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The Aspinall Unit allows water to be stored during spring runoff and released when needed to meet downstream needs and produce hydropower. The hydrograph on the following page shows how Unit operation has influenced average monthly streamflows downstream, compared to flows that occurred before the Unit existed (pre-Aspinall; 1911-1965). Unit regulation since 1969 has reduced Gunnison River flows during spring runoff and increased flows during the non-runoff months. Comparing the graph line for the most recent period (1992-1999) to the other two periods shows that coordination of operations has had the effect of returning the shape of the hydrograph for this short period to mimic more natural, seasonal flow patterns represented by the pre-Aspinall hydrograph line. The 1966-68 years were not included as this was when Blue Mesa was filling. Even with regulation, however, flows vary significantly depending on the amount of snowfall. For example, annual flows through the National Park averaged 396 cfs during 1977 and 2,943 cfs during 1984. Long-term changes in climatic conditions explain some of the post- versus pre- Aspinall differences (overall, the 1969-1991 period appears drier than the other two periods), but changes in the seasonal distribution of flows depicted by the hydrographs are due mostly to reservoir release patterns. Endangered Fish The Gunnison River provides habitat for two endangered fish species, the Colorado pikeminnow and the razorback sucker. The 50 miles of river downstream from Delta, Colorado, are designated as critical habitat for these species. These and two additional endangered fish species-the humpback chub and bonytail-occur in the Colorado River downstream from the mouth of the Gunnison River. Operation of the Unit is important in determining the flow pattern of the Gunnison River, and to a lesser extent, the Colorado River. Between 1990 and 1997, a major consideration in coordinating operation of the reservoirs was to provide flows to support scientific research studies on the habitat and life history of these fishes in the Gunnison and Colorado rivers. These studies are part of the Recovery Implementation Program for Endangered Fish of the Upper Colorado River Basin (RIP), a cooperative effort of private, state, and federal interests. Data from a number of individual scientific studies are now being compiled into a synthesis report that is scheduled for completion in late 2000. These studies include inventories of native and non-native fish populations; monitoring endangered fish recruitment; inventories of backwater habitats; characterizations of the relationships between flows and changes in channel morphology, habitat availability, and habitat maintenance; and monitoring of endangered fish stocking efforts. The synthesis report will include flow recommendations for critical habitat. The flow recommendations will be set for the Whitewater gage on the lower portion of the river. It should be noted that only 50 percent of the basin above this gage is regulated by the Unit. Therefore, downstream tributary flows from the remainder of the basin are important in meeting recommendations. Reclamation, in cooperation with the NPS and the Fish and Wildlife Service, is developing a RiverWare hydrology model that will be used to analyze the effects of the flow recommendations A-5