third of the capacity, the equivalent of a year's water requirements, will be occupied by silt, endangering the water supply for irrigation over a period of several dry years such as is common in the region. For a number of years the rainfall on the watershed of the Roosevelt Reservoir has been below normal. In 1925 the water supply became very low. If the part of the reservoir capacity occupied by silt had been available for water, the shortage would not have been so great. During the dry period rains washed much of the surface soil from slopes into small gullies and occasionally as far as main channels. In 1927 torrential rains fell rapidly and, with the vegetation depleted by drought and overgrazing, caused terrific cutting of slopes. The several years' soil accumulations in the channels were carried farther down the main stream courses, usually to be dropped into the reservoir. Plate 15, B, shows strikingly the enormous quantity of silt, in the upper parts of the reservoir basin, being washed out by erosion of the silt banks and carried down nearer the dam. On experimental plots on the range where the vegetation had made normal growth the roots bound the soil, with the result that soil cutting was not so severe and much water-carried soil coming into the plots from above lodged in grass or brush. REDUCED PRODUCTIVITY OF RANGE LANDS In the midst of such great destruction by floods and silting to valley farm lands, irrigation and other reservoirs, roads, bridges, and other public works, the excessive damage to the productivity of the range lands for livestock grazing has hardly been appreciated. In their original condition the slopes and valleys, except in those arid parts where rainfall was very light, were well carpeted with valuable grasses and a small percentage of other herbaceous and shrubby plants. The decaying vegetable matter had built up the surface soil into a friable condition and added to it a large quantity of rich organic matter. The mulch of decaying vegetable matter acted as a sponge, and the friable humic character of the soil allowed a maximum moisture penetration. The result was that the forage plants made the most of the rainfall and the fertile soil and produced abundantly. When erosion removes the top layer of soil it robs the plants. If erosion continues, the soil may become incapable of producing the stand it once supported. The experiments at the Great Basin Experiment Station (21) showed that noneroded soil was much richer than eroded soil in lime, phosphoric acid, and total nitrogen; that the water-holding capacity was greater; and that the water required by representative plants to produce a pound of dry matter was less. A great many more leaves, greater stem and leaf length, and more dry matter are produced on the noneroded than on the eroded soil, even with a notably smaller supply of water. The conclusion was drawn that erosion is detrimental to plant growth chiefly because it brings about two conditions of soil impoverishment: (1) Lack of adequate soil moisture for full plant development and seed production and (2) lack of adequate plant nutrients in the soil for good growth. Furthermore, reestablishment of the vegetative cover is made more difficult. A.-Water accumulated in the wagon tracks and cut these gullies (Montana) A.-Flood control fence constructed with iron posts to reduce velocity of flow, induce silting, and prevent bank cutting in a large drainage (Arizona) B.-The establishment of grass in a gully tended to check further cutting (Arizona) |