Low Summer Flow Experiment

LTEMP Experimental Action: Aquatic Resource-Related Experimental Treatments (BA, pages 30-41) [1] Low summer flows may be tested in the second 10 years of the LTEMP period, for the purpose of achieving warmer river temperatures (≥14°C) to benefit humpback chub and other native species. Under low summer flows, daily fluctuations would be less than under base operations (e.g., approximately 2,000 cfs). Investigating the anticipated effects of and options for providing warmer water temperatures in the mainstem Colorado River through Grand Canyon is an identified management action in the Humpback Chub Recovery Goals (USFWS 2002a).

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Description of the Low Summer Flow Experiment Low summer flows (minimum daily mean 5,000 to 8,000 cfs) for three months (Jul.–Sep.) to target ≥ 14°C at Little Colorado River confluence Objectives Increase humpback chub growth Trigger Initial experiment: in the second 10 years of the LTEMP period (2026-2036) when target temperature of ≥14°C can be achieved only with low summer flow Subsequent experimental use if: initial test was successful, humpback chub population concerns warrant their use, water temperature appears to be limiting recruitment, and target temperature of ≥14°C could be achieved only with low summer flow. Offramps Low summer flows do not increase growth and recruitment of humpback chub Increase in warmwater nonnative species or trout at the Little Colorado River Longterm unacceptable adverse impacts on the resources listed in Section 1.3 are observed Or sufficient warming does not occur as predicted Risks Reduce food availability to humpback chub by desiccating the nearshore environment with the reduction in flow between June and July, and reducing drift with the reduction in daily fluctuation. Increasing bioenergetic demands of humpback chub via warmer water while decreasing food supply and availability. Increasing sediment transport during the high releases in the months prior to the LFS test in order to get the annual volume commitment to the Lower Basin. Increasing the potential for nonnative fish and parasites to proliferate throughout the Grand Canyon. Implementation Issues Modeling release temperatures far enough in advance to start making releases to meet annual release requirements Scheduling monthly releases in order to meet annual release requirements while dealing with uncertanties in inflow forcasts. Science planning to monitor affects

Links The 2000 Low Summer Steady Flow Experiment Papers and Publications Summary report of responses of key resources to the 2000 Low Steady Summer Flow experiment, along the Colorado River downstream from Glen Canyon Dam, Arizona, 2011 USGS-GCMRC Open-File Report by Barbara E. Ralston Water temperatures in select nearshore environments of the Colorado River in Grand Canyon, Arizona, during the Low Steady Summer Flow experiment of 2000 Other Stuff