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Show TABLE 15.-Federal grazing district lands in Colorado Basin States (1948) State Number of districts Public lands (acres) Leased lands (acres) Arizona. . 4 8 5 6 11 5 9, 764, 000 7, 489,000 33,668,000 14, 267, 000 23, 648, 000 13,504,000 2, 232, 000 633, 000 Colorado......... Nevada.. New Mexico Utah..... 461, 000 Wyoming........... 3, 093, 000 The causes of deterioration.-Competent students disagree on the causes, and therefore the extent of remedial measures possible in land treatment within the basin. The principal point of difference relates to the extent of man-induced erosion. The difference further is concerned mainly with those areas in the basin which receive on an average less than 15 inches of rainfall. This includes the greater part of tihe area of Arizona, Nevada, California, Utah, New Mexico, and Wyoming lying within the basin. A. major part of the basin section of Colo- rado has more than 15 inches of precipitation. The place of "normal" or geologic erosion in the basin's difficulties.-There can be no doubt of the pervading effects of normal wind and water erosion on the movement of earth materials in the basin. The existence of the Grand Canyon and other im- pressive water-carved land features of the basin is unmistakable testimony of the force of these proc- esses. Tfcius even under an undisturbed natural vegetation cover, sediment probably would be moved from many areas in great quantity, particu- larly under desert conditions.36 Most of the watershed problems are found in the arid and semiarid sections of the basin, where there is a delicate balance between climate and vegeta- tion. Thornthwaite and others of the Soil Conser- vation Service have stated:37 Fox all given climatic conditions there is a certain minimum plant cover needed to protect the land against destructive erosion. In arid lands with little or no vegetation and low pre- 88 See Silt Problems of the West, J. C. Stevens, American Geophysical Transactions, 1939, pt. I. 87 Climate and Accelerated Erosion in the Arid and Semiarid Southwest, with special reference to the Polacca Wash Drainage Basin, Arizona. Technical Bulletin No. 808. Department of Agriculture, May 1942. 420 cipitation, there may never be sufficient vege- tation to offer protection from the infrequent rains. The dryness of the ground and lack of well marked water channels may prevent rapid runoff but a permanent inadequacy of cover is characteristic. In semiarid lands the plant cover may be barely adequate to resist the impact of the climatic elements^ * * * They also point out that: Owing to the delicate adjustment of vegeta- tion to climate in the Southwest, a succession of even a few dry years may so impoverish the plant cover that rains of heavy, or even mod- erate, intensity can initiate a period of accel- erated erosion. Climatic records and news re- ports show that from time to time parts of the region have been visited by storms of excep- tional intensity. These are as characteristic of the Southwest as is its semi-arid climate. Specific storms listed in the above reference in- cluded thundershowers and cloudbursts over lim- ited areas at scattered locations in August 1868, August 1872, September 1880, August 1881, Octo- ber 1881, August 1882, March 1884, August 1886, July 1887, and September 1887. Intense storms also occurred in the next decade, although severe flooding in the wettest year, 1905, was the result of less intense but more widespread precipitation. It was during this year that the Colorado broke out of its banks and flowed into the Salton Sink. The most intense storm of recent record occurred in Sep- tember 1939. The same study by the Soil Conservation Service further indicates the occurrence of accelerated ero- sion in the arid and semiarid Southwest coincidental with heavy grazing which began about 1850 and reached a peak in the 1880's. Overgrazing of the grasslands, trampling, and trail cutting by large herds of cattle and sheep removed protective cover and induced flow concentrations resulting in severe sheet erosion, gullying, and sediment overflows wherever high intensity rainfall occurred. Timber cutting in the higher elevations and fires also de- nuded large areas, making! them susceptible to accelerated erosion. It is the belief of some that most of the erosion in the areas of less than 10 to 15 inches of rainfall in the Colorado River Basin has been and con- tinues to be of geologic nature. The main streams of the basin are deeply entrenched in sections of their reaches. For example, the Colorado River within the Grand Canyon is over a mile below the |