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. . . 000386 PART XL COMPARISON OF VIRGIN AND mSTORlC STREAMFLOW AT GAGE NEAR mGHWAY 666 Tables 27 and 28, respectively, are summaries of the estimated virgin and historic streamflow at the gage near Highway 666. The table of historic streamflow has been corrected by adding 1,400 acre-feet to the gage records each year to compensate for the irrigation diversions during May through September that historically bypassed the gage. The difference between the virgin and historic streamflow is attributable to the activities of man in the drainage basin upstream of the gage. Although municipal and domestic water use have had a negligible effect on streamflow, diversions for irrigation on up to 11,500 acres have had a significant impact. The effect of irrigation on the flow of the Mancos River is expressed by the following relationship: (6) Historic Streamflow = Virgin Streamflow - hrigation Depletions The difference between virgin and historical stream flows is summarized in Table 29, and varies from 4,700 to 30,300 acre-feet per year, with an average of 19,100 acre-feet. As described in Part V of this report, it is estimated that overall historic irrigation depletions have averaged about 18,800 acre-feet per year. Thus, it is concluded that the virgin streamflow estimates for the Mancos River are accurate within the limits of the available data. As shown in Table 29, irrigation in the Mancos River basin has resulted in large depletions to the Mancos River flows from April through July during the spring runoff period when large amounts of streamflow was available for storage and irrigation use. Since groundwater return flows from irrigation reach the river on a lagged basis, the return flows typically resulted in a net accretion to the river in November through March as indicated by the negative numbers in Table 29. -40- ,~,