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marized by Stanford and Ward (1986a). Various portions of the river system originated from 3.3 to over 20 million years ago (Minckley et al. 1986). Human occupation of the Colorado River Basin dates to some of the earliest records of man as a nomadic hunter in the Western Hemisphere over 10 000 yr ago. Agricultural Anasazi, Fremont, Mogollon, and Hohokam cultures flourished at various times into the 1200s, creating exten- sive irrigation systems (Masse 1981) and exploiting fishes and game along watercourses (Bolton 1919). They declined and were succeeded by their descendants (Hopi, Taos, Zuni, and Pima-Papago) and immigrant Navaho, Apache, South- ern Paiute, Havasupai, and Hualapai people (Watkins 1969; Fradkin 1981; Graf 1985). Spanish explorers in search of riches encountered the lower Colorado River in 1540 (LaRue 1916). The river was given several names before Father Kino applied the name 'Colorado' on a 1705 map of his passage westward in search of religious converts (Bolton 1919; Hughes 1967). White trappers continued exploration of the canyon country in the early 18oos, and William Ash- ley's party first navigated the upper Green River in 1825 (LaRue 1916 ; Watkins 1969). Mormon colonizers estab- lished towns along the river, and John Lee was sent by church leaders to establish a crossing below Glen Canyon (Fradkin 1981). Lee's ferry site has played a prominent role in the law of the river. John Wesley Powell's scientific investigations of the Colorado began in 1869, when he floated from Green River" Wyoming, through the Grand Canyon (Stegner 1982). Watkins (1969) and Fradkin (1981) have contributed excellent comprehensive reviews of subse- quent Colorado River history. The first high dam on the mainstream river (Hoover Dam) was closed in 1935, mark- ing the free-flowing Colorado's demise (Stanford and Ward 1986a). In the remainder of this paper, we summarize the status of the Colorado River ecosystem prior to 1935, brie- fly describe human manipulation of the basin's waterways, and assess the current condition and future of the river sys- tem and its biota. The Colorado River before 1935 Prior to 1935, the river flowed essentially unchecked from its sources to the sea, ending in a "live delta" of mean- dering streams, silt, and shifting land patterns (Hundley 1966). Then, as now, much of its basin consisted of rela- tively barren deserts. High relief, sparse vegetation, and desert storms combined with montane snowmelt to pro- duce spectacular variations in discharge (Minckley 1979; Table 1). The upper basin produced most of the river's dis- charge, and peak flows occurred after snowmelt in spring and early summer (Bishop and Porcella 1980). Maximum runoff in the lower basin often followed winter rainstorms (Sykes 1937). Lowest discharge typically occurred in mid to late summer, but long periods oflow flows accompanied droughts. At times of moderate flows and during droughts, the river ran clear, but sediment transport was remarkably high dur- ing floods. In high-gradient reaches, alluvial rubble from side channels resulted in formation of rapids between long, sand-bottom pools (Stanford and Ward 1986a). Unstable flows and other conditions resulted in arroyo cutting in small and medium-sized streams, and floods and droughts resulted, over time, in alternating braided and meandering 222 TABLE 1. Selected physicochemical characteristics of the Colorado River before 1935 (Deacon and Minckley 1974; Dolan et al. 1974; US Geological Survey 1975; Weatherford and Jacoby 1975; Pillsbury 1981; Graf 1985; Stanford and Ward 1986a). Discharge, range at Yuma, AZ ....,....0 to 7 000 m3.s-1 Virgin flow, long-term annual mean from upper basin ..............16.65 km3 Sediment transport, daily maximum in Grand Canyon ...................25 .106r Seasonal water temperature ranges: Headwaters .. . , . . . . . ' . . . . . . . . . . . . . .0-20oC Low-elevation desert streams. . . . . . . ' . .5- > 300C TDS: Headwaters,.. ' . ... . ' ... . ... ., . .. .. >50 mg.L-1 Lower mainstream. . . . . . . . . . . . . . . . . .250-380 mg.L-1 channels (Graf 1985). Oxbow lakes and extensive, transi- tory marshes formed where the lower river occupied broad valleys. Minckley (1979) characterized the lower Colorado before regulation as "a broad, meandering, sandy- bottomed, periodically erosive, yet often aggrading stream' , . Water temperatures in the pristine Colorado River proba- bly resembled those recorded today in reaches far from mainstream reservoirs, and wide diurnal fluctuations were common (Minckley 1979). Chemical conditions before 1935 must have been almost as variable as discharge. Dis- solved solids (TDS) concentrations differed with the geol- ogy of various sub-basins, but calcium, sulphate, and bicarbonate were predominant ions (Stanford and Ward 1986a). Local oxygen depletions probably were common in deeper backwaters along the lower river (Minckley 1979). The biota of the Colorado River before 1935 is generally poorly documented. Headwater streams probably harbored plant and invertebrate communities like those currently present. Riverine algae were diverse but sparse, and nutri- ent levels were adequate to support considerable primary production during clear flows (Stanford and Ward 1986a). Autochthonous production was probably particularly important in areas on the lower river where marshy back- waters and oxbow lakes provided plant production that sup- plied organic detritus (Minkley 1979). Ward et al. (1986) stated that a highly adapted riverine zoobenthos existed at potamon sites in the upper basin but not in the Lower Main- stem Colorado. The trophic structure of the lower river probably was direct and simple. Chironomids and oligochaetes were the predominant benthic organisms. Only soft bottoms of backwaters and woody debris in main chan- nels could have supported diverse and abundant invertebrate communities. The Colorado River Basin has an unique indigenous fish fauna. The drainage was established long ago and has had no major connections with surrounding river basins for mil- lions of years (Behnke and Benson 1983). As Molles (1980) noted, the Colorado River System may be considered "an aquatic island in a terrestrial sea". Because of long isola- tion, the fish fauna consists of species distinctly different from their nearest relatives, and relationships are evolution- arily and geographically distant (Miller 1959; Minckley et al. 1986). Many native genera probably existed in the basin by Miocene time, and present-day species occurred by the Plio-Pleistocene (Smith 1978; Minckley et al. 1986).