Forests And Floods
( Originally Published 1922 )
Forests are necessary at the headwaters of streams. The trees break the force of the rain drops, and the forest floor, acting as a large sponge, absorbs rainfall and prevents run-off and floods. Unless there are forests at the sources of streams and rivers, floods occur. The spring uprisings of the Mississippi, Ohio and Missouri Rivers are due largely to the lack of forests at their headwaters. In the regions drained by these streams the rim-off water is not absorbed as it should be. It flows unimpeded from the higher levels to the river valleys. It floods the river courses with so much water that they burst their banks and pour pell-mell over the surrounding country. Many floods which occur in the United States occur because we have cut down large areas of trees which formerly protected the sources of streams and rivers.
A grave danger that threatens western farming is that some time in the future the greater part of the vegetation and forest cover on the watersheds of that section may entirely disappear. Such a condition would cause floods after every heavy rain. The available supplies of rain-water which are needed for the thirsty crops would be wasted as flood waters. These floods would cause great damage in the valleys through which they rushed. The freshets would be followed by periods of water famine. The streams would then be so low that they could not supply the normal demands. Farmers would suffer on account of the lack of irrigation water. Towns and cities that depended on the mountain streams for their water supplies would be handicapped severely. In a thousand and one ways, a deficient water supply due to forest depletion would cause hardships and suffering in the regions ex-posed to such misfortune.
The important part which forests play in the development of our country is shown by the fact that from the streams of the National Forests over 700 western cities and towns, with an aggregate population of nearly 2,500,000, obtain their domestic water supply. The forests include 1266 irrigation projects and 325 water-power plants, in addition to many other power and irrigation companies which depend on the Government timberlands for water conservation and the regulation of rain water run off and stream flow.
The National Forests aid greatly in conserving and making available for use the precious limited rainfall of the arid regions. That is why settlers in irrigated districts are deeply interested in the cutting of timber in the Federal woodlands. Destructive lumbering is never practiced in these forests. In its place has been substituted a system of management that assures the continued preservation of the forest-cover. Uncle Sam is paying special attention to the western water-sheds which supply reclamation and irrigation projects. He understands that the ability of the forest to regulate stream flow is of great importance. The irrigation farmers also desire a regular flow, evenly distributed, throughout the growing season.
One of the chief reasons for the establishment of the National Forest was to preserve the natural conditions favorable to stream flow. In a treeless country, the rise of the streams is a very accurate measure of the rainfall. In the region where forests are frequent, an ordinary rain is scarcely noticed in its effect on the stream. In a denuded district no natural obstacles impede the raindrops as they patter to the ground. The surface of the soil is usually hard. It is baked and dried out by the sun. It is not in condition to absorb or retain much of the run-off water, consequently, the rain water finds little to stop it as it swirls down the slopes. In torrents it rushes down the stream beds, like sheets of water flowing down the steep roof of a house.
Conditions are very different in a region where forest cover is abundant. In the forests, the tops of the trees catch much of the rain that falls. The leaves, twigs, branches and trunks of the trees also soak up considerable moisture. The amount of rainfall that directly strikes the ground is relatively small. The upper layer of the forested ground consists of a network of shrubs, and dead leaves, branches, and moss. This forest carpet acts like an enormous sponge. It soaks up the moisture which drops from the trees during a storm. It can absorb and hold for a time a rainfall of four or five inches. The water that finally reaches the ground sinks into the soil and is evaporated or runs off slowly. The portion that is absorbed by the soil is taken up by the roots of the trees and plants or goes to supply springs and watercourses.
The power of the trees and forest soil to absorb water regulates the rate at which the rain-fall is fed to the streams and rivers. Frequently it takes weeks and even months for all the waters of a certain rain to reach these streams. This gradual supplying of water to the streams regulates their flow. It prevents floods and freshets. Careful observation and measurements have shown that unforested regions will discharge rain water at least twice as fast as will forested districts.
The stealing of soil by erosion occurs where run-off waters are not obstructed by forest growth. Silt, sand, and every other kind of soil are swept from their natural positions and spritted away by the foaming waters as they surge down the steep slopes. The stream or river which is flooded by these rushing waters roars down its narrow channel, tearing loose and under-mining the jutting banks. In some cases, it will break from its ordinary course to flood exposed fields and to carry away more soil. As the speed of the stream increases its power to steal soil and carry it off is increased. Engineers report that the carrying power of a stream is increased 64 times when its rate of flow is doubled. If the flow of a river is speeded up ten times, this raging torrent will be able to carry one million times as much foreign material as it did when it was flowing at a normal rate of speed, causing inexpressible damage and destruction of life and property.
The protection afforded by forests on the water-sheds of streams furnishing the domestic water supply for cities and towns is becoming more fully realized. A large number of cities and towns have purchased and are maintaining municipal or communal forests for this very reason.