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. . Historical Geomorphology An historical approach to stream channel geometry and change provide a good avenue into understanding the fluvial geomorphology of this system. All major tributaries of the Colorado in southern Utah experienced arroyo cutting in response to large floods between 1862 and 1909, with record flooding in the Colorado, San Juan and Little Colorado River basins. Watershed changes resulting from grazing and clearing of vegetation from the floodplain may have been partly responsible for arroyo cutting, but climatic changes also played a major role. The Escalante River provides a typical example of channel geometry change in the context of human land use patterns on the Colorado Plateau (Webb and Baker 1987). Early settlers in the Escalante River basin in south-central Utah described the "Upper Va1ley" of the Escalante "grassy meadows" in 1872, and significant settlement took place by 1875 with streamflow apparently increased because of land use practices. By 1880 large herds of cattle were present, and large herds of sheep had been introduced by 1886. The upper valley was dominated by sagebrush and greasewood by 1893. A major summer storm in 1909, and additional flooding between 1909 and 1932, eroded an arroyo channel 15 to 17 m deep and up to 300 m wide. This erosional ~henomenon was induced by increased precipitation and severity of flooding, resulting in a deeply incised, rectangular channel profile. Such channels gradually changed after formation through bank collapse, channel widening and reinvasion of riparian ve~etation (Webb 1987). Floods after 1940 were smaller in magnltude and influenced channel morphology less strongly. Hereford (1984 throu~h 1987) described three phases that have dominated short-term f1uvlal processes since 1880 in the Little Colorado and Paria drainages: (1) arroyo development and stream entrenchment 1880 to about 1940; (2) aggradation through floodplain alleviation beginning in the early 1940's; and (3) incision of the flood plains since about 1980. A change in the magnitude of flooding and related to decreased frequency of severe rainstorms apparently caused the shift from erosion to aggradation (Hereford 1984). Sediment yield of the Little Colorado River declined after 1942 or so, while the amount of runoff did not change. In addition, El Nino events may trigger increased snowfall in the Rocky Mountains and thereby influence discharge. Record snowpack in the Rocky Mountains attributed to an unusually severe E1 Nino event produced widespread spring flooding in 1983 and 1984 in the upper basin. . . . . . . . . 31 .