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Therefore, one of the factors that is included in this reasonable and prudent alternative is an attempt to "mimic" the natural hydrograph. Since development began in the San Juan River Basin, and particularly since the construction of Navajo Dam, the natural pattern has been disrupted by the storage of the peak runoff and subsequent release of water through the summer, while return flows, flood control releases, and trout releases have contributed to higher base flows in the fall and winter. Storage in Navajo Reservoir has been high during the period because all of the Reservoir yield has not yet been committed. The high storage level results in releases at times that may not be beneficial for the endangered fish. Therefore, the main objective of the operation of Navajo Reservoir is to increase the spring and early summer peak flows by reducing the fall and winter flows and utilizing the active storage capacity of Navajo Reservoir. 2. Modeled in May and June One of the key elements of the reasonable and prudent alternative (RPA) was the release of a block of water from Navajo Dam during the spring to mimic or replicate the pattern of natural unregulated flows. Iterative modeling was performed with various combinations of minimum flows for trout and spring releases for squawfish, varying between 100 and 500 cfs and 100,000 and 500,000 AF respectively. These combinations produced spring releases that occurred with differing long term frequencies. Biologists associated with the San Juan River selected the combination of 300 cfs and 300,000 AF for use in the RPA, which at the current level of depletions in the basin resulted in high spring releases during 96 percent of the years modeled. During the 1991 modeling, this volume was released in the month of June, producing releases of 5,000 cfs for the entire month. During wet years, reservoir spills often occur during the month of May, resulting in full reservoir conditions. Maximum releases (for the RPA) during the following month of June often caused a significant drawdown of the reservoir, affecting storage and releases in succeeding years. Biologists believed that the pattern could be better if divided between the months of May and June, but the computer model was incapable of depicting such a distribution at the time. In the 1995 modeling, the computer algorithms were modified to allow the endangered fish a volume of 300,000 AF to be spread equally between May and June. In wet years, if the reservoir naturally spilled due to hydrologic conditions, these spills were included as part of the 150,000 AF released for the endangered fish, thus reducing slightly the total volume released during the year. This affects the reservoir in succeeding years as a result of generally fuller reservoir storage conditions, but produces a more naturally shaped hydrograph. The percentage of years that the spring release would be made has not changed. Graphs of the flows at Farmington, 6