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mix of willows (Salix sp.), cottonwood (Populus sp.), mesquite (Prosopis sp.), seepwillow (Baccharis salicifolia), and arrowweed (Tessaria sericea). Specific communities were described in detail by Brown et al. (1977) and Stanford and Ward (1986a). The exotic tamarisk (also known as salt cedar), Tamarix gallica, had been introduced to the basin and occurred in riparian communities of most drainages by 1935 (Graf 1985). A well-developed riparian community along the lower Colorado provided allochthonous organic input to the stream and furnished habitat for terrestrial insects which served as fish food. Ohmart et al. (1977) esti- mated that at least 2 023 ha of cottonwood communities existed along 322 Ion of potentially suitable habitat on the lower river in the 1600's. Overgrazing and cutting of cot- tonwoods and mesquite for fuel had caused significant changes in pristine riparian communities before regulation of the river (Ohmart et al. 1977). Riparian vegetation was important then, as now, in supplying habitat for terrestrial vertebrates (Johnson and Jones 1977). Management of the Colorado River System Native Americans began diverting water from rivers in the Colorado River Basin to irrigate crops around 1000 AD, and sophisticated canal systems existed on floodplains of the Salt, Gila, San Juan, and other streams by 1200 (Graf 1985). The early irrigators vanished by 1400, and interest in divert- ing water for agriculture in the Colorado basin was not renewed until the middle to late 18oos, when small diver- sions abounded and discussions of major diversion projects began (Fradkin 1981). Levees were constructed to contain the lower Colorado River, and jetties and frontworks were built to deflect currents and protect streamside develop- ment. Major diversions of Colorado River water to the Imperial Valley, California, began just after the turn of the century, and the Salton Sea was created when the flooding Colorado broke through restraints in 1905 (Sykes 1937; Watkins 1969). TABLE 3. Selected decisions and projects influencing the Colorado River System (Hughes 1967; Watkins 1969; Spofford 1980; Fradkin 1981; Carlson and Carlson 1982; Harris et al. 1982; Plummer 1983; Valentine 1983; Coats 1984; Upper Colorado River Commission 1984; Graf 1985; Welsh 1985; Hundley 1986; Bureau of Reclamation data). A. Laws, compacts, treaties and other decisions B. Dams, diversions, and other structural modifications Year Action Rationale I effect Year Action Rationalel effect 1902 Reclamation Created U.S. 1892 Grand Ditch First diversion of (Newlands) Act Reclamation service (Colorado R. to water from passed (Bureau of Recla- eastern slope, CO) Colorado basin mation) completed 1903 Salt River Project Irrigation agriculture 1901 Imperial Canal com- Irrigation of (Colorado R. to near Phoenix, AZ; pleted (Colorado R. Imperial Valley central AZ) authorized near Yuma, AZ, to authorized Roosevelt Dam Imperial Valley, 1904 Yuma Project Irrigation of Yuma CA) (Laguna Dam) Valley, AZ/CA 1909 Laguna Diversion Irrigation of Yuma authorized Dam (Colorado R., Valley, AZ/CA 1908 Grand Canyon CAI AZ) completed National Monument 1911 Roosevelt Dam and First high dam established Power Plant com- (multipurpose) in 1915 Dinosaur National pleted (Salt R., basin b Monument and AZ) Rocky Mountain 1913 Strawberry Dam Water storage for National Park (Strawberry R., Provo, UT, area established UT) completed (Great Basin) 1919 Grand Canyon 1915 First Imperial Irriga- Ensuring minimum National Park tion District tem- flows to imperial established porary diversion Valley 1922 Colorado River Divided Colorado R. dam completed Compact signed water between 1935 Hoover Dam com- Multipurpose (water (AZ joined 1944) upper and lower pleted (Colorado storage, flood con- basins a R., NV/AZ) trol, and power 1929 Boulder Canyon Authorized Hoover generation) Project Act passed Dam and all- 1938 Imperial Dam Desilting of irrigation American Canal; (Colorado R., AZI water to AZ and Congress approved CA) and Parker CA; water storage Compact of 1922 Dam (Colorado R., for southern CA 1934 Fish and Wildlife Provided that federal CAI AZ) completed and central AZ; Coordination Act water project flood control passed planning must 1940 All-American Canal Irrigation of Imperial consider impacts (Imperial Reservoir Valley on wildlife to Imperial Valley) 1944 Mexican Water Assured Mexico of opened Treaty signed 1.85 km3.yr-1 of 1941 Colorado River Municipal water for Colorado R. Aqueduct (L. Los Angeles and 228