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<br />4) USGS Streamflow Analysis <br /> <br />Streamflow data provide an alternative approach for investigating extreme storms. <br />Through the integration of rainfall magnitudes (depth), storm area and duration, <br />streamflow provides important evidence of both the existence and the extent of heavy <br />precipitation. <br /> <br />Streamflow data available from the U.S. Geological Survey were utilized in this study to <br />examine large storm events by identifYing the magnitude and extent of observl'..d high <br />flows. Streamflow data were used in this study in two different ways. First, streamflow <br />records from all portions of Colorado were examined to identifY possible extreml~ storms <br />that had not been detected by precipitation reports. Secondly, streamflow records were <br />used in conjunction with extreme precipitation repOlts to help identifY potentially suspect <br />and erroneous precipitation reports. It is known and understood that extreme rainfall does <br />not equate directly to extreme high flows so that rainfall may not be strictly inferred or <br />verified solely from records of peak streamflow events. The storm area and duration <br />along with basin geology, vegetation and land use all influence the amount of str~:arnflow <br />resulting from a specified magnitude ofrainfali. ]'01' the purposes of this study, however, <br />storms producing high streamflow were given gn:ater weight than storms with similar <br />maximum reported precipitation but yielding much lower streamflows. <br /> <br />Analyses of streamfIow records, including both direct (gauged) and indirect (manually <br />surveyed) observations, were conducted by John Enr)and, a graduate student in Civil <br />Engineering at Colorado State University at th~~ time of the project. Dr. Robert Jarrett <br />originally developed this set of peak flow measurements. Dr. Jarrett of the U.S. <br />Geological Survey in Denver, Colorado, along with Dr. Thomas McKee and Nolan <br />Doesken of the Colorado Climate Center offef(~d guidance and review. The results of this <br />work are included in Appendix C. <br /> <br />This investigation of streamflow records produced several results and conclusions. The <br />magnitude of observed peak flows associated with storms on this storm list were highly <br />variable ranging from extreme peak flows of record for events such as the Big Thompson <br />flood of 1976 and Plum Creek and related stonns in June 1965 to relatively minor peaks <br />associated with other large storms. For the pUlpose of selecting a final list of most <br />extreme storms for future consideration, priority was given to storms that includ~~d both <br />very heavy rainfall reports and large peak flows. <br /> <br />Not enough work was done in this project to fhlly utilize streamflow records to help <br />identifY very intense and usually quite localized convective storms that were not previously <br />identified based on precipitation records. Several large unit discharges that could be <br />associated with local storms of two to four inches of rainfall in short time periods (30 <br />minutes to two hours) were observed from vef1J small basins but were not looked. at <br />closely since streamflow volumes farther downstream on larger rivers were not <br />significantly affected. Had there been more time and resources allotted for streamflow <br />analyses, undoubtedly many more candidate Local Convective storms could have been <br />identified and added to the list. This was not pursued, however, due to the relatively short <br /> <br />16 <br />