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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 mi1es eastward from the Continental Divide to emerge fram 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 Longment. Boulder Creek drains an area of 440 square mdles. Slightly more than one-half of the drainage area is in the mountains. South Boulder Creek is a major tributary to Boulder Creek. This stream runs a parallel course to Boulder Creek through the mountains and emerges at El Dorado Springs. South Boulder Creek then turns north and joins Boulder Creek approximately 2 miles east of Boulder. 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 gropus, as classified by the BeS (Reference 14). The entire reaches of Bear Canyon, Skunk, Fourmile Cayon, and Elmers Twamile Creeks consist of well-defined channels. Wonderland creek has a well-defined channel, except for the reach southeast of Valmont Road. Both Twamile and Goose Creeks have extensive reaches without well-defined channels. The Lefthand Creek watershed extends about 30 mdles eastward from its headwaters in the Roosevelt National porest to its confluence with St. Vrain Creek at Longment. Most of the watershed lies in the mountains and varies in elevations 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 Longment. Lefthand Creek passes within 5 miles of the City of Boulder's northern corporate limits and extends through the expanding suburbs of LODgment. Until recently, the 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 precipi tation is 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 areas east of the foothills receive more precipitation (a few 8