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<br /> <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 observed 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 extreme storms <br />that had not been detected by precipitation reports. Secondly, streamflow records were <br />used in conjunction with extreme precipitation reports 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 streamflow <br />resulting from a specified magnitude of rainfall. For the purposes of this study, however, <br />storms producing high streamflow were given greater weight than storms with similar <br />maximum reported precipitation but yielding much lower streamflows. <br /> <br />Analyses of streamflow records, including both direct (gauged) and indirect (manually <br />surveyed) observations, were conducted by John England, a graduate student in Civil <br />Engineering at Colorado State University at the 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 offered 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 storms in June 1965 to relatively minor peaks <br />associated with other large storms. For the purpose of selecting a final list of most <br />extreme storms for future consideration, priority was given to storms that included both <br />very heavy rainfall reports and large peak flows. <br /> <br />Not enough work was done in this project to fully 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 very 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 />