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• • • • i • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Is Base Flows - Reach Two August through Febuary I (10101) ? ? -- 8000 6000 4000 ?000 tOri ?0'7" 40% 6051 80'h 10051 Percentage Exceedance Summary This report details the refinements and modifications that have been made over the past three months to the Flaming Gorge model. These refinements were made in response to comments received on the modeling report issued in October, 2001. The way the model was originally developed, Flaming Gorge Dam was operated such that it was inconsistent with how the 2000 Flow Recommendations and 1992 Biological Opinion were intended to be implemented. The modifications made over the past three months attempt to solve this problem. The results described in this report show significantly reduced impacts to reservoir related resources. Of all of the modifications made since October, the most significant was the implementation of the daily model to react to estimated Yampa River flows. This modification substantially reduced the volume of the spring releases made by the Action alternative, which in turn, decreased the drawdown effects associated with the spring release. The Action alternative now yields reservoir elevations that are significantly higher than those presented in the October report. While the frequency of bypasses in the Action alternative has not changed very much from those reported in October, the bypass volumes have been significantly reduced. In October, there was about a 20% chance that any given year would have a bypass in excess of 300,000 acre-feet. With the modifications made since October, there is now a 20% chance in any given year of a bypass in excess 150,000 acre-feet. This report is not comprehensive in terms of the model results analysis presented. It is an attempt to provide some useful analysis for the purposes of determining other resource impacts. Statistical No Action - - Action - Historical (71-?)1 ) Pre Dam (4( _- ,_6I )