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25 <br />• Data were also provided which showed that razorback sucker spawning <br />coincides with the peak runoff and occurs in warm, off-channel ponds <br />and inundated floodplain habitats. The explanation for low razorback <br />sucker reproduction was that, currently, flows often do not reach <br />levels high enough to inundate low-lying floodplain features where <br />spawning takes place. The Service also reported that in the absence <br />of high spring flows, important backwater habitats filled with silt <br />and sand, tamarisk colonized sand and cobble bars, and nonnative <br />minnows capable of preying on or competing with larval endangered <br />fishes greatly increased in numbers. <br />The Service's earlier flow recommendations for spring identified the <br />magnitude and frequency of a range of minimum peak flows and the mean <br />monthly flows capable of producing these peaks and of maintaining the <br />natural shape of the hydrograph, Refinement of these earlier flow <br />recommendations for spring was based on the results of new streambed <br />monitoring studies. The results of one study showed that the spring <br />runoff in 1993, which peaked at 25,900 cfs, was capable of moving <br />coarse bed materials thereby winnowing accumulated fines from the <br />channel substrate. Osmundson et al. (1995) suspected that the earlier <br />recommended peak of 23,500 cfs would also be capable of moving bed <br />materials. The results of another study showed that a spring runoff <br />with a peak of about 12,900 was capable of flushing accumulated fine <br />sediments from the bottoms of backwaters thereby restoring their <br />• depth. Based on this information, the Service developed recommended <br />peak spring flows: <br />>23,500 cfs (5 in 20 years) <br />21,750 cfs (5 in 20 years) <br />16,700 cfs (6 in 20 years) <br />12,900 cfs (4 in 20 years) <br />The Service's 1995 year-round flow recommendations are summarized in <br />Table 1 as mean monthly discharges. Spring (April-July) <br />recommendations are further subdivided into 10-day increments and are <br />reported in Table 2 as volumes of water needed for each of twelve <br />10-day time periods. Variation in precipitation levels from year to <br />year is taken into account and recommendations are provided for years <br />of high, above-average, below-average, and low snow fall. With the <br />exception of winter, recommendations are for flows considerably lower <br />than historic levels but somewhat higher than recent levels. <br />Table 1. Recommended mean monthly flows for the top of the 15-mile <br />reach in cubic feet per second. Rate is the percent of years that the <br />recommended flows should be provided based on winter snowpack levels. <br />For example, in the wettest 25 percent of years, flows in June should <br />• average at least 15,660 cfs; stated another way, this recommendation <br />should be met in five of every 20 years. During low-water years, June <br />