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<br />OOlJ120 <br /> <br />bases offered for the instantaneous peak flow recommendation was the work of Stanford. <br />Stanford and others have stated the need for a natural hydro graph for restoring river systems. <br />This concept is relevant to the Gunnison and Colorado rivers flow recommendations, but does <br />not explicitly provide guidance on the magnitude of peak and base flows needed to restore river <br />functions or endangered fish habitats. <br /> <br />Stanford did not state or imply that an unregulated hydro graph was needed to restore <br />· rivers. A natural hydro graph can simply mean a spring rise in flow preceded and followed by <br />relatively lower base flows. Recommendations for specific magnitudes and durations of spring <br />peak flows should be based on specific functions to be performed and the threshold values that <br />perform those functions. Those threshold values are provided in the Pitlick et al. reports and <br />consist of half-bankfull and bankfull flows. We believe that if the Aspinall Unit were operated to <br />provide the recommended Pitlick flow durations, many of the characteristics of a "natural <br />hydro graph" including instantaneous peak flows and peak flow variability would be provided by <br />inter-annual variation in hydrology and flow from intervening tributaries below the Aspinall <br />'^. Unit. One can examine the historic period of operation to determine the amount of variation that <br />can be expected when implementing the Pitlick recommendations. <br /> <br />Suggested Presentation in the Revised Gunnison Flow Recommendations Report <br /> <br />Given the uncertainties associated with any peak flow recommendations for the Gunnison <br />and Colorado Rivers, the status of endangered fish populations in these rivers, and the impacts of <br />altered flows on power and water users, we believe that it is important to adhere closely to <br />existing flow recommendations that are based on site-specific data. In the report, peak flow <br />recommendations should be stated simply as the long-term average duration of flows above half- <br />bankfull and bankfull flow (as in the original Pitlick et al. report) without specifying the number <br />of days above these thresholds in each hydrologic category. If peak flow durations are specified <br />for particular hydrologic categories (as currently presented in the revised tables under <br />consideration by the Biology Committee), it should be explicitly stated that the distribution <br />~ among hydrologic categories is just one example of how Pitlick's recommendations could be <br />implemented. <br /> <br />To our knowledge, there is no specific scientific basis for recommending instantaneous <br />peak flows above the Pitlick thresholds. Any recommendation for an instantaneous peak should <br />be supported with specific justification rather than more nebulous support such as adherence to a <br />natural hydro graph. As an alternative to recommending specific instantaneous peaks, variability <br />~ in peak flows could be identified as desirable and the historic range of peak flows that have <br />occurred post-Aspinall could be provided to indicate the desirable range of peaks flows. <br /> <br />The flow recommendations report should include implementation guidelines that include <br />how peak flow durations and instantaneous peak flow magnitudes are determined. Year-to-year <br />implementation of peak flow magnitude and duration should be left to a multi-agency working <br />committee that evaluates and incorporates the full spectrum of information that can be used to <br />describe annual resource conditions. <br />