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14 BIOLOGICAL REPORT 24 these low velocity habitats may not be as produc- tive as higher velocity reaches because of fluctu- ating flows caused by regulation. Measurements are needed to more firmly establish cause and effect. The problem is complicated because site- specific velocities vary with flow, which is pre- cisely why channel geomorphology is so complex and dynamic in time and space. I conclude that throughout their life cycles these fishes are highly adapted to variations in flow velocity, depth, tur- bidity, and food web structure and function asso- ciated with this spatially and temporally dynamic biophysical interaction. They simply move around as flow varies, constantly seeking the best energy return on energy invested in foraging. In the case of squawfish, large size apparently provides for considerable movement, which allows them to ef- ficiently use a highly variable environment. An- thropogenic activities, such as revetment of flood- plains and erratic regulation ofbaseflows by dams and diversions, change the natural biophysical variability and reduce the variety of habitats available, thereby compromising the life history energy balance of the fishes (Ward and Stanford 1989). Influences of Stream Regulation Flows in the Green and Colorado River subbas- ins have been depleted by diversions and further regulated by hydroelectric releases from large storage reservoirs (Figs. 1, 7, 8, and 9). Of the larger tributaries, only the Yampa remains essen- tially free flowing, although regulation of the White River is not severe (i.e., the mainstem dam is a low-head structure, and water depletions are about the same as on the Yampa). To examine the rationale for provision of flows to recover the en- dangered fishes, one must understand how the river ecosystem has been changed by regulation. The ecological effects of stream regulation have been extensively reviewed and summarized (cf., Ward and Stanford 1979; Lillehammer and Saltveit 1984; Petts 1984; Stanford and Ward 1986b; Craig and Kemper 1987; Carlson and Muth 1989; Gore and Petts 1989). I discuss only salient aspects of the problem here. Alteration of Flow, Temperature, and Sediment Regimes Regulation has reduced the spring peaks of the snowmelt-dominated rivers of the Upper Colorado River Basin and increased the baseflows (see fig- ures in Stanford and Ward 1983 and Andrews 1986). Hydroelectric operations also have in- creased short-term (hourly, daily) flow variability (e.g., Figs. 10-14). Note that extreme hourly vari- ation may be masked by presentation of flow as daily means (compare Figs. 12 and 13 with August and September data in Fig. 14). Daily means are usually plotted in analyses of flow durations be- cause hourly data are reduced to daily means in the long-term data bases for stream flows main- tained by the U.S. Geological Survey. Rivers regulated by hypolimnial (bottom) re- lease dams (e.g., Aspinall Units on the Gunnison) are cooler in summer and warmer in winter for many miles downstream from the dam than be- fore impoundment (Stanford and Ward 1983), al- though Flaming Gorge Dam was retrofitted with a selective withdrawal system to ameliorate nega- tive effects of cold temperatures on fish growth downstream from the dam (Stanford and Ward 1986a). Retention of sediments within impoundments such as Flaming Gorge and the Aspinall Units has reduced suspended sediment concentrations and bedloads downstream from the dams. Moreover, loss of peak flows has reduced the transport power of the river. Therefore, sediment discharges from tributaries downstream from the point of regula- tion are more persistent; alluvium and colluvium entering the river channel are not moved down- stream with predam efficiency (personal observa- tion in the Upper Colorado River Basin and docu- mented in the Grand Canyon by Dolan 1978 and others). Thus, riverine sediment budgets and channel elevations may change significantly after regulation. In the Green River, mean annual sedi- ment discharge decreased by 54% at Jensen and 48% at Green River, 169 and 467 river kilometers downstream from Flaming Gorge Reservoir (An- drews 1986). A new quasi-equilibrium between sediment supply and transport has been attained in the Green River (Lyons and Pucherelli 1992), resulting in a decrease in the bankfull channel of 6% (Andrews 1986) to 10% (Lyons and Pucherelli 1992). Loss of channel area is attributed to forma- tion of new islands and increased island size and loss of side channels that filled with bed materials (Lyons and Pucherelli 1992). In the Gunnison Gorge of the Gunnison River downstream from the Aspinall Units, summer thunderstorms in 1991-92 caused debris flows in normally dry side- flow channels. This episodic inflow of rocks and soil created large alluvial fans out into the river, which have persisted owing to insufficient peak