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<br />SUMMARY OF YEAR-ROUND RECOMMENDA nONS
<br />
<br />.
<br />
<br />Year-round recommendations provided in this report are summarized in Table 8 on a mean
<br />monthly discharge basis. Spring (April-July) recommendations are further subdivided into 10.day
<br />increments and are reported in volumes of water needed for each of twelve 10-day time periods
<br />(fable 9). Variation in precipitation levels from year to year is taken into account and recommen-
<br />dations are provided for years of high, above-average, below-average, and low snow falL, With the
<br />exception of winter, recommendations are for flows considerably lower than historic levels but
<br />somewhat higher than recent levels (Fig. 27). '
<br />
<br />We utilized methodologies that relied on data specific to the IS-mile reach. This included data on
<br />adult Colorado squawfish habitat preferences, stage vs. habitat quantity and quality relationships,
<br />and discharge thresholds for sediment transport, Methodologies used here for developing the
<br />summer and winter recommendations take habitat needs and preferences into account in a more
<br />direct way than did the earlier recommendations. Earlier July recommendations were based on a
<br />hypothesis that a significant reduction in July flows would result in greater over-winter survival via
<br />an extended first-year growing season. Results from subsequent studies failed to bear this
<br />relationship out. Factors that went into developing the earlier spring recommendations are still
<br />considered valid and were retained as the basis for recommendations in this report Here we
<br />provide infonnation regarding flow needs for habitat maintenance and cleansing of coarse substrate
<br />that was lacking in the previous report. '
<br />
<br />This report represents another evolutionary step in the development of flow recommendations for
<br />the 1 S-mile reach. We have refined the recommendations through a combination of replacing
<br />some methodologies with new ones, filling in infonnation gaps with the collection of new data and
<br />incorporating results from studies conducted by other researchers. As additional studies are
<br />completed, our knowledge of the relationship between discharge and fish habitat will continue to
<br />evolve and recommendations will no doubt continue to be refmed. For now, we are satisfied that
<br />the recommendations provided here are an improvement over those provided earlier and feel
<br />strongly that timely implementation of these recommendations wiIl make a significant contribution
<br />to the recovery of endangered Colorado River fish.
<br />
<br />.
<br />
<br />Table 8. Recommended mean monthly flows for the top of the IS-mile reach in cubic ftIsec. Rate
<br />is the percent of years that the recommended flows should be provided based on winter snowpack
<br />levels. For example, in the wettest 2S% of years, flows in June should average at least 15,660 cfs;
<br />stated another way, this recommendation should be met in five of every 20 years. During low-
<br />water years, June flows should average no less than 6,850 cfs, and such a minimum should occur at
<br />a rate of no more than 4 in 20 years (20%). '
<br />
<br />---------------------.-------
<br />
<br />Rate Exceedance JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
<br />
<br />-------------------------------------------- -------------------------------- -------------------------------------- f
<br />25% 25% 1,630 1,630 1,630 3,210 10,720 15,660 7,060 1,630 1,630 1,630 1,630 1,630 -3 -9 ""~;
<br />25% 50% 1,630 1,630 1,630 2,440 9,380 14,250 ?,370 1,630 1,630 1,630 1,630 1,630 z,&' B
<br />30"10 80"/0 1,630 1,630 1,630 2,260 7,710 11,350 3,150 1,240 1,240 1,240 1,630 1,630 z-, (7-
<br />20% 100% 1,240 1,240 1,240 1,8~ 7,260, ~ 1,480 810 810, 81~ 1,240 1,240 / I ~7 ~\o.
<br />-----------------~c;:-~~z. --- ~o-----------.6t~\0-------- ________c____________________________ .
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<br />II 65 II
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