The Colorado-Big Thompson Project (C-BT) is the single largest diverter of Colorado River water in Colorado and is the biggest transmountain diversion in the state. The C-BT collects the headwaters of the Colorado River in Grand County, annually sending in excess of 200,000acre-feet of water through the Alva B. Adams tunnel on the eastern end of Grand Lake to farms and cities in northeast Colorado. The C-BT is a complex system of reservoirs, pumps, pipelines, canals and other water structures for collecting and distributing water and generating hydroelectric power. Its operations are set out in Senate Document 80,passed on June 24, 1937 by the 75th Congress, which specifies that the purpose of the C-BT was to provide a supplemental water supply for northeast Colorado. The project began construction in 1938 and was completed in 1957.Agricultural users owned 85% of the shares at the time the project was completed, however, growing municipalities now own the majority of the water shares. Green Mountain Reservoir in Summit County is part of the Colorado-Big Thompson project. Of its152,000 acre-foot capacity, 100,000 acre-feet has been set aside for the benefit of the West Slope.