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March 1985 Marsh-Incubation Temperature and Fish Survival 137 to a lesser extent. Estimates of development rate (V) in particular are inde- pendent of net hatch estimates since calculations was based only upon hatching time. Since low temperatures reduce and may preclude reproductive success of these fishes, it is useful to examine historic and present-day thermal data from their known habitats in the Colorado River basin. Early reports (e.g., Davis, 1897; Lippencott, 1900; Lee, 1904; 1905; Ross, 1923) dealt with supply and chemical water quality, with no mention of river temperatures. In the Salt River, AZ, for the period 1959-1965 upstream from the influence of reservoirs, temperatures were >15°C from Apr to mid-Oct and >20°C from early May to early Oct (Rinne, 1973). Depending upon location, water temperatures during 1976-1978 in the Gila River upstream from reservoirs were >15°C from Feb-Apr through Sep-Oct and >20°C from Mar-May through late Aug-Sep. Major tributaries had annual extremes of 3-34°C (San Francisco River), 5-28°C (Eagle Creek), and 5-33°C (Bonita Creek), with sea- sonal pattern similar to that of the mainstream (Minckley and Sommerfeld, 1979). Temperatures in the mainstream Colorado River prior to closure of Hoover (Boulder) Dam in 1935 were within a range of 10-30°C, similar to temperatures recorded today in reaches unaffected by reservoir discharges (Minckley, 1979). Observed temperatures in unimpounded reaches of these rivers thus bracket ranges within which razorback sucker, bonytail chub, and Colorado squawfish may successfully reproduce. Yet, they are largely extir- pated from these habitats (Minckley, 1973). Hypolimnetic discharges today result in downstream temperatures in most of these streams that are relatively cool in summer and warm in winter. The Salt River below a series of dams upstream from Phoenix, AZ, rarely exceeds 25°C in summer (USGS, 1950-1980b). Much of the Gila River was dried in the early 1900s by consumptive diversion, thus precluding use by native fishes. In the upper Colorado River, data compiled by Vanicek (1967) show alter- ations of temperature regimes in Green River, UT, as a result of operations at Flaming Gorge Dam. Preclosure (1957-1959) temperatures near the dam- site at Greenlee, UT, ranged from 0.6°C (Dec-Feb) to 21.1°C (Jul) while the range was only 2.7°C (Feb) to 12.2°C (Oct) in the four-year period following closure. Discharges from Glen Canyon Dam (Lake Powell) also have a sig- nificant impact on water temperatures in the mainstream Colorado. Paulson et al. (1980) report historical water temperatures in the Grand Canyon rang- ing from 2 (Jan) to 26°C (Jul) during 1947-48; temperatures >20°C prevailed from May into Sep-Oct. Following formation of Lake Powell in 1963 temperatures have been between 9-15°C year-round, with excursions rarely exceeding 15°C (from their Fig. 5.3.1). Kubly and Cole (1979) reported annual river temperatures between 7 and 10°C at Lee's Ferry, AZ, 24 km downstream from Glen Canyon. Influence of this cold water discharge remains evident at the headwaters of Lake Mead, 440 km downstream (Paul- son et al., 1980). Thermal conditions suitable to reproduction prevail in the upper Colo- rado River basin in reaches downstream of, but remote from dams. About 160 km downstream from Flaming Gorge Dam at Jensen, UT, annual pre-