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

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...transitions from August to September in some years have consisted of a transition from a lower limit of 10,000 cfs in August to an upper limit of 10,000 cfs in September. Such a transition results in a river stage level that is below the varial zone of the previous month's flow, and may be detrimental to fishes and food base for fish. Reclamation has committed to adjusting daily flows between months to attempt to attenuate these transitions such that they are more gradual, and to studying the biological effects of these transitions, in particular to humpback chub. However, studying potential effects of these transitions on food base or fish would require a series of experimental transition flows that would confound and complicate interpretation of data from the ongoing FSEF. Previous research on stranding in Grand Canyon, which was conducted during the 1980s when daily changes in discharge were much greater than presently allowed (that is, change from 25,000 to 5,000 cfs in roughly 6 hours), revealed that only a small percentage of backwaters contain isolated pools when discharge is drastically reduced over a short time period (Maddux and others, Arizona Game and Fish Department, written commun., 1987). Humpback chub -(0.2 percent) and native fishes collectively (4 percent) represented a small percentage of the 1,923 fish that were stranded in isolated pools during a systemwide survey conducted from October 19 -21, 1984 (Maddux and others, Arizona Game and Fish Department, written commun., 1987). The average size of native fish stranded was also small ( <55 mm), indicating that adult native fish are not susceptible to stranding. The GCMRC has concluded that the comprehensive study by the Arizona Game and Fish Department (Maddux and others, Arizona Game and Fish Department, written commun., 1987) provides sufficient information for managers to make an informed decision about whether transitions should be gradual (to reduce potential stranding of native fish) or if transitions can be abrupt (because changes in monthly volume are much more gradual now than during Maddux study). As a result, the GCMRC concludes that additional study of stranding is not warranted. If managers are interested in studying the effects of abrupt transitions in monthly volume on food base or fish growth, the GCMRC is ready to help design the necessary experimental hydrographs. But any transition experiments and associated studies should be conducted after 2012 when the FSEF will be completed. Conducting this research after 2012 would allow transition studies to benefit from the experience and insights gained by FSEF studies (that is, what techniques worked, whether process -level measurements sensitive enough to detect transition effects, etc.). Further, designing transition experiments would not be constrained by the ongoing FSEF. High -Flow Experiments The number and size of backwaters has greatly diminished since the March 2008 HFE (T. Kennedy, U.S. Geological Survey, personal observations, 2008, 2009). In April 2008, immediately after the HFE, there were at least six large backwaters between the Little Colorado River and Lava Chuar (the reach for the nearshore ecology project), and they were all present across the full range of April flow fluctuations. A year later, there was only one backwater in this reach that was present across the full range of flow fluctuations in July 2009 (M. Yard, U.S. Geological Survey, personal observations, 2009). There are three other backwaters in this reach, but they are only isolated during the night when discharge is low; however, during the day when discharge is high, the return - current channel bar is overtopped. As such, the nearshore ecology project is evaluating the interactions between flows, fish growth, and habitat type in a reach where the amount of backwater habitat is minimal. If managers are interested in having more W