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. [Illustration: 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.] In this kind of protection work, as in the case of forest fires, it has been found that preventive measures are much more effective and much less costly than remedial measures. The regulations under which the Forests are administered give the Secretary of Agriculture power to institute preventive measures. To insure the sufficiency and purity of the water supply of a municipality or of an irrigation district, or to prevent floods and snowslides, the use of watersheds for grazing, timber, special uses, or settlement is especially restricted when such restriction is found to be necessary. On steep grass or timber-covered mountain slopes both grazing and timber sales are prohibited, if necessary.