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00D384 . . . of changing precipitation and runoff patterns, with most of the gains occurring during the spring runoff period of February through June. An estimate of monthly gains for the February through June period was made through use of the stream depletion estimates set forth in Part V and summarized in Table 2. Thus, river gains during February through June are expressed as the following: (4) Monthly River Gains = Historical Flow at Highway 666 (Appendix A) + Bypass of Flows around Gage (from Section IV) + Irrigation Depletions (Table 2) + River Losses (Table 12) - Virgin Flow at Upper Gages (Appendix C) - Short-Term Runoff (Table 16) During the remaining months of the year, it was estimated that inflows from springs and small intermittent creeks in the areas downstream of the upper gages contribute a small continuous base flow. On the basis of the water balance calculations, it was estimated that the base flow is about five cfs, with some variation depending on whether antecedent precipitation during the year was below average, average, or above average. A summary of estimated river gains is presented in Table 21. As shown in the table, annual gains have varied from about 2,900 to 46,000 acre-feet, with an average gain of about 10,600 acre-feet per year. This estimate of the average annual gain derived on a monthly basis compares favorably with the estimate derived on an annual basis from equation 3 above (10,300 acre-feet). From inspection of Table 21, it is apparent that snowmelt inflow in the Mancos River basin originating from tributary areas downstream of the conflu- ence of the West, Middle, and East Mancos rivers, generally peaks in March, April, and May, with only a relatively small amount occurring in June. This runoff occurs relatively early in the spring because nearly all of the drainage area contributing to the runoff below the confluence is at an elevation of less than 8500 feet. -38- ,j,