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I I I I I I I I I I I I I I I I I I I Those areas studied by detailed methods were chosen with consideration given to all proposed construction and forecasted development through 1998. 2.2 Community D~:scription The Town of Silverthorne is in the center of Summit County, in north central Colorado, approximatdy 70 miles west of Denver, on Interstate Highway 70. Silverthorne lies at an elevation of 8790 feet, just downstream of the Dillon Reservoir on the Blue River. Incorporated in 1961, Silverthorne began as a residential area approximately 2 miles north of the old Town of Dillon. State Highway 9 generally parallels the Blue Riv~:r and provides access to Interstate Highway 70; U.S. Route 6, just south of the town; and Green Mountain Reservoir and Kremmling to the north. The Blue River flows north through the town and is the principal river in Summit County, having a drainage area of 377 square miles near the north end of town at Willow Creek. The Dillon Reservoir regulates the discharge of the Blue River through Silverthorne. The average annual peak discharge downstream of the reservoir is approximately 1100 cubic feet per second (USGS, 1995). 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 13.5 square miles. The principle 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. According to the 1990 census, the town had a population of 1768 (Bureau of Census, 1991). In recent years, it has grown substantially to its current permanent population of2982. In addition, during the winter months, there is a noticeable increase in population due to the many nearby skiing areas. The majority ofreeent 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 35 degrees F, with minimum daily temperatures averaging approximately 16 degrees F and maximum daily temperatures averaging approximately 52 degrees F (U.S. Department of Commerce, 1964). The lowest yearly temperature averages approximately --45 degrees F in the winter and the highest yearly temperature averages approximately 89 degrees F in the summer. 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 th~: Blue River, Straight Creek, and Willow Creek normally occurs from April to July. The most prevalent type of flood flow results from 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 of