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STORM RUNOFF The State of Colorado and U. S. Geological Survey operate 11 stream-.gaging stations on the main-stern South Platte River and four major tributaries whose confluences are in or near the Denver metropolitan area. Supplementary to these stations, nine miscellaneous sites were selected for indirect dete,rmin- ation of the May 6 peak discharge. In addition, 19 urban area runoff gages were operating during the storm. The locations of these data collection points are showo in figure 1. Hourly and bi-hourly stage-discharge data are presented for 10 of the 11 stream-gaging stations in Appendix 2. The stage-discharge relation has not been developed at this time for the station on Plum Creek near Louviers. Table 3 includes peak flow data for the 11 stream-gaging stations emd the corresponding approximate recurrence intervals. Also included are his- toric peak flow data at each of the stations for comparison purposes. Table 4 lists peak flow data as calculated by indirect methods for the nine miscellaneous sites. The approximate recurrence interval is also listed. The recurrence intervals in tables 3 and 4 are estimates based on a regional flood-frequency analysis by Matthai (1968). The basic information needed to estimate the recurrence intervals at specific sites are geographic location, drainage area, and peak discharge. Accurate determination of drain- age area boundaries within a metropolitan area, such as Denver, is a con~lex and time-consuming task. Consequently, many drainage area values used in these flood-frequency evaluations are approximate and qualified as indicated in Appendix 1. Eight of the miscellaneous peak flow determination sites are on or near the hydrologic boundary used in Matthai's report. The approximate recurrence intervals at these sites are determined by straight-line inte,rpo- lation between the calculated results for each of the adjacent areas and additional information where available. The recurrence interval values for major gaged tributaries to the South Platte River range from 5 years at Cherry Creek near Franktowo to 20 years at Bear Creek at mouth, at Sheridan. Values for miscellaneous sites on cre,eks and gulches in the Denver metropolitan area range from less than 2 years at Sanderson Gulch at W. Arkansas Ave. to IS years at Big Dry Creek at Sout.h Santa Fe Dr. and Lena Gulch near Taft St. Another expression commonly used for frequency of floods is the inverse of the recurrence interval, or the probability of occurrence in any year. For example, a 5-year flood has a 20 percent chance of occurring in any given year, while a 20-year flood has a 5 percent chance. It is conceivable t.hat a 20-year flood could occur several times in a year and then not recur for many years. 14