DEPARTMENT BULLETIN No. 1430

Washington, D. C.

August, 1926

FINANCIAL LIMITATION IN THE EMPLOYMENT OF FOREST COVER IN

PROTECTING RESERVOIRS

By W. W. ASHE,' Assistant District Forester, Forest Service

The influence of the forests in protecting reservoirs used for purposes varies in different regions with the differences in soil, surface, and precipitation. In this bulletin those regions in which this influence is most important (pl. 1) are pointed out, also the value of forest cover and the financial limitations in its employment to prolong the utility of a reservoir are discussed.

POSSIBILITIES IN THE STORAGE OF STORM WATER FOR POWER

The development of water power in the United States has reached a point where hydroelectric engineers are no longer satisfied to use merely the minimum flow of a stream. Many power companies are now planning the storage of storm water or the spring and summer melt of snow.

The water-power resources of the United States can be greatly increased through storage of storm waters, it being estimated by

The writer desires to acknowledge the many valuable suggestions and criticisms given by Thorndyke Saville in connection with the preparation of this bulletin.

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M. O. Leighton, of the United States Geological Survey, that with a minimum of 36,000,000 horsepower without storage there might with storage "eventually be established a total power installation of at least 200,000,000 horsepower and probably much more."

The minimum water-power resource of the eight southern Appalachian States, without storage, is placed by the United States Geological Survey at 3,340,000 horsepower. If storage of storm water is employed, this minimum can be increased, at a conservative estimate. to 5,678,000 horsepower. This increase is based upon the figures of the Geological Survey of Tennessee 2 indicating the possible increase in that State. Plans which have been worked out for the development of water power in Tennessee show that the power could be increased by storage 70 per cent above the minimum dry-season flow. Since the Tennessee River, on which these figures are based, is a large stream, its tributaries having their sources in six other States. it can reasonably be assumed that the water-power resources of the Appalachian and Piedmont regions as a whole might be increased in the same proportion.

A vast power plan which has been outlined by Harold C. Fiske for the full development of the Tennessee River calls for the construction of about 100 dams on the river above Knoxville. It is estimated that under this development there might be produced about 4,000,000 horsepower on the river at and above Muscle Shoals. The economic feasibility of this plan would depend largely upon the possibility of procuring at low prices much of the land which would be submerged by the reservoirs, many of which would be vast lakes.

This proposed plan may never be entirely consummated. In any event, if the enormous resource of this river at and above Muscle Shoals is to be adequately developed, it must be through a series of storage reservoirs, the ultimate value of which will depend upon the maintenance of storage capacity through reducing siltage.

NECESSITY OF PERMANENCY OF POWER RESERVOIRS

The value of a storage reservoir depends largely upon its life, that is, upon the permanency of its storage capacity. The rate at which erosion of soil is taking place on the basin of the supplying stream is the all-important factor in determining the period of service, or the number of years which will elapse before the soil removed by heavy rains from the lands on its catchment areas will fill the reservoir. (Pl. 2, figs. 1 and 2.)

Many engineers, accepting siltage in certain regions as a natural condition, customarily determine the life of a reservoir and ascertain whether the earnings will provide for the amortization of its costs. There are regions, as will be pointed out, where methods for the control or reduction in silting are impossible or impracticable.

2 SWITZER, J. A. THE WATER POWERS OF TENNESSEE. Tenn. State Geol. Survey Bull. 17, 137 pp., illus., 1914.

In most cases it is not possible to remove economically the silt deposited above the dam. To do so would often mean merely passing it to a reservoir lower down on the stream. It is suggested that the proposed Boulder Canyon Reservoir on the Colorado River be flushed, after draining it, once in about 10 years by means of a reservoir located above it and employed only for this purpose. It is doubtful, however, if this can actu ally be done since the river at present carries its full load of silt and the load could not be materially increased by flushing. There are likewise mechanical difficulties. But if such a system could ever be employed it might also be used most advantageously on the streams of middle and western Texas with their high silt burden.