Our national forests

by Richard H. D. Boerker

Figure 49. Wrecked farm buildings due to flood of May 21, 1901, Nolichucky River, near Erwin, Tenn. This is one result of denuding the Appalachian Mountains of their forest cover.

Figure 50. When steep hillsides are stripped of their forest growth, erosion results. Erosion has been especially serious in the Appalachian Mountains. View taken in Madison County, North Carolina.

Water Supply. Undoubtedly the greatest value of the mountain forests of the West, most of which are within the National Forests, lies in their influence upon the regularity of the water supply. In many States these mountains afford the only water supply for domestic use, for irrigation, and for the development of power. The future development of the entire region depends, therefore, upon a regular water supply. It is not so much the amount of water as the manner in which it flows from the mountains that is important. To insure this regularity, the vegetative covering is an important factor. For this reason, Congress made the preservation of conditions favorable to stream flow one of the principal objects in the establishment and administration of the National Forests.

Many of my readers who have lived out-of-doors a great deal have learned by common observation the simple problem of how the forest regulates stream flow. Any one who has been in a treeless region after a heavy rainstorm can recall how suddenly the streams swell and flood their banks, and how soon these same streams return to their former flow. On the other hand, a severe rainstorm in a forested region will hardly have an appreciable effect upon the streams. The difference is not very hard to explain. In a treeless region there are no natural obstacles which might delay or prevent the raindrops from reaching the ground. The soil is usually hard and dry, and the water runs off as though from a gable roof. In a forest, we well know, the crowns of the trees intercept most of the rain that falls; very little strikes the ground directly. The rain that strikes the crown is dissipated on the leaves or needles, on the twigs and branches, and on the trunk. It must travel a long way before it reaches the ground, and all this delay helps in preventing a rapid run-off or flood. The soil in the forest is covered by a living ground cover of flowers, shrubs and young trees, and by a dead cover composed of leaves, twigs, dead branches, fallen trees, all of which interrupt the raindrop's journey to the ground. Even after the rain reaches the ground, only a small part of it goes off as surface run-off. The soil in the forest is loose and full of holes and channels made by decaying roots, earth worms, etc., so that the water is absorbed as fast as it reaches the soil. Also the soil in the forest contains a large amount of organic matter, resulting from decaying leaves and branches, and this organic matter acts as a great sponge, because it is capable of holding several times its own weight of water. As a result of the living and dead ground cover, the crown cover, and the organic matter in the soil, the rainfall is fed to the streams gradually through weeks and months, instead of a few hours, and the nearby rivers have a steady, equable flow, instead of alternate stages of floods and low water.

Closely bound up with the protection of watersheds is the erosion problem. Without a forest cover, rain runs off mountain slopes very rapidly, often carrying with it silt and sand, and, in severe floods, even rocks and bowlders. A well known physical law states that the carrying capacity of a stream increases as the sixth power of its velocity. In other words, double the velocity of a stream and you have multiplied its carrying power by 64; increase its velocity ten times, and you multiply its carrying power by a million. The delay caused by the forest cover in each raindrop's journey down a mountain side not only prevents floods, but also preserves the fertility of the fields in the valleys below.

Many streams in the West carry such enormous amounts of silt that the storage capacity of reservoirs has been seriously impaired, even within a comparatively short time. Then, also, there is the added difficulty and expense of keeping the diversion works—the ditches and canals—free from an excess of this material. Studies which have been carried on to determine in what way the administration of the National Forests can keep the destructive processes of erosion at a minimum have shown that the balance between the stability of the soil and rapid erosion on many slopes is so delicate that only a slight abuse may result in complete loss of the fertile top soil and permanent changes in the character of the vegetation.

In August, 1909, the town of Ephraim, on the Manti National Forest, Utah, experienced a disastrous flood from Ephraim canyon, which was attributed in part to the overgrazed condition on the watershed. An examination made the next spring clearly demonstrated that the severity of the flood was a direct result of deterioration of forest, brush, and grass cover, due to overgrazing during a long period of years. The canyon was therefore closed to grazing as an immediate protective measure. Plans were thereafter made to restore the forest cover of the canyon by planting.

Figure 51. A fertile corn-field covered with sand, gravel, and débris brought down from the mountains by floods. These farm lands are ruined beyond redemption. This could have been prevented by preserving the forests on the watershed of this river.