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<br />a. Magnitude of Peaks Compared to Historic <br /> <br />The level of depletions in the San Juan River Basin has increased <br />since the pre-Navajo Dam era, Navajo Dam is storing water during the <br />peak runoff, therefore the peaks are lower than historic. During wet <br />years, for instance, the historic peaks in 1938 and 1949 were <br />11,478 cfs and 12,566 cfs respectively. Under full depletions, <br />Navajo Dam can be operated to achieve peaks of 8,920 cfs and <br />10,425 cfs respectively. These peaks are still comparable to the <br />1987 peak of 9,026 cfs. <br /> <br />During average years, the historic peaks in 1936 and 1945 were <br />6,905 cfs and 7,419 cfs respectively. Under full depletions, Navajo <br />Dam can be operated to achieve peaks of 6200 cfs and 7391 cfs <br />respectively. <br /> <br />In dry years, under the full level of depletion, the spring peak <br />cannot be enhanced with storage releases from Navajo Dam. Historic <br />peaks in 1946 and 1951 were 3,427 cfs and 3,169 cfs respectively. <br />Under full depletions, the spring peak flows were 1,525 cfs and <br />1,187 cfs respectively. These peaks are considerably lower than the <br />historic peaks in a dry years. <br /> <br />b. Shape of hydrograph (ascending/descending limbs) <br /> <br />While there is little change in the shape of the hydrograph between <br />current depletion and current depletions plus Animas M&I there is a <br />noticeable change when full depletions are considered. With full <br />depletions the shape of the hydrograph is altered significantly, the <br />hydrograph shifts downwards in every month except May and June when a <br />large spike is evident. The spike is only obtainable 33 percent of <br />the time and consequently the preservation of a natural hydrograph <br />with a high June peak would be a significant change which will be <br />evaluated during the study period. <br /> <br />When the spike is evident, it has a steep ascending and descending <br />limbs because the flows in the months before and after the peak have <br />been reduced significantly to store water in Navajo Reservoir to <br />accommodate depletions. <br /> <br />When the peak is not evident, the flows are lower in all months of <br />the year with only a small peak, which is mainly provided by <br />unregulated tributaries. <br /> <br />Under full development there is also an increase in the peaks in dry <br />years, which will also need further study. <br /> <br />13 <br />