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CHAPTER 4 Flow Analysis of Duchesne River Introduction The purpose of the flow analysis was to determine historical flows on the Duchesne River for the period of USGS flow record and to extend that record to earlier years, if possible. The actual USGS flow records at the Randlett gauge exist from 1943 to 1994. Flow statistics developed for this period of record include average flows, percent exceedance flows, recurrence intervals, and extreme flows. . An examination of impacts with changing upstream operation was done by dividing the record into two periods (pre-1970, post-1970) to develop statistics for each period, The pre- project period was assumed to represent historical flow regime and the post-project period was assumed to reflect current flow regime with increased diversions from such projects as CUP and Provo River Project. There is no defined date when the flow regime changed from historical flow regime because the change was incremental. Major changes occurred with the CUP project coming online in 1970. Thus, the pre-project and post-project periods are assumed to be pre-1970 and post-1970, respectively. To assist in determining pre-project flows and the effects of longer term records, the My ton gauge was used to extend the flow record of the Randlett gauge to earlier periods with less development. An analysis of flows was also performed to differentiate the impacts of projects from the hydrologic effects in the post-project period. Gauges not significantly impacted by development were used to accomplish this task. The information is summarized in the tables and figures at the end of the chapter. The information can be used to make statistical inferences and draw conclusions to the historical flows. Flow Statistics at Randlett for 1943 through 1994 Average monthly flows for the Randlett gauge are shown in Tables 4-1 and 4-2 in both CFS and acre-feet, respectively. Flows statistics for the various periods are summarized in the table below. Period Oct Nov Dee Aug Sep Yearly Total AC-FT 1943-1994 299 382 410 401 443 492 419 1,022 1,881 468 206 191 398,578 1943-1969 254 389 449 442 485 517 511 1,301 2,127 482 213 172 442,068 .1970-1994 347 374 368 356 397 465 320 722 1,615 453 198 224 ':tC:1 c:.n7 ......,.,vv. The average flow is a useful measure of the central value of flows. The post-project average flow was determined to be approximately 20 percent less than the pre-project flows. The post-project average monthly flows were less than the pre-project flows in all months SlC\ 1341 07\REPT1.DOC 4-1