Study Plan - Biological Resource Responses to Fall Steady Experimental Flows Feruary 2010

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Evaluating the impacts of changing the lower limit of discharge on biological resources also was raised in the EA. A 2008 biological opinion (BO) released by the U.S. Fish and Wildlife Service (U.S. Fish and Wildlife Service, 2008) called for two conservation measures that are addressed in this plan in response to the experimental flows. The conservation measures under the 2008 BO include the nearshore ecology project and a study of the impacts of flow transition, that is, the impacts of the flows fluctuating in August followed by reduced, steady flows in September during the experimental period. The BO suggested that the long -term impacts of the experimental flows would benefit humpback chub by creating and improving rearing habitats and increasing growth and survival. The BO also calls for monitoring the impacts of the March 2008. HFE and the fall steady flows on humpback chub. There was not sufficient time to prepare this plan and implement the activities described herein during the 2008 FSEF; the GCMRC and its cooperators were preparing to undertake a substantial body of research associated with the March 2008 high -flow experiment (HFE) in February when the EA was released. However, there were ongoing studies during the 2008 FSEF, including a pilot project to start collecting data for the nearshore ecology project. Additionally, select biological resources were studied in association with the March 2008 HFE, and final reporting for all projects is nearly complete (Melis and others, 2010). Some baseline data from these studies are available and will be incorporated into the longer term studies. Previous Experiments and Studies Stable and low discharge is expected to benefit native fish in the Colorado River below Glen Canyon Dam by improving the quality of nearshore habitats that are occupied by juvenile life stages of native fish. Warming of nearshore water temperatures, stability of nearshore environments, and increases in food resources are the main habitat parameters that are hypothesized to improve with stable and low discharge. A number of these hypotheses have been investigated and the findings are summarized below. Nearshore Water Temperature The water temperature of nearshore habitats in the Colorado River is influenced by a variety of factors, especially the temperature of releases, air temperatures, the residence time of water in the mainstem, and the residence time of water in the nearshore environment (Korman and others, 2006; Wright and others, 2008). The elevation of Lake Powell and the thickness of the epilimnion have a major influence on release temperatures. In particular, the temperature of water discharged from Glen Canyon Dam during the summer of 2005 was 4 °C warmer relative to the summertime average from 1989 to 2000 because of low Lake Powell elevations. The mainstem water temperatures observed between Glen Canyon Dam and the Little Colorado River in 2005 were warmer than temperatures observed in 2000, despite fluctuating flows in 2005 and steady flows in 2000, because of the lower elevation of Lake Powell in 2005 (B. Ralston, U.S. Geological Survey, written commun., 2009).