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HYDROLOGY APPENDIX The purpose of this Hydrology Appendix is to document the model runs, assumptions made, and the data and analysis used by the U.S. Fish and Wildlife Service to make determinations set forth in the Upper Colorado River programmatic biological opinion, BACKGROUND The reach of the Colorado River between the diversion headgate of the Grand Valley Irrigation Company and the confluence of the Colorado River with the Gunnison River is known as the IS-Mile Reach. The IS-Mile Reach has been identified as important habitat for endangered fish, The Service and others involved in development of the Recovery Program have been investigating various proposals to maintain flows for the endangered fish in this habitat as part of their responsibility under the Endangered Species Act. Various alternatives for flow enhancement in this reach have been suggested by the technical working groups. This Hydrology Appendix documents the evaluation of some of those alternatives using a surface water modeling system (STATEMOD) for the Colorado River developed as a part of the State of Colorado's - Colorado River Decision Support System (CRDSS). ST A TEMOD was developed in part to evaluate the effect of scenarios of future water use and reservoir operations on flows. STATEMOD for the mainstem of the Colorado River Basin was used by the staff of the Colorado Water Conservation Board in cooperation with Service staff to model and evaluate scenarios as described below for the biological opinion. The CWCB staff has provided the following documentation of the CRDSS modeling undertaken for the biological opinion (the documentation has been edited slightly for continuity with terms used in the biological opinion). STATEMOD IS-Mile Reach Modeling Documentation Introduction STATEMOD was run on a monthly basis for the study period of 1975 to 1991 (water years- October 1 through September 30). The following scenarios were implemented. > Base Case (defined in the biological opinion as baseline) > C1 scenario > C1 + 60,000 acre-feet future depletion > C1 + 60,000 acre-feet future depletion + RIPRAP items > C1 + 120,000 acre-feet future depletion > C1 + 120,000 acre-feet future depletion + RIPRAP items These scenarios are described below.