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for these evaluations were recorded by the U.S. Geological Survey and <br />were retrieved from magnetic tape storage in the National Water Storage <br />and Retrieval System files. The period of record varies between stations, <br />but most data are continuous from before 1920 to the present. The sta- <br />tions examined were: <br />o Colorado River near Cisco, Utah <br />o Green River near Green River, Utah <br />o Gunnison River near Grand Junction, Colorado <br />o San Juan River near Bluff, Utah <br />o White River near Watson, Utah <br />o Yampa River near Maybell, Colorado <br />Two of these stations (the Green River near Green River, Utah, and the <br />Gunnison River near Grand Junction, Colorado) provide continuous flow data <br />from 1897. <br />Five-year averages of flows recorded during the months of May (spring <br />runoff) and November (minimum withdrawal at low flow) were plotted to il- <br />lustrate historical trends (Figures 1.4, 1.5, 1.6, 1.7, 1.8, 1.9). Five- <br />year averages were used to reduce point scatter resulting from short-term <br />climatic variations. Calculated virgin flows were provided by the Bureau <br />of Reclamation, Salt Lake City, Utah, and were plotted on the same graphs <br />to illustrate changes caused by man's activities. Some of the virgin <br />flow values for the period prior to 1920 are lower than the recorded <br />flows, and it is apparent that either the computed virgin flows or the <br />recorded flows are incorrect. Therefore, trends prior to 1920 are not <br />used in the discussion which follows. <br />The six rivers studied exhibit different degrees of flow alteration. <br />The Yampa and White Rivers show little variation from virgin flows, in- <br />dicating few depletions (Figures 1.4 and 1.5). Relatively small amounts <br />of land are irrigated in these two basins; little industrial development, <br />and no major dams, exist on either the White or Yampa Rivers. Small re- <br />servoirs on the White River have little apparent effect on flow. Deple- <br />tions have not increased significantly during the period of record, which <br />indicates that little additional irrigated acreage has been added since <br />1920. The White River shows more depletio-,.s in May than the Yampa, sug- <br />gesting it supports a larger agricultural community. <br />19