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,~~~ 5 r 6--r t1A1 (;; r d y. lUa Vul /1 ??~'^- '"', 01-urd tom +- \Alard lq1?(o~ 9. The Colorado River system to J. A. Stanford & J. V. Ward Introduction The American Southwest has long captured the awe of wayfarers, struck by the apparent hostility of its eroded wastelands. The area contains the Mohave and Sonoran deserts and on centre stage is the Colorado River, whose silt-laden waters have eroded magnificent canyons in lowland regions of the United States and northern Mexico (Fig. 1; Crampton 1964). Fed by snowmelt from the southern Rocky Mountains, the Colorado drains 1/12th of the United States, although its annual discharge is relatively low. The river carved the Grand Canyon, the largest gorge in the world, and its waters have spawned the economic development of the Southwest (Williams 1951; Fradkin 1981). In the late 1600s the first Spanish explorers ventured from Mexico into the Colorado Basin via the San Pedro River, but were sobered by the hostile environment. Along the middle Gila River they found a sophisticated network of canals that had irrigated prehistoric crops. The only remnants of these early river regulators were wandering bands of Apache and Piman Indians whose ancestors, the Hohokam, once fished the rivers for giant minnows and farmed the desert with water diverted from the Gila. The Hohokam and other advanced, prehistoric cultures of the American Southwest may have failed during a dry period, when the rivers did not supply enough irrigation water (Euler et al. 1979). The Spaniards returned to Mexico; development awaited hardy Mormon pioneers 300 years later. In the late 1800s settlers began in earnest to divert Colorado waters into the deserts. By 1903, water was flowing via new canals into the valleys of southern California. A Federal reclamation law allowed construction of further diver- sions and storages, and in 1910 Roosevelt Reservoir was constructed on the Gila River. Diversion projects began also in the Colorado headwaters. In 1935 the construction of Hoover Dam to form Lake Mead (Fig, 1) harnessed' spring floods from headwater snowmelt. The economic benefits of Lake Mead are countered by !he realisation that the riverine ecosystem evolved in harmony n.l. {Tile Ecology 'of River Systems. edited by B, R, Davies & K. F Walker IQ !j.) 1986, Dr W, Junk Publishers, Dordrecht, The Netherlands 353 ,~, '~ ~ ~ '<.Z'~ 7 , ,~ 7"