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8 WATER AND CHOICE IN THE COLORADO BASIN powerful agents of erosion; boulders weighing tons are moved during flash floods, canyons are scoured, and weathering debris is swept downstream by waves of water. Because of the high gradient of the master streams of the Colorado system, the abundant detritus supplied to them is readily removed from the basin, except where it is caught and held by dams. Despite the dependence of the smaller features of this arid landscape on geologic structure, the main streams cross the major geologic structural features in independent fashion, sawing deep notches-of which the Grand Canyon of the Colorado is the most spectacular-into uplifted segments of the earth's crust as they follow courses whose origins remain obscure. Aside from its striking scenery, the principal features of the basin are its aridity, its temperature extremes, and its small proportion of cultivated land (only 3.3 percent of Utah, for example, is farmed). A ROUGH AND RUDDY RIVER The steep and turbulent Colorado is neither large nor clean. Its estimated average annual virgin flow of about 15 maf is small compared with the 180 maf of the Columbia or the 440 maf of the Mississippi; it is the same as that of the Delaware, which is only 390 miles long and drains a much smaller basin of 12,300 square miles (Figure 1).* In 1917, before the construction of any large dams on the Colorado, 500,000 tons of sediment a day passed Yuma in the river water. The sediment so colored the river that the Spanish explorer Garces named it Colorado, Spanish for ruddy; the huge delta the river has built with the sediment has partly filled and divided a tectonic depression that includes the Gulf of California and the Imperial Valley. Although it drains a basin of about the same size as that of the Columbia River (258,000 square miles), the Colorado's flow is much smaller (Figure 1), and the contrast in discharge to the ocean is a critical point in the growing debate on the feasibility of large-scale water transfer from the Columbia to California or to the Colorado basin; the Columbia's "unused" discharge is more than 12 times the virgin flow of the Colorado. Precipitation in the basin is seasonal, and the flow of the river varies greatly from month to month and year to year (Figure 3). Before the building of Hoover Dam, violent flooding in the lower reaches of the Colorado was a common hazard, and it still is a threat, though infrequent and erratic, on *If all the water that flows in each river during an average year were spread evenly over the area drained by the river, the depths would be: Delaware 20.9 in.; Columbia 13.1sin.; Mississippi 6.7 in.; Colorado 1.15 in.