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Ground water is an important source of supply for municipal and industrial (M&I) and agricultural uses in the planning area. Aquifers suitable for supplying high-capacity wells occur within the valley alluvium of the Cache la Poudre and South Platte River Basins. It is estimated that about 1,700 wells have been installed to tap the ground-water supplies. However, it is not known whether the ground.water resource is fully developed. The Bureau of Reclamation estimates that a yearly average of 178,000 acre. feet of water was pumped during the 1950.1970 period studied. This represents about 24 percent of the water used for M&l and irrigation purposes. The estimated present average annual pumpage from each drainage area is shown below: Basin c-~~--- Annual pumpage Number (1,000 of acre- wells feet) Percent- age Cache la Poudre 1,510 Big and Little Thompson 110 St. Vrain 70 154 87 15 9 8 5 Basin area totals 1,690 178 100 In addition to the South Platte, Cache la Poudre, and Big Thompson Rivers, St Vrain Creek, and their tributaries, water is provided to the three-county area through importations from other basins (table 11-3) and by ground water. There are presently 11 transmountain diversion systems that import water into the Front Range study area. The schematic water budget for the Front Range Unit for the 1950-to.1970 period (exhibit IV-6, Part IV) shows the major streams and their average annual flows. Colorado-Big Thompson Project (C-BT) This complex multipurpose transmountain water diversion project was designed and constructed by the Bureau of Reclamation. Almost all facilities, except the major control facilities, have been transferred to the Northern Colorado Water Conservancy District for operation and maintenance. Project construction began in 1938 at Green Mountain Dam, and electrical energy was first generated in May 1943 at Green Mountain Powerplant. More than 100 features plus about 700 miles of transmission lines are located over an area of 150 miles east-west and 65 miles north-south. The project system is designed to provide a supplemental water supply to irrigation districts by diverting up to 310,000 acre.feet of water per year from the Colorado River Basin to the project service area, but annual diversions to date have averaged about 205,000 acre.feet. The project service area encompasses over 700,000 acres in the South Platte River Basin, and water is delivered to about 125 water-user organizations. Municipalities have always used large quantities of the project water, with 44,950 acre-feet originally being allocated to nine 11.6