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. . Chapter 1. Introduction The Colorado Plateau The Colorado Plateau is an arid basin covering 362,000 km2 between the Rocky Mountains and the Uinta Mountains in western North America (Fig. 1). The Plateau extends from 340 to 43.50 N latitude, and from 105.50 to 1140 W longitude, including portions of southwestern Wyoming, western Colorado, eastern Utah, northwestern New Mexico and northeastern Arizona. The elevation of the Colorado Plateau ranges from 1,200 to 4,600 m, with about 45% of its area less than 1,850 m (West 1965; Hunt 1974), and it contains six isolated mountain ranges over 3,050 m in e1evation: the La Sal, Abajo, and Navajo Mountains (all east of the Colorado River), the laccolithic Henry Mountains west of the river, and the San Francisco and White Mountains south of the Colorado River in Arizona. These ranges, the 4,115 m high Uinta Mountains and the still higher Rocky Mountains, contribute annual spring snowmelt, the primary source of water for the Colorado River. The Colorado River The Colorado Plateau is drained by the many tributaries of the Colorado River, which is the 12 largest river in the United States (Fi~. 1). The Colorado River heads on the east slope of 3962.5 m hlgh Mt. Richthofen on the Continental Divide. The river is joined by the Gunnison River at Grand Junction and the Dolores shortly thereafter. The river travels 1,030 km to its confluence with the Green River at the head of Cataract Canyon. To reach Cataract Canyon, the Green River has traversed an equally complex 1,130 km course from the Wind River Range in southwestern Wyoming. Like the Colorado, it is also the composite flow of several major tributaries, including the Yampa, Duchesne, White and other rivers. At the foot of Cataract Canyon, the river dies quietly in the arms of Lake Powell, which also receives the San Juan River from the Four Corners area. Ice cold and clear, the river emerges from Glen Canyon Dam and plunges downstream through the Grand Canyon, its finest sculptural achievement. Passing nearly 450 km through the Sonoran Desert, the river exits the Colorado Plateau at the Grand Wash Cliffs, entering Lake Mead and the fractured terrain of the Basin and Range geologic province. At that point the river has traveled nearly 1,850 km and descended 3,600 m. Biotic Diversity The Colorado River hosts a phenomenal diversity of aquatic and terrestrial plant and animal life (Appendix A). Many of these species are associated in complex communities which depend directly and indirectly on the river's water supply, water quality and flow regime for survival and development. Although its wetlands comprise from 0.1 to less than 7% of the landscape, . . . . . . . . 1 .