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2.1.3 Spill Between Fourmile Canyon Creek and Wonderland Creek The LOMR includes an hydraulic analysis of the amount of spill occurring from Fourmile Canyon Creek to Wonderland Creek which occurs near the upper study limit of the two Creeks. The location and amount of discharge from Fourmile Canyon Creek are identified using the HEC-RAS analysis of Fourmile Canyon Creek. The spill arca betwcen the two Creeks is defincd using a separate HEC-RAS model. 2.2 Community Description The City of Boulder is about 24 sq. mi. in area with a population is about 100,000. It is located within central Boulder County along the Foothills of central Colorado. Please refer to the Vicinity Map, Figure 1. The climate is relatively dry and averages approximately 20 inches of precipitation per year. The average elevation is 5344 feet and rangcs from about 5150 to 8500 feet. The upslope areas are covered with a variety of rock outcrop, thin residual soils on bedrock, and thicker debris, alluvium, and slope wash deposits and are vegetated with grasses, trees, and shrubs. Deeper soils and wetland vegetation are found on alluvial deposits adjacent to streams. The City includes residential and commercial development and open space. Development has altered historic channels, stormwater flow paths, runoff characteristics, and surface water quality. Fourmile Canyon Creek is an intermittent stream which flows from west to east through the City and County with a drainage area of approximately 10.2 sq. mi. at the confluence with Boulder Creek. The upper portion of the catchment is steeply sloped and located in the foothills region of Colorado. The lower portion flows at a more moderate slope. The channel flows over an alluvial fan deposited at the base of the foothills. Much of the catchment in the City consists of commercial and residential development. The catchment in the county is mostly rural residential dcvelopment and open space. Open space occupies part of the upper part of the catchment. Wonderland Creek flows to the east/southeast through the City and the catchment is approximately 2.1 sq. mi. in area. It is a minor tributary of Goose Creek which eventually joins Boulder Creek. The Creek passes through Wonderland Lake which is located at the base of the foothills. Upstream of Wonderland Lake, slopes are steep and much of thc catchment is located within open space. Below Wonderland Lake, catchment and channel slopes are moderate, and the Creek flows thorough developed lands within the City of Boulder. In addition to storm flows generated in the Wonderland Creek Catchment, Wonderland Creek receives flood flows which spill from Fourmile Canyon Creek during major storm events. -6-