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Straight Creek, a tributary flowing west to the Blue River, is approximately 8.5 miles long and has a drainage area of 20 square miles. Willow Creek, also a tributary, flows east to the Blue River. It is approximately 4.2 miles long and has a drainage area of I3. 5 square miles. The principal industry in Silverthorne is tourism, with many residents self-employed in recreation-related businesses. The Henderson Mine, and general construction throughout the county, employs most of the remaining townspeople. In 1998, the town had a population of 1,768 (Reference 3). During the winter months, there is a noticeable increase in population due to the many nearby skiing areas. The majority of land development is adjacent to State Highway 9 on the north side of town and consists of small subdivisions of single-family residences. Development in this area can be attributed to land availability. The climate of Silverthorne is typical of a Colorado alpine valley. The mean annual temperature is approximately 350P, with minimum daily temperatures averaging approximately 160P and maximum daily temperatures averaging approximately 520P (Reference 4). The lowest yearly temperature averages approximately -450P in the winter and the highest yearly temperature averages approximately 89'P in the summer (Reference 3). Total annual precipitation is approximately 18.4 inches, with the total snowfall at approximately 140 inches per year. 2.3 Principal Flood Problems Flooding along the Blue River, Straight Creek, and Willow Creek normally occurs from April to July. The most prevalent type of floodflow is a result of the melting of the winter accumulation of snow. As the temperature rises, the snowmelt runoff increases, generally reaching a peak in mid-June and receding back to normal flow by mid-July or August (Reference 5). The rainfall that occurs during these months usually has a minor effect on the runoff during the period before the peak. After the snowmelt peak, rainfall usually increases the runoff, but rarely causes floodflows (Reference 6). There have been no serious flood problems in Silverthorne since the Dillon Reservoir began operating in 1963. The highest discharge has been 2,010 cfs, which is less than the 100year flood. The longest flood on record prior to the construction of the dam was in 1918. The combined flow of the Blue River, Ten Mile Creek, and the Snake River was 3,500 cfs, just upstream from the present location of Silverthorne. According to U.S. Geological Survey (USGS) records, high flows have been recorded during May and June of several other years on all three streams. However, no significant damage was reported (References 5 and 7). Floodwaters can breach a private road to the south of Willow Creek, causing a ponding effect along State Highway 9. 3