Influence du déboisement des montagnes sur les fleuves et les rivières.

CHAP. 13: Influence of the deforestation of mountains on rivers and streams.

It will be readily recognized that every river is the common property of the various populations that occupy its basin; that the proprietors of the lower lands cannot make any work on it to slow its course, in such a way as to harm the proprietors of the upper lands; and that the latter can make use of it, whether to carry away the waters that fall on their lands, or to export their foodstuffs, or to have arrive in their country the things they need, and which they can draw only from abroad.

But will it also be recognized that the populations formed in the lower parts of the basin, in those that were the first to be converted into private properties, and which are the best cultivated, the richest, the most populous, are likewise proprietors in undivided ownership of the river on whose bank their agriculture, their industry, their commerce, have developed, and that the populations by which the plateaus and the slopes of the mountains are occupied cannot deteriorate it, by modifying the lands that pour their waters into it?

This question is much more important than that which was treated in the preceding chapter; the works that the inhabitants of a city can execute on the river that traverses it can never cause great damage to the properties situated above them, if moreover they are not an obstacle to navigation; but the proprietors of the highest lands, those who possess the plateaus and the slopes of the mountains, can, by the manner in which they dispose of these lands, cause considerable damage to the lower properties and to the populations that occupy the bottom of the basin.

The water of which a river is formed is only the water of rain or snow, which falls annually in its basin, which infiltrates slowly into the lands, and which, by the obstacles it encounters, is obliged to reappear on the surface of the lower lands; in very high places where the heat never lasts long enough to completely melt the snows or the ice that accumulate there during a part of the year, one finds, it is true, running waters that do not come from infiltrations; but it is not of those that we have to occupy ourselves here; human industry can neither increase nor sensibly diminish their quantity.

If rainwater evaporated as it fell, there would be no more infiltration, and consequently the springs and rivers would dry up. If, instead of evaporating, it fell on steep slopes, stripped of vegetation and of all matter proper to retain it, it would rush with force into the valleys, and the rivers would be only torrents. It is necessary, therefore, for rivers or streams to have an equal and regular course, and for them to be truly useful, that the water that results from the fall of snows or rains infiltrates into the earth in a very slow manner. In a country that counted, for example, in the course of a year, eighty days of rain and two hundred and seventy-four of drought, it would be necessary for the rivers to be always in good condition, that the time necessary for infiltration have three or four times more duration than the rainy season.

One now understands how the populations that possess the plateaus and the slopes of the mountains can, by acting on the lands of which they are in possession, cause great damage to the properties situated in the most inferior parts of the basins; it suffices for them, to make the water destined to supply the rivers evaporate, or to convert it into torrents, to destroy the trees and the vegetation that prevent evaporation, or that retain the soils on the mountain slopes.

It is easy to conceive, moreover, that the dangers of this kind are more or less great, according to whether one is placed under a more or less ardent sky, and according to whether the mountains that form the edges of the basins are more or less extensive, more or less steep; evaporation happens in a more rapid manner on the lands placed between the tropics, than in the islands of Great Britain or in Denmark; and the rain more easily forms torrents in the mountains of Switzerland, of some parts of Italy and of France, than in countries where the lands have little slope.

These observations are not only the result of an induction drawn from the nature of things; they are the product of experiments made in various times and in various countries; and the more one reflects on the causes of the decadence or the prosperity of peoples, the more one will find that they are of importance. The disastrous effects of the deforestation of mountains have manifested themselves in all countries; but it is particularly under hot climates that one has promptly perceived them.

In the island of Trinidad, it has been remarked that the rains diminished as the clearings made progress, that is to say as the forests disappeared. In a space of fifteen or sixteen years, the water of the rivers whose basins were stripped of trees has sometimes been seen to decrease in a sensible manner; while the neighboring basins, whose trees were preserved, continued to be watered by the same quantity of water [^64].

The same phenomenon has been remarked in Martinique: the mountains have been stripped of their forests, and, since that time, the basins of which these forests were a part are deprived of breezes, of rains, of fountains, of abundant dews [^65].

In Saint-Domingue, the same causes have produced similar effects: the colonists have stripped the mountains of the forests that crowned them, principally on the windward side of the island; and since that time drought has devoured everything.

These woods, says a traveler, arrested the clouds, aspirated the vapors, maintained the coolness and humidity under their shade, fed the springs that gushed from the feet of their mornes; but, since these mornes have been stripped of their useful vegetation, the fertilizing vapors have ceased to stop there. The vapors there have become rare; thus the drought and aridity of the mornes has dried up the sources of the fertility of the surrounding plains. The clouds falling back to leeward no longer stop except toward those high peaks, neighbors of Saint-Pierre, where they dissolve into repeated and abundant rains [^66].

Deforestation has produced in Île de France (today Mauritius), the same effects as in the islands of Trinidad and Saint-Domingue.> “However abundant the rains may still be in Île-de-France,” says Péron, “it is a generally established opinion, throughout the country, that they have greatly diminished in the last twenty-five or thirty years, and everyone blames the considerable clearings, which, especially in recent times, have been made in too injudicious a manner. This sentiment is shared by all the most enlightened and oldest cultivators; all claim that the rivers today carry noticeably less water than before; that several springs have dried up; that vegetation is no longer as active; and this last effect, they attribute far less to the exhaustion of the soil than to the lack of habitual moisture. Certainly, it is not impossible that the injudicious felling of forests has indeed contributed much to diminishing the absolute quantity of rainfall, but it is also quite possible that, this quantity remaining the same, it is nevertheless no longer sufficient for the needs of vegetation, because the first effect of the denudation of the soil is to make evaporation quicker, and above all more considerable [^67].”

The effect produced in the West Indies by the deforestation of mountains has been felt in the parts of the American continent where the mountains have been stripped of the woods that covered them. In the United States, it has long been observed that the cutting of forests, particularly on the heights, generally diminishes the mass of rainfall, and of the fountains that result from it, by preventing the clouds from fixing and distilling themselves on high places.

“Kentucky itself,” says Volney, “offers proof of this, as do all the other states of America, since a multitude of streams are already cited there that did not run dry fifteen years ago, and which now lack water; others have totally disappeared, and several mills, in New Jersey, have been abandoned for this reason [^68].”

A celebrated naturalist, M. Alexander von Humboldt, has made, on the effects produced by the deforestation of mountains, observations similar to those of Volney. His testimony is of such great weight here that I will be forgiven for reporting it in its entirety.

“By felling the trees that cover the summit and the flank of the mountains,” he says, “men, in all climates, prepare two calamities at once for future generations, a lack of fuel and a lack of water.

“Trees, by the nature of their transpiration and the radiation of their leaves toward a cloudless sky, envelop themselves in a constantly cool and misty atmosphere: they act on the abundance of springs, not as has been so long believed, by a particular attraction for the vapors that are diffused in the air, but because by sheltering the soil from the direct action of the sun, they diminish the evaporation of rainwater.

“When forests are destroyed, as the European colonists do everywhere in America with an imprudent precipitation, the springs dry up entirely or become less abundant. The beds of the rivers remain dry for a part of the year, and are converted into torrents each time great downpours fall on the heights.

“As with the brushwood, the turf and the moss are seen to disappear on the crest of the mountains, the rainwater is no longer retained in its course; instead of slowly increasing the level of the rivers by progressive infiltrations, it furrows, at the time of great showers, the flank of the hills, carries away the eroded soil, and forms those sudden floods that devastate the countryside.

“It results from this that the destruction of forests, the lack of permanent springs, and the existence of torrents, are three phenomena closely linked to one another. Countries situated in opposite hemispheres, Lombardy, bordered by the chain of the Alps, and lower Peru, confined between the Pacific Ocean and the Cordillera of the Andes, offer striking proofs of the correctness of this observation [^69].”

The islands that have been completely stripped of trees, like Easter Island, have been reduced to having neither ravines, nor streams, nor springs; a horrible drought has destroyed the plants and shrubs, and they have become almost uninhabitable [^70].

The destruction of woods and the disappearance of springs and rivers have produced, in some parts of the ancient world, still more fatal effects; some parts of Persia and of Upper Egypt have been transformed into arid deserts; men and animals have disappeared from them with the vegetation [^71].

The changes that the soil of the various states of Europe has undergone go back to times too remote and too barbarous for it to have been possible to observe and ascertain the effects they have produced on the springs, on the rivers, on the streams, and on the lands susceptible of cultivation. One can hardly doubt, however, that these effects have been analogous to those that have been remarked in America and in other parts of the world, and that the rivers have diminished and become more irregular, as the woods have disappeared from the plateaus and the slopes of the mountains.

In the last days of the Roman republic, Gaul and Germany were covered with immense forests that have been in large part destroyed [^72]. If this fact were not ascertained by Roman writers, it would be by the numerous monuments of the Druids that still exist on plateaus completely stripped of trees, and which were formerly in the middle of forests. Now, it is impossible that these forests, situated on the plateaus or on the crest of the mountains, could have disappeared from them without any effect resulting on the springs that the waters of snow or rain produced. These springs have certainly become less abundant, rarer, more irregular; some have probably been replaced by torrents.

Several historians of the former province of Franche-Comté have thought that the rivers that descend from the Jura were formerly more considerable than they are in our time; they have believed that the Doubs, for example, was navigable at a point where it is not today; and there still exist, near the village of Mandeure [^73], a league and a half from Pont-de-Roide, vestiges of a bridge, which support this opinion [^74].

The uncultivated lands that exist in the department of Indre and whose extent is 204,746 arpents, were formerly ancient forests that fires have devoured or that the hands of men have destroyed [^75]. In some parts of the department of Deux-Sèvres, vast expanses of forests have likewise disappeared, and the springs they fed have dried up [^76].

There are departments in which the destruction of the forests that covered the mountains, and whose waters fed the rivers and carried fertility into the plains, does not date from a very remote epoch. When, in the years 1787, 1788, and 1789, Arthur Young made a journey in France, to observe its agriculture and its various resources, he was witness to devastations that he would not have believed possible, if he had not seen them with his own eyes. He deserves all the more confidence, in his testimony in this regard, as in his opinion the price of wood was still too low at that epoch, and he believed it useful to convert forests into arable land, until the moment when the product of an arpent in wood would be equal to the product of an arpent in cereals or in fodder. The devastations of which he was a witness and which he bitterly deplored, were taking place in the Pyrenees.

“A great part of these mountains,” he says, “is covered with woods, and a much greater part has been; for the destruction that is made of them every day is not believable for those who have not seen it. I frequently passed through several woods near Bagnères de Luchon, in which men were at work, cutting and splitting young birches, to make barrel hoops. I was shocked to see the consumption they made of them, which would not have been more devastating and more prodigal in the middle of an American forest.... This beautiful and noble forest of Lartigues has experienced a devastation so general, that it is almost entirely destroyed; there are no young shoots to replace the trees that have disappeared, and, in ten or twelve years, it will be nothing more than a bare mountain, with some miserable shrubs browsed by goats or by other animals.

“In certain parts that I visited, a few leagues distant, toward the lands traversed by the Spanish flocks, there are forests destroyed in so shameful a manner, that it is incredible for the citizen of a country in which wood has some value. Several score of acres were so completely ruined that not a single tree remained standing; and yet it was still a whole forest of trunks three, four, and six feet high, a sad and shocking spectacle to see! On all sides, the torrents carry away as much wood as stones, and present similar ruins; the roads are formed with fragments of trees, and are protected against the precipices by whole trees that are placed there and left to rot. One does not advance a few steps without plunging his cane into tree trunks that are rotting or are already rotten. Everything is ruin, devastation, desolation; it is the aspect of a forest where an enemy army, in a fit of license and wickedness, would have destroyed everything” [^77].

In deploring these devastations, Arthur Young saw in them only the loss that resulted immediately, that of the wood; he did not seem to suspect the effects that were to be the consequence for the springs and the rivers, nor the damages that could result for the most fertile lands.

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Notes

[^64]: Dauxion Lavaysse, Voyage aux Îles de la Trinidad, vol. 1, ch. 2, p. 96-97. [^65]: Robin, Voyage dans la Louisiane, vol. 1, ch. 15, p. 228. [^66]: Robin, vol. 1, ch. 6, p. 89 and 90; ch. 15, p. 228. [^67]: Péron, bk. 1, ch. 4, p. 51. — La Pérouse, vol. 2, ch. 4, p. 93 and 94. [^68]: Voyage aux Etats-Unis, vol. 1, ch. 3, p. 26 and 27. [^69]: Voyage aux régions équinoxiales, bk. V, ch. 16, vol. 5 p. 172-174. [^70]: La Pérouse, vol. 2, ch. 4, p. 92-94. [^71]: See Traité de législation, bk. III, ch. 23, vol. 3, p. 147-148. [^72]: See the Commentaries of Caesar. [^73]: The village of Mandeure was, in Roman times, a considerable town that bore the name of Epamandudorum. Many vestiges and ruins of this town still remain. [^74]: Statistique générale de la France, published by order of the Government. Department of Doubs, ch. 1, p. 3. [^75]: Statistique générale de la France. Department of Indre, p. 173-257. [^76]: Statistique générale de la France. Department of Deux-Sèvres, p. 132. [^77]: Travels during the years 1787, 1788 and 1789 undertaken more particularly with a view of ascertaining the cultivation, wealth, resources, and national prosperity of the Kingdom of France, by Arthur Young, vol. 2. ch. p. 26, p. 106.