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. . The U.S. Geological Su~vey, in a 1939 field investigation of the 31 May 1894 flood, estimated a peak discha~ge of 9,800 cubic feet pe~ second fo~ this event, which was t~eated as a high outlie~. Since Button~ock Dam has cont~olling effect on peak discha~ges at the Lyons gauge, only the period of ~ecord p~eceding its full time ope~ation was used in the discharge probability analysis. Button~ock Reservoir was in the process of filling du~ing the 7 May 1969 flood and an estimate of the uncontrolled peak was made for use in the frequency analysis. The 2,900 cubic feet per second peak discharge recorded at the gauge was adjusted to 6,000 cubic feet per second by using unit hydrograph ratios. The gauge at the mouth nea~ Platteville has fifty-th~ee yea~s of reco~d. The ~ecords for 1905, 1906, and 1927 through 1977 were used in the f~equency analysis. The 1969 event was adjusted f~om 10,300 cubic feet per second to 13,400 cubic feet per second in order to reflect an uncontrolled condition. Peak discha~ges since 1969 appear to have been generated from the lower portion of the basin and did not require adjustment to reflect effects from Buttonrock Reservoir. the estimated 1894 peak discha~ge of 15,500 cubic feet per second was treated as an histo~ic event. Rainfall values fo~ the 10-, 50-, and 100-year 6-hour events were obtained from the "Precipitation-Frequency Atlas of the Western United States, Atlas 2, Volume III, Colorado", published by the National Oceanic and Atmosphe~ic Administration (NOAA) in 1973. In each case, the 5-yea~ and 500-year value was estimated by ext~apolating the selected Atlas values on semi-logarithmic paper. Adjustments for depth-area were based on the depth-area relationship presented on the NOAA publication. Adjustments for length of record were made on logarithmic probability paper using Beard's table of expected probability adjustments for various lengths of record. The spatial variation of the ~ainfall potential shown in the Atlas for the St. Vrain Creek basin was simulated by SWMM through the introduction of several hyetog~aphs. The time va~iation of the 6-hour rainfall for each hyetog~aph was based on a study of hourly precipitation data reco~ded during major sto~ms on the South Platte Rive~ basin. The rainfall dist~ibution is given in Table 3 below. Detention storage values of 0.15 inch mountains and 0.10 inch plains were selected by trial and error. Mountain infilt~ation rates we~e set at a uniform ~ate of 1 inch per hour. In order to calib~ate the St. V~ain model to the Platteville gauge, it is necessary to use a plains area infiltration'rate of 0.5 inches per hour. The f~equency analysis of the gauged data at Lyons was used in the calib~ation of the SWMM model of North and South St. V~ain Creeks fo~ conditions in the basin prior to the const~uction of Button~ock Dam. An average annual June base flow of 500 cubic feet per second was added to obtain the modeled frequency curves at the Lyons gauge. The average annual June base flow of 600 cubic feet per second was added to obtain the modeled discharges at the mouth. 5