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Construction and operation of Glen Canyon Dam has dramatically impacted the flow of the Colorado River through Glen, Marble, and Grand Canyons. Extremes in both streamflow and water temperature have been suppressed by controlled releases from the dam. Trapping of sediment in Lake Powell, the reservoir formed by Glen Canyon Dam, has also dramatically reduced the supply of suspended sediment entering the system. These changes have altered the riverine ecosystem and the habitat of native species, including fish such as the endangered humpback chub (Gila cypha). Most native fish are adapted to seasonally warm water, and the continuous relatively cold water released by the dam is one of the factors that is believed to limit humpback chub growth and survival. While average mainstem temperatures in the Colorado River are well documented, there is limited understanding of temperatures in the nearshore environments that fish typically occupy. Four nearshore geomorphic unit types were studied between the confluence of the Colorado and Little Colorado Rivers and Lava Canyon in the summer and fall of 2010, for study periods of 10 to 27 days. Five to seven sites were studied during each interval. Persistent thermal gradients greater than the 0.2 °C accuracy of the instruments were not observed in any of the sampled shoreline environments. Temperature gradients between the shoreline and mainstem on the order of 4 °C, believed to be important to the habitatseeking behavior of native or nonnative fishes, were not detected. Introduction The construction and operation of Glen Canyon Dam (GCD) has greatly changed the character of the Colorado River through Glen, Marble, and Grand Canyons. Dam operation for water storage and production of hydropower has resulted in reduced seasonal variations in flow, but much larger daily fluctuations (Topping and others, 2003). The average predam annual peak flow was about 87,000 cubic feet per second (ft/s) and winter low flows were typically under 5,000 ft/s (Topping and others, 2003). Current flows typically fluctuate by 5,000 to 8,000 ft/s on a daily basis, with daily maximum flows less than about 22,000 ft/s and daily minimum flows greater than about 6,000 ft/s. Because the water intake structure is at a fixed elevation deep in the reservoir, the water released from Glen Canyon Dam is cool and typically varies between 8 and 12 °C in comparison with predam seasonal variations of between 0 and 30 °C (Wright and others, 2009). The changes in daily, seasonal, and annual temperature extremes have affected the aquatic habitat used by native species, including the humpback chub (Gila cypha). Temperature gradients along the shorelines are not well documented; previous work on mainstem shorelines (Korman and others, 2006; Vernieu and Andersen, unpublished data) has been limited to short durations of one to three days. Most temperature data collection has been in backwaters and shoreline cavities with some degree of isolation from the mainstem. A better understanding of both

Nearshore thermal gradients of the Colorado River near the Little Colorado River confluence, Grand Canyon National Park, Arizona, 2010