Colorado-Big Thompson Project, Water infrastructure system in Northern Colorado, United States.

The Colorado-Big Thompson Project is a federal water-diversion system in northern Colorado that moves Colorado River headwater flows east under the Continental Divide through the 13.2-mile Alva B. Adams Tunnel. Its 11 reservoirs, about 18 dams and dikes, and six power plants span roughly 150 miles east-west. On exposed hillsides, light-gray steel pipe sections joined end to end carry water downslope, bending into a short vertical segment on a small support.

Construction began in 1938 under President Roosevelt's authorization to address water shortages affecting northeastern Colorado. The work extended over decades as engineers completed this complex infrastructure system.

The project enabled farmers to cultivate land throughout the year and played a key role in the development of communities along the Front Range.

The system spans about 150 miles east-west and 65 miles north-south, so pick one area rather than trying to see it all. The Alva B. Adams Tunnel runs 13.1 miles beneath Rocky Mountain National Park, with its west portal near Grand Lake and its east portal southwest of Estes Park. Water then drops roughly 2,800 feet to the foothills west of Loveland, passing Mary's Lake and Estes Lake.

The Alva B. Adams Tunnel runs 13.1 miles (21.1 km) beneath the Continental Divide and Rocky Mountain National Park. When the two drilling crews met on 10 June 1944, the Northern Colorado Water Conservancy District reported that the centrelines of the two headings were off by less than the width of a penny.