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as 1.8 m in a few minutes, but potential for ecological dam- age in the Grand Canyon has no bearing on the dam's oper- ating criteria (Coats 1984). Less than one percent of the river's virgin flow now reaches its mouth (Petts 1984). Dams have also changed the capacity of streams in the Colorado River System to transport sediments. Sediments previously moved by streams are deposited in reservoirs, and space intended for water storage is gradually reduced as they accumulate (Graf 1985). Flows released from dams are relatively clear as well as seasonally constant; an excel- lent example is provided by data collected before and after closure of Glen Canyon Dam (Table 4). Studies at several mainstream Colorado River dams (Petts 1984) have demonstrated that rapid degradation of channels may extend for many kiIometres downstream from dams releasing frequent and prolonged outflows of clear water. Regulated flows have, thereby, changed channel forms and armoured stream bottoms in tailwaters (Graf 1985; Stanford and Ward 1986a). Numbers and sizes of mid-channel bars or islands and channels ide bars or beaches have also been reduced below dams (Graf 1985). By limiting ability of streams to move coarse material, flow regulation has led to stabilized rapids downstream from dams (Graf 1985; Stan- ford and Ward 1983) and accumulation of sediments dis- charged from tributaries to mainstream channels (Dolan et aI. 1974; Howard and Dolan 1981). Reductions in peak flow and channel and bank modifications have reduced the extent of backwalers and marshes. Regulation has lowered mainstream water temperatures 10-150 C and resulted in cooler summer and warmer winter water temperatures below dams (Stanford and Ward 1986a). Lowered summer temperatures have adversely affected native fishes below Flaming Gorge and Glen Canyon dams (Kaeding and Zimmerman 1983). By 1957, natural salt levels (about 250 mgoL -1) at Lee Ferry had doubled (Graf 1985) . Welsh (1985) reported a salt concentration of 600 mg 0 L -1 below Lake Powell and noted that the Central Arizona Project will extract water with 750 mgoL -1 from below Lake Mead. Paulson and Baker (1983) reported salinity of 825 mgoL -1 at Imperial Dam. Sulfate constitutes nearly half of the TDS in the Colorado River but has little effect on agriculture or munici- pal water uses. Paulson (1983) discussed means of reducing TDS in and evaporation from Lake Mead by regulating releases from Lake Powell at Glen Canyon Dam. Lake Powell construc- tion and regulation have affected a marked reduction in phosphorus transport to and productivity in Lake Mead 450 km downstream (Evans and Paulson 1983; Prentki and Paulson 1983; Stanford and Ward 1986b). A decline in the fishery of Lake Mead has been partly attributed to the reser- voir's diminished fertility (Baker and Paulson 1983). Production in Colorado River reservoirs is strongly influ- enced by physicochemistry of river inflows (Stanford and Ward 1986b). More uniform water temperatures and reduction in back- water and marsh habitat can be expected to diminish the fre- quency and severity of oxygen depletions in the Colorado River System. However, river regulation has introduced potential for downstream release from reservoirs of waters supersaturated with air gases; these could have detrimental effects on aquatic life (Holden 1979). Aquatic flora and invertebrate fauna of few mainstream reaches of the Colorado River have been studied. Ward et al. (1986) stated that the filamentous green alga, Cladophora glomerata, is common on solid surfaces for several kilometers below dams in the basin. The cooler and less turbid waters in the Grand Canyon are now character- ized by dense bottom mats of this alga, which is little used by most invertebrates but provides food for introduced fishes (Carothers and Minckley 1981; Carothers and Dolan 1982). The alga provides habitat and food for diatoms and the non-native Gammarus lacustris, a common mainstream invertebrate. Benthic invertebrate productivity and diversity in the Grand Canyon are low, and the main river lacks many common invertebrate groups found in tributaries. Ward et aI. (1986) reviewed studies of lotic zoobenthos in the mainstem Colorado River and at 34 tributary loca- TABLE 4. Hydrological and sediment transport characteristics of the Colorado River below Glen Canyon Dam (modified from Dolan et al. 1974 and Petts 1984). Lee Ferry Grand Canyon (24 km downstream) (165 km downstream) Pre-dam Post-dam Pre-dam Post-dam Daily average flow equalled 102 156 113 167 or exceeded 95 % of the time (m3os-I) Median discharge (m3.s-I) 209 345 232 362 Mean annual flood (m3.s-l) 2434 764 2434 792 10 year's flood (m3os-I) 3481 849 3453 1 132 Annual maximum stage (m): Mean 5.04 3.56 6.89 4.79 Standard deviation 0.96 0.17 0.35 0.15 Annual minimum stage (m): Mean 1.76 1.46 0.46 0.70 Standard deviation 1.40 0.23 0.85 0.45 Mean sediment concentration (mg. L - I) 1500 7 1250 350 Sediment concentration equalled 21 000 700 28 000 15 000 or exceeded 1 % of the time (mgoL-I) 231