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 rainfall 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.