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Boulder Creek is a steep mountain stream draining a portion of the eastern slope of the Rocky Mountains in Boulder County. The creek extends 22 miles eastward from the Continental Divide to emerge from the mountains and pass through the City of Boulder. Boulder Creek then flows 18 more miles to enter St. Vrain Creek 5 miles east of Longmont. Boulder Creek drains an. area. of 440 square miles. Slightly more than one-half of the drainage area is in the mountains. . South Boulder Creek is a stream runs a parallel mountains and emerges at then turns north and joins of Boulder. major: tributary to Boulder Creek. This course to Boulder Creek through the Eldorado Springs. South Boulder Creek Boulder Creek approximately 2 miles east The Bear Canyon, Skunk, Twomile, and Fourmile Canyon Creek basins originate in the foothills west of the City of Boulder. The terrain of these upstream subbasins is steep and rocky, and consists of topsoils in the C and D hydrologic soils groups, as classified by the SCS (Reference 14). The entire reaches of Bear Canyon, Skunk, Fourmile Canyon, and Elmers Twomile Creeks consist of well-defined channels. Wonderland Creek has a well-defined channel, except for the reach southeast of Valmont Road. Both Twomile and Goose Creeks have extensive reaches without well-defined channels. The Lefthand Creek watershed el<tends about 30 miles eastward from its headwaters in the Roosevelt National Forest to its confluence with St. Vrain Creek at Longmont. Most of the watershed lies in the mountains and varies in elevation from 5,600 to 11,000 feet. The remainder of the watershed lies in the high plains. St. Vrain Creek is a continuous-flowing stream that drains the east slope of the Continental Divide ,by way of North and South St. Vrain Creeks. From the confluence of these creeks at Lyons, Colorado, St. Vrain Creek flows southeasterly through Longmont, then northeasterly to the South Platte River. Lefthand Creek, Dry Creek No.1, Spring Gulch, and Loomiller Basin join St. Vrain Creek within Longmont. Lefthand Creek passes within 5 miles of the City of Boulder's northern corporate limits and extends through the expanding suburbs of Longmont. Until recently, ~he floodplain was devoted entirely to agriculture. Now, because of expanding population and industrialization, urban development has begun at both ends and in the middle of the study reach. . The climate of the study area is classified as semiarid. The average annual precipitation lS 18.3 inches, which includes an average annual snowfall of 83 inches (Reference 2). Location with respect to the foothills west of Boulder has a slight influence on the total precipitation depths in the study-area subbasins. Those a~eas east of the foothills receive more precipitation (a few 8