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EXECUTIVE SUMMARY The life history and ecology of the humpback chub (Gila ~ in the Colorado River in Grand Canyon was studied from October 1990 through November 1993, as part of the Glen Canyon Environmental Studies (GeES). The purpose of the investigation was to describe the ecological requirements of the species and evaluate the effects of Glen Canyon Dam operations on the various life stages,. The objectives of the study were to describe and identify limiting factors to distribution, abundance, survivorship, movement, habitat, food habits, and reproductive capacity. Important biotic interactions and the life history schedule for the species were also described. This life history information was used to identify management options and recommendations for core research and monitoring with respect to dam operations. This investigation was designed to integrate with other ~ 18 companion studies and provide the GI ~ 16 framework for an integrated scientific d, ~ 14 report by GeES. ~ 12 THE COLORADO RIVER ! 10 f? 8 III '5 6 III Q 4 INTRODUCTION Preceding the summary of the life history of the humpback chub is a characterization of the changes to the Colorado River following the construction of Glen Canyon Dam, as a perspective of the physical and chemical changes to the aquatic ecosystem in Grand Canyon. The physical, chemical, and biological nature of the Colorado River in Grand Canyon was changed dramatically after it was impounded by Glen Canyon Dam in March ofl963. Since impoundment, water has been released through the dam under six operational scenarios, each with unique characteristics but all with common features. The dam eliminated spring floods and increased low base flows in late summer, fall, and winter. Daily fluctuations for hydropower generation became a normal feature of the hydrograph (Fig. 1), and flows ~ ~ 40,000 ~ ii: ~ 30,000 'iij Q c III 20,000 == of over 31,500 cfs and below 1,000 cfs were eliminated. Daily flows varied by as much as 30,000 cfs. Interim flows implemented in August 1991 decreased high flows to 20,000 cfs and increased lows to 5,000 cfs, with reduced daily fluctuations. The physical and chemical nature of the river were also substantially altered by the dam, primarily from retention of sediments in Lake Powell and cold hypolimnetic releases. The predam temperature range at Lees Ferry of2 to 260C was modified to a range of 7 to 12 0 C, resulting in colder spring, summer, and fall temperatures, and warmer winter temperatures. This temperature pattern did not 22 Annual Discharge 20 o Predam III Postdam 2 o 1920 1930 1940 60,000 Mean Daily Flow 50,000 -Predam - Postdam 10,000 o Oct Aug Sep ~ ~ ~ ~ ~ - - ~ ~ Fig. 1. Annual discharge rNY 1922-92) and mean daily predam rNY 1922-62) and postdam rNY 1965-92) flow of Ute Colorado River at Lees Ferry, AZ.