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Show canyon rim. The downstream reaches of the San Juan and Little Colorado Rivers are also deeply entrenched. The down cutting of these streams has been from flows originating in the humid areas drained by their headwaters and moving over steep gradients to base level at the stream mouth. The arid and semiarid parts of the basin are in a stage of geologic immaturity. The upper reaches and headwaters of the stream system draining these localities are far above base level, slopes are steep, and heavy geologic erosion is potential. Where intensive rainfall has occurred it has left a pattern of severe erosion within the limits of its influence. It is evident that the principal factor lacking in producing extremely active erosion in these areas is precipitation sufficient to produce flows capable of downgrading the topography. While man's activities without doubt have provided a measure of trigger action partially accounting for accel- erated erosion in these areas, according to this view the high potential for geologic erosion leads to the conclusion that geologic factors are primarily responsible for the present state of erosion. This view further maintains that while proper watershed management would be partially effec- tive in arresting the spread of erosion, it would still be incapable of completely arresting erosion, much less restoring historical watershed conditions, except at excessive cost. It is suggested that too little is known of the over-all costs involved to per- mit sound judgment as to the necessity for adoption of an all out watershed management program in the basin at this time, particularly in the part of the basin with less than 10 inches of precipitation. The case against overgrazing.-Other equally competent observers stress the effect that over- grazing has had on the increase in erosion almost throughout the basin. According to this view, there are areas which indisputably will continue to erode regardless of land treatment, but they comprise a minor part of the basin area. For all of the re- mainder, destruction of the vegetation by over- grazing or other unsuited land-use practices has been a significant cause of the erosion now taking place. Correction of range and land abuse, ac- cording to this view, would therefore reduce erosion and sediment production greatly. Evidence supporting this view can be chosen from many examples. Much of it points to a major change in the condition and composition of the plant cover in the basin since about 1880. From that date, gullying became prevalent, severe erosion commonplace, and profound changes took place in the regimen of the streams and the volumes of sedi- ments they carried. Acceleration of erosion followed close on the heels of the great herds. Wide valleys were deeply entrenched, great arroyos were cut in gentle swales, and gullies became commonplace. The methods of handling stock, particularly sheep, the trailing of herds for great distances, and the no- madic characteristic of the livestock industry all contributed to the damage. What the industry started was accelerated by practices around set- tlements. Roads, ditches, canals, diversion dams, and efforts to divert water by means of levees or walls resulted in great washes which ruined many thousand acres of bottom lands. The balance between grazing, vegetal cover, and climate can easily be upset in a region where cli- matic conditions are extremely irregular. In a series of years with normal or greater than normal rainfall, the range could carry a heavy load with little detrimental effect. With the coming of a dry season or of a series of such years, the deple- tion became so great that there was almost complete loss of cover and starvation of the animals resulted. Several periods in which starvation occurred are known, as in 1892 when more than 75 percent of the stock in the Gila Basin died on the range. Under conditions of severe cover depletion, ex- cessive or intensive rains initiate an erosive process which once set in motion progresses rapidly through many stages to almost complete ruin. The head- ward extension of gullies and the starting of new gullies continue year after year. Travel accounts, explorations, and diaries all present evidence of the condition of the range before the cover was depleted. Soils were undis- turbed, most creeks were clear, and beaver were abundant.38 Livestock first moved into the headwaters of the Santa Clara Creek of southern Utah in 1863. Seven years later, the mountain meadows began to erode vigorously. Kanab and Long Valleys were settled in 1868; deep channeling began in 1883. Escalante was settled in 1875, and 15 years later the valley flats were being deeply gullied. Kitchen 38 G. W. Thornthwaite, C. F. S. Sharpe, and E. F. Dosch, op cit. Lockett and Snow, Along the Beale Trail, U. S. Office of Indian Affairs, 1938. Also National Re- sources Planning Board, Upper Gila River Basin (mimeo), 1940; John R. Bartlett, Personal Narrative of Explorations and Incidents, 1854; Thomas M. Marshall, St. Vrain's Expedition to the Gila in 1826, 1917; R. C. Thorn- thwaite's Pattie's Personal Narratives, 1905. 421 |
