OCR Text |
Show away from the mountains, the accumulation of vegetable litter, the formation of new soil, and the healthy growth of a protective covering of grass originally occurred on almost all slopes and flats. It is true that in the Colorado River Basin the lowering of stream beds and canyon bottoms by natural erosion, and the reduction in size of plateaus and buttes by the weathering and collapse of their vertical- walled perimeters, proceeded at a rate that, geologically speaking, was fairly rapid, when compared with that of some other regions. The top surface of the country also was extensively planed down during the lapse of millions of years. Nevertheless, in terms of year- to- year continuity, the top- soil was healthy, and maintained a wealth of plant cover and animal life in startling contrast to what is found today. Natural erosion originally was no insuperable obstacle to fertility and abundance even in the desert. For example, in the great expanse of sandstone soils of the Little Colorado River Basin, " where the cover of grass, sage, and other low growth has not been disturbed by grazing, erosion is of no importance; but under conditions of depleted vegetation, as a result of overgrazing, sheet erosion, gullying, and wind erosion are active over large areas." 39 The soil has sickened and commenced to die only after the balance has been disturbed by the coming of the white man. Man- caused or " accelerated erosion" results when grazing and other agricultural activities strip away the plant cover and allow the wind and water to wear away the topsoil more rapidly than nature ever did. In many places, this living surface layer, product of hundreds of centuries of microscopic accumulation, has completely disappeared, and the land has died within the space of 20 years. 40 The misconception of those who fail to distinguish between natural and man- caused erosion, imagining that the consequences of the latter are of little importance, is like that of a person who would ignore the fact that his house had caught fire, on the theory that fire always had existed as a purely natural phenomenon. Franklin Flat, in the San Simon Valley of Arizona, was covered with grassy, flower- spangled meadows in 1882. Grass was stirrup- high along the continuously flowing river, which had practically 39 Bennett, 1939, p. 794. 4° Lord, 1938, p. 7. no banks. In 1895, 50,000 cattle were grazing in the valley. By 1934, as a result of overgrazing, the green meadow cover had been replaced by drifting sand and the river, now an intermittent stream, had cut a chasm through the heart of that once- fertile land. 41 Oraibi Wash, once a small stream tributary to the Little Colorado River in Arizona, has been cut by silt- laden, runaway waters into a twisting gully 80 miles long and from 20 to 80 feet deep. 42 The adjacent Polacca Wash commenced to deepen after 1868, when immense numbers of livestock were crowded upon the newly created Navajo Reservation. Now the wash is dissected by a continuous 90- mile gully from 10 to 50 feet below the valley floor, with great tributary gullies that continually eat into the slopes of Black Mesa. Today, " overgrazing has altered the plant association on the mesa areas of the Polacca Wash drainage and has so increased the run- off that all rains except those of very low intensity cause soil erosion". 43 Most of the hundred- foot- deep channel in the Jadito Wash has been cut since 1914. Chaco Canyon had no gully in 1849 and contained a continuous, clear stream. By 1877 an ar- royo 16 feet deep and 40 to 60 feet wide had developed. In 1924 this arroyo was 30 feet deep and 200 to 300 feet wide. There is no stream there now except during flash flood. 44 The Santa Cruz River did not develop a continuous channel until 1890.45 Until about 1888, the ( San Pedro) valley consisted of a narrow strip of very fertile sub- irrigated fields. Beaver dams retarded the flow and prevented channel cutting. Trapping of the beavers and removal of the grass from the hillsides allowed such an increase in erosion that by 1892 or 1893 a channel 3 to 20 feet deep had been cut almost the whole length of the river. 46 In 1846, even the Gila River, which, like the Colorado River, receives much sediment from natural erosion, flowed clearer. Its tributaries commenced to erode rapidly only after 1870. 41 Lord, 1938, p. 112; Soil Conservation Service, 1935, p. 5. 42 Lord, 1938, p. 109. 43 Thornthwaite et al, 1942, p. 122. 44 Thornthwaite et al, 1942, p. 103. 45 Schwalen, 1942, p. 427. 46 Thornthwaite et al, 1942, p. 103. 64 |