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11-2 is a massive flat, fault-bounded arch approximately 185 mi les long and 25 to 45 mi les wide. The main body of the Front Range consists of Precambrian crystal I ine formations. Structural rei ief of the Precambrian surface ranges from approximately 14000 feet above sea level along the Continental Divide to approximately 8000 feet below sea level in the adjacent Denver Basin. The Denver Basin is an asymmetrical structural basin. Its axis closely parallels and is located near the Front Range uplift. The deepest portion of the Basin is located near the City of Denver and is estimated to represent more than 13,000 feet of sediments. Along the western edge of the Denver Basin uplifted paleozoic, mesozoic and tertiary sediments are exposed and are known as the Foothi I Is Belt. 5. Water Resources Sources of water used in the approximate 480-sq-mi St. Vrain Basin come from native surface runoff, trans-Basin diversions, and groundwater. These sources are quantified briefly herein; however, a more detai led discus- sion is presented in Chapter II I. The long-term average native (vi rgin) surface water runoff of the St. Vrain Creek Basin exclusive of the Boulder Creek drainage is estimated to be 117,600 ac-ft per year (U.S. COE, 1977). This native surface water runoff of the St. Vrain Creek is estimated from the combined flow records of gaging stations located on St. Vrain Creek near Lyons and Left-Hand Creek near Boulder. Surface water runoff from areas below these gaging stations is not included in these estimates, but runoff from the lower basin represents a small volume on a long-term average basis. Native surface water runoff during the 1953 to 1956 4-year drought period averaged 75,800 ac-ft per year, or approximately 64% of the long-term average. The largest peak discharge of record on St. Vrain Creek at Lyons prior to 1980 was 10,500 cfs on June 22, 1941 (U.S. COE, 1977). Another source of water to the Basin comes from trans-Basin diver- sions. The Colorado-Big Thompson (C-BT) Project is a trans-Basin diversion project constructed by the Bureau of Reclamation in the 1940's and 1950's, and operated (except for power generation features) by the Northern Colorado Water Conservancy District. At the time the Study began, it was the only trans- Basin diversion conveying water to the Study Area; however, the new Windy Gap Project wi II deliver western slope water beginning in 1985 by use of the C-BT del ivery system. Water users wi thin the St. Vrain Basin di.vert from the C-BT system an average of approximately 31,000 ac-ft per year wi th approximately 13,500 and 17,500 ac-ft going to municipal and agricultural uses, respective- ly. Since the C-BT system is a supplemental water supply system, del iveries from the system are generally inversely proportional to native flow in the St. Vrain Basin, i.e. the highest del iveries of C-BT system water to the Basin occur in drought years. I I I I I I I I I I I I I I I I I I I