamities, have but rare and imperfect opportunities of observing the destructive causes in action. Still more rarely can they compare the past with the actual condition of the provinces in question, and trace the progress of their conversion from forest-crowned hills, luxuriant pasture grounds, and abundant cornfields and vineyards well watered by springs and fertilizing rivulets, to bald mountain ridges, rocky declivities, and steep earth-banks furrowed by deep ravines with beds now dry, now filled by torrents of fluid mud and gravel hurrying down to spread themselves over the plain, and dooming to everlasting barrenness the once productive fields. In surveying such scenes, it is difficult to resist the impression that nature pronounced a primal curse of perpetual sterility and desolation upon these sublime but fearful wastes, difficult to believe that they wore once, and but for the folly of man might still be, blessed with all the natural advantages which Providence has bestowed upon the most favored climes. But the historical evidence is conclusive as to the destructive changes occasioned by the agency of man upon the flanks of the Alps, the Apennines, the Pyrenees, and other mountain ranges in Central and Southern Europe, and the progress of physical deterioration has been so rapid that, in some localities, a single generation has witnessed the beginning and the end of the melancholy revolution.

I have stated, in a general way, the nature of the evils in question, and of the processes by which they are produced; but I shall make their precise character and magnitude better understood by presenting some descriptive and statistical details of facts of actual occurrence. I select for this purpose the south-eastern portion of France, not because that territory has suffered more severely than some others, but because its deterioration is comparatively recent, and has been watched and described by very competent and trustworthy observers, whose reports are more easily accessible than those published in other countries. [Footnote: Streffleur (Ueber die Natur und die Wirkungen der Wildbuche, p. 3) maintains that all the observations and speculations of French authors on the nature of torrents had been anticipated by Austrian writers. In proof of this assertion he refers to the works of Franz von Zallinger, 1778, Von Arretin, 1808, Franz Duile, 1826, all published at Innsbruck, and Hagenus Beschreibung neuerer Wasserbauwerke, Konigsberg, 1826, none of which works are known to me. It is evident, however, that the conclusions of Surell and other French writers whom I cite, are original results of personal investigation, and not borrowed opinions.]

The provinces of Dauphiny and Provence comprise a territory of fourteen or fifteen thousand square miles, bounded north-west by the Isere, north-east and east by the Alps, south by the Mediterranean, west by the Rhone, and extending from 42 degrees to about 45 degrees of north latitude. The surface is generally hilly and even mountainous, and several of the peaks in Dauphiny rise above the limit of perpetual snow. Except upon the mountain ridges, the climate, as compared with that of the United States in the same latitude, is extremely mild. Little snow falls, except upon the higher mountains, the frosts are light, and the summers long, as might, indeed, be inferred from the vegetation; for in the cultivated districts, the vine and the fig everywhere flourish; the olive thrives as far north as 43 and one half degrees, and upon the coast grow the orange, the lemon, and the date-palm. The forest trees, too, are of southern type, umbrella pines, various species of evergreen oaks, and many other trees and shrubs of persistent broad-leaved foliage, characterizing the landscape.

The rapid slope of the mountains naturally exposed these provinces to damage by torrents, and the Romans diminished their injurious effects by erecting, in the beds of ravines, barriers of rocks loosely piled up, which permitted a slow escape of the water, but compelled it to deposit above the dikes the earth and gravel with which it was charged. [Footnote: Whether Palissy was acquainted with this ancient practice, or whether it was one of those original suggestions of which his works are so full, I know not, but in his treatise, Des Eaux et Fontaines, he thus recommends it, by way of reply to the objections of "Theorique," who had expressed the fear that "the waters which rush violently down from the heights of the mountain would bring with them much earth, sand, and other things," and thus spoil the artificial fountain that "Practique" was teaching him to make: "And for hindrance of the mischiefs of great waters which may be gathered in a few hours by great storms, when thou shalt have made ready thy parterre to receive the water, thou must lay great atones athwart the deep channels which lead to thy parterre. And so the force of the rushing currents shall be deadened, and thy water shall flow peacefully into his cisterns."—Oeuvres Completes, p. 178.] At a later period the Crusaders brought home from Palestine, with much other knowledge gathered from the wiser Moslems, the art of securing the hillsides and making them productive by terracing and irrigation. The forests which covered the mountains secured an abundant flow of springs, and the process of clearing the soil went on so slowly that, for centuries, neither the want of timber and fuel, nor the other evils about to be depicted, were seriously felt. Indeed, throughout the Middle Ages, these provinces were well wooded, and famous for the fertility and abundance, not only of the low grounds, but of the hills.

Such was the state of things at the close of the fifteenth century. The statistics of the seventeenth show that while there had been an increase of prosperity and population in Lower Provence, as well as in the correspondingly situated parts of the other two provinces I have mentioned, there was an alarming decrease both in the wealth and in the population of Upper Provence and Dauphiny, although, by the clearing of the forests, a great extent of plough-land and pasturage had been added to the soil before reduced to cultivation. It was found, in fact, that the augmented violence of the torrents had swept away, or buried in sand and gravel, more land than had been reclaimed by clearing; and the taxes computed by fires or habitations underwent several successive reductions in consequence of the gradual abandonment of the wasted soil by its starving occupants. The growth of the large towns on and near the Rhone and the coast, their advance in commerce and industry, and the consequently enlarged demand for agricultural products, ought naturally to have increased the rural population and the value of their lands; but the physical decay of the uplands was such that considerable tracts were deserted altogether, and in Upper Provence, the fires which, in 1471 counted 897, were reduced to 747 in 1699, to 728 in 1733, and to 635 in 1776. [Footnote: These facts I take from the La Provence au point de vue des Bois, des Torrents et des Inondations, of Charles de Ribbe, one of the highest authorities.]

Surell—whose admirable work, Etude sur les Torrents des Hautes Alpes, first published in 1841, [Footnote: A second edition of this work, with an additional volume of great value by Ernest Cezanne, was published at Paris, in two 8vo volumes, in 1871-72.] presents a most appalling picture of the desolations of the torrent, and, at the same time, the most careful studies of the history and essential character of this great evil—in speaking of the valley of Devoluy, on page 152, says: "Everything concurs to show that it was anciently wooded. In its peat-bogs are found buried trunks of trees, monuments of its former vegetation. In the framework of old houses, one sees enormous timber, which is no longer to be found in the district. Many localities, now completely bare, still retain the name of 'wood,' and one of them is called, in old deeds, Comba nigra [Black forest or dell], on account of its dense woods. These and many other proofs confirm the local traditions which are unanimous on this point.

"There, as everywhere in the Upper Alps, the clearings began on the flanks of the mountains, and were gradually extended into the valleys and then to the highest accessible peaks. Then followed the Revolution, and caused the destruction of the remainder of the trees which had thus far escaped the woodman's axe."

In a note to this passage the writer says: "Several persons have told me that they had lost flocks of sheep, by straying, in the forests of Mont Auroux, which covered the flanks of the mountain from La Cluse to Agneres. These declivities are now as bare as the palm of the hand."

The ground upon the steep mountains being once bared of trees, and the underwood killed by the grazing of horned cattle, sheep, and goats, every depression becomes a water-course. "Every storm," says Surell, page 153, "gives rise to a new torrent. [Footnote: No attentive observer can frequent the southern flank of the Piedmontese Alps or the French province of Dauphiny, for half a dozen years, without witnessing with his own eyes the formation and increase of new torrents. I can bear personal testimony to the conversion of more than one grassy slope into the bed of a furious torrent by baring the hills above of their woods.] Examples of such are shown, which, though not yet three years old, have laid waste the finest fields of their valleys, and whole villages have narrowly escaped being swept into ravines formed in the course of a few hours. Sometimes the flood pours in a sheet over the surface, without ravine or even bed, and ruins extensive grounds, which are abandoned forever."

I cannot follow Surell in his description and classification of torrents, and I must refer the reader to his instructive work for a full exposition of the theory of the subject. In order, however, to show what a concentration of destructive energies may be effected by felling the woods that clothe and support the sides of mountain abysses, I cite his description of a valley descending from the Col Isoard, which he calls "a complete type of a basin of reception," that is, a gorge which serves as a common point of accumulation and discharge for the waters of several lateral torrents. "The aspect of the monstrous channel," says he, "is frightful. Within a distance of less than two English miles, more than sixty torrents hurl into the depths of the gorge the debris torn from its two flanks. The smallest of these secondary torrents, if transferred to a fertile valley, would be enough to ruin it."

The eminent political economist Blanqui, in a memoir read before the Academy of Moral and Political Science on the 25th of November, 1843, thus expresses himself: "Important as are the causes of impoverishment already described, they are not to be compared to the consequences which have followed from the two inveterate evils of the Alpine provinces of France, the extension of clearing and the ravages of torrents. ... The most important result of this destruction is this; that the agricultural capital, or rather the ground itself—which, in a rapidly increasing degree, is daily swept away by the waters—is totally lost. Signs of unparalleled destitution are visible in all the mountain zone, and the solitudes of those districts are assuming an indescribable character of sterility and desolation. The gradual destruction of the woods has, in a thousand localities, annihilated at once the springs and the fuel. Between Grenoble and Briancon, in the valley of the Romanche, many villages are so destitute of wood that they are reduced to the necessity of baking their bread with sun-dried cow-dung, and even this they can afford to do but once a year.

"Whoever has visited the valley of Barcelonette, those of Embrun, and of Verdun, and that Arabia Petraea of the department of the Upper Alps, called Devoluy, knows that there is no time to lose—that in fifty years from this date France will be separated from Savoy, as Egypt from Syria, by a desert." [Footnote: Ladoucette says the peasant of Devoluy "often goes a distance of five hours over rocks and precipices for a single [man's] load of wood;" and he remarks on another page, that "the justice of peace of that canton had, in the course of forty-three years, but once heard the voice of the nightingale."—Histoire, etc, des Hautes Alpes, pp. 220, 434.]

It deserves to be specially noticed that the district here referred to, though now among the most hopelessly waste in France, was very productive even down to so late a period as the commencement of the French Revolution. Arthur Young, writing in 1789, says: "About Barcelonette and in the highest parts of the mountains, the hill-pastures feed a million of sheep, besides large herds of other cattle;" and he adds: "With such a soil and in such a climate, we are not to suppose a country barren because it is mountainous. The valleys I have visited are, in general, beautiful." [Footnote: The valley of Embrun, now almost completely devastated, was once remarkable for its fertility. In 1800, Hericart de Thury said of it: "In this magnificent valley nature had been prodigal of her gifts. Its inhabitants have blindly revelled in her favors, and fallen asleep in the midst of her profusion."—Becquerel, Des Climats, etc., p. 314.] He ascribes the same character to the provinces of Dauphiny, Provence, and Auvergne, and, though he visited, with the eye of an attentive and practised observer, many of the scenes since blasted with the wild desolation described by Blanqui, the Durance and a part of the course of the Loire are the only streams he mentions as inflicting serious injury by their floods. The ravages of the torrents had, indeed, as we have seen, commenced earlier in some other localities, but we are authorized to infer that they were, in Young's time, too limited in range, and relatively too insignificant, to require notice in a general view of the provinces where they have now ruined so large a proportion of the soil.

But I resume my citations.

"I do not exaggerate," says Blanqui. "When I shall have finished my description and designated localities by their names, there will rise, I am sure, more than one voice from the spots themselves, to attest the rigorous exactness of this picture of their wretchedness. I have never seen its equal even in the Kabyle villages of the province of Constantine; for there you can travel on horseback, and you find grass in the spring, whereas in more than fifty communes in the Alps there is absolutely nothing.

"The clear, brilliant, Alpine sky of Embrun, of Gap, of Barcelonette, and of Digne, which for months is without a cloud, produces droughts interrupted only by diluvial rains like those of the tropics. The abuse of the right of pasturage and the felling of the woods have stripped the soil of all its grass and all its trees, and the scorching sun bakes it to the consistence of porphyry. When moistened by the rain, as it has neither support nor cohesion, it rolls down to the valleys, sometimes in floods resembling black, yellow, or reddish lava, sometimes in streams of pebbles, and over huge blocks of stone, which pour down with a frightful roar, and in their swift course exhibit the most convulsive movements. If you overlook from an eminence one of these landscapes furrowed with so many ravines, it presents only images of desolation and of death. Vast deposits of flinty pebbles, many feet in thickness, which have rolled down and spread far over the plain, surround large trees, bury even their tops, and rise above them, leaving to the husbandman no longer a ray of hope. One can imagine no sadder spectacle than the deep fissures in the flanks of the mountains, which seem to have burst forth in eruption to cover the plains with their ruins. Those gorges, under the influence of the sun which cracks and shivers to fragments the very rocks, and of the rain which sweeps them down, penetrate deeper and deeper into the heart of the mountain, while the beds of the torrents issuing from them are sometimes raised several feet in a single year, by the debris, so that they reach the level of the bridges, which, of course, are then carried off. The torrent-beds are recognized at a great distance, as they issue from the mountains, and they spread themselves over the low grounds, in fan-shaped expansions, like a mantle of stone, sometimes ten thousand feet wide, rising high at the centre, and curving towards the circumference till their lower edges meet the plain.

"Such is their aspect in dry weather. But no tongue can give an adequate description of their devastations in one of those sudden floods winch resemble, in almost none of their phenomena, the action of ordinary river-water. They are now no longer overflowing brooks, but real seas, tumbling down in cataracts, and rolling before them blocks of stone, which are hurled forwards by the shock of the waves like balls shot out by the explosion of gunpowder. Sometimes ridges of pebbles are driven down when the transporting torrent does not rise high enough to show itself, and then the movement is accompanied with a roar louder than the crash of thunder. A furious wind precedes the rushing water and announces its approach. Then comes a violent eruption, followed by a flow of muddy waves, and after a few hours all returns to the dreary silence which at periods of rest marks these abodes of desolation. [Footnote: These explosive gushes of mud and rock appear to be occasioned by the caving-in of large masses of earth from the banks of the torrent, which dam up the stream and check its flow until it has acquired volume enough to burst the barrier and carry all before it. In 1827, such a sudden eruption of a torrent, after the current had appeared to have ceased, swept off forty-two houses and drowned twenty-eight persons in the village of Goncelin, near Grenoble, and buried with rubbish a great part of the remainder of the village."