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. flr)'~;8,~ 9 - '. U .};_ . PART IV. DITCH DIVERSION RECORDS . Study Period A study period of 1951 through 1980 was chosen for use in this analysis. Ditch diversion records, aerial photographs of irrigated lands, and USGS stream- flow gaging station records throughout the San Juan River basin are generally available during the study period. The availability of reliable data decreases for the years prior to 1950. . . The 1951 through 1980 study period includes a number of wet and dry years that are representative of the long-term regional climate as evidenced by available gage records in the San Juan River basin and tree-ring studies of long-term precipitation trends. The gaging station with the longest period of record in the San Juan River basin in Colorado is located on the Animas River at Durango. Annual discharge records are available for 79 years during the period of 1897 through 1984. Since there are few diversions for irrigation up- stream of Durango, the Animas River gage can be used as a reference to compare the 1951 through 1980 study' period with the longer term period of record. A comparison of the annual streamflow in the Animas River is provided below: . . (Values in Acre-Feet) . Number of years Mean annual streamflow Median annual streamflow Minimum 1Z-month streamflow Minimum streamflow May through Sept. Maximum 1Z-month streamflow 79 604,000 587,000 1897-1950, 1981-1984 Period 49 644,000 68Z,OOO Z38,OOO 137,000 l,1Z9,OOO 1951-1980 Period 30 538,000 488,000 Z13,OOO 137,000 934,000 . 1897-1984 Period . Streamflow during 1951 through 1980 was generally lower than during the other 49 years for which records are available. The dry year flow in 1977 during the study period was only slightly more severe than a critical drouth in the mid 1930's. -5- . :-ii~...~