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. ft3/slmi2 during 1984, which was a record snowmelt runoff year. Low unit runoff in Elkhead Creek Basin primarily results from substantially lower snowpack accumulation than Buffalo Pass and mountain ranges that substantially reduce the flow of moist air into northwestem Colorado, Analysis of extreme rainfall data for Colorado that has been collected at five sites for nearly 100 years and rainfall-bucket survey data indicates a distinct lack of intense rainfall amounts in northwestem Colorado. Maximum 24-hr rainfall in northwestem Colorado is about 3.25 in., and there are no known f100d- producing rainstorm having an areal extent of more than about 30 mi2. Interestingly, the maximum monthly precipitation for northwestem Colorado is 8.15 in. recorded in September 1961 at Steamboat Springs. Because of a belief that rainfall has produced large floods in northwestem Colorado, an analysis of the two largest, historic rainstorm in the region were made. This belief has resulted in substantial increases in flood magnitudes used in flood-plain management and design-flood hydrology including dam safety. A 2.57 in. rainstorm occurred on September 9-1 0, 1915 at Columbine at an elevation of 8,680 ft The rainstorm covered about 10 mi2. At the Elk River at Clark streamflow gage, which is located about 10 mi downstream from Columbine, there was no apparent rainfall runoff on September 9 or 10th, None of the streams in the area have any evidence of out-of-bank flows, On June 14,1921, the largest daily rainfall of 2.57 in. was recorded in Steamboat Springs (record collection began in the early 1900s), The rainstorm occurred coincident with annual peak snowmelt runoff, which produced the flood of record of 6,820 ft3/s in . the Yampa River at Steamboat Springs (period of record 1904 to present). Analysis of temperature, rainfall, streamflow, and paleoflood data indicated thai the rainstorm primarily was limited to about 20 to 30 mi2 in the Steamboat Springs area. Estimates of net peak flows resulting from rainfall runoff were made for streamflow gages in operation in 1921 in northwestem Colorado. At the Yampa River at Steamboat Springs, rainfall runoff was estimated to have contributed between 950 to 2,000 ft3/s (15 to 30 percent) to the annual peak; other streams had 800 ft3/S to almost no rainfall runoff, If the rainfall runoff is subtracted from the 1921 annual peak flow, peak flows in 1921 still would have been the third to fifth largest peak flows for each gage in northwestem Colorado. The volume of the rainfall runoff was about the same for the Yampa River at Steamboat Springs as the gage near Maybell, which supports the conclusion that the 1921 rainstorm was limited to the Steamboat Springs area, Out-of-bank flooding was limited to small streams in Steamboat Springs and lowlands along the Yampa River, Analysis of rainfall-runoff contributions for the Yampa River at Steamboat Springs from 1904 to present indicated that 1921 had the largest rainfall-runoff contribution and only three other years had contributions more than 1,000 ft3/sI mi2, The maximum paleoflood for the Yampa River at Steamboat Springs is about 10,000 ft3/S since glaciation. Although rainfall can contribute to peak flows, the contribution is much smaller than has been used in past flood studies, . Flood-frequency relations, which incorporated the paleoflood data, were developed for several gaged and ungaged sites in northwestem Colorado and have an accuracy of about +/- 30 percent These relations can be used for a risk-based analysis of hydrologic aspects of dam safety, The 1 OO-yr flood estimate at Elkhead Creek Dam is about 3,000 ft3/s. Extrapolation of magnitude-frequency relations can be 33