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<br />construction of Navajo Dam. This commitment has both short-term research <br />and long-term operation implications. <br /> <br />The 7-year research period is designed to allow biologists to test the <br />relationship between varying hydrographs and the reproductive and <br />recruitment success of endangered fish. This will involve the testing of <br />a wide range of flows. Depending on the natural contribution of <br />tributaries such as the Animas River, varying volumes of water will be <br />required to be delivered from Navajo Dam. <br /> <br />At the end of the approximately 7-year research period, the Navajo Dam <br />and Reservoir would be used to mimic a natural hydrograph for the life of <br />Navajo Dam based upon the research. Mimicry of the natural hydrograph <br />consists of high spring flows that provides a gradually ascending limb <br />followed by a gradually declining recession limb to low, stable flows <br />throughout the summer, fall, and winter. Based on results of the <br />hydrologic modeling and consideration of biological information, it was <br />determined that an additional 57,100 AF depletion would not appreciably <br />affect~eclamation's ability to mimic the natural hydrograph under <br />current levels of development on the San Juan River. <br /> <br />C. Computer Modeling of Water Availability <br /> <br />An important part of providing suitable habitat conditions for the <br />endangered fish is the assurance that water will be available at the end <br />of the 7-year research period should the research indicate that storage <br />water from Navajo Reservoir is needed to supplement the spring peak <br />flows. <br /> <br />A computer model was prepared which simulated the hydrologic conditions <br />on the San Juan River, both at current depletion levels and with <br />additional future depletions. Annual releases for endangered fish from <br />100,000 AF to 500,000 AF were modeled, and the results were studied by <br />fishery biologists familiar with the endangered fish and the San Juan <br />River. <br /> <br />Releases as high as 500,000 AF could be provided in a significant number <br />of years,but the water level in Navajo Reservoir would be lower in <br />general more of the time than it would be for smaller annual releases. <br />This would result in lower spring spills that contribute to more natural <br />flow patterns most of the time and higher spring flows a smaller <br />percentage of the time. Service biologists, after review of this data <br />believe that, under the current level of depletion and with the <br />Animas-La Plata Project depleting 57,100 AF annually, an annual release <br />of 300,000 AF most closely resembles the natural spring flow pattern. <br /> <br />1. Attempt to Mimic Natural Hydrograph <br /> <br />The Service believes, based upon current biological information, that <br />the magnitude, flow pattern, and frequency of the natural hydrograph <br />was likely a significant factor in maintaining a healthy fish <br />population historically throughout the Colorado River Basin. <br /> <br />5 <br />