C-BT History

The Colorado-Big Thompson Project is the largest transmountain water diversion project in Colorado.

Built between 1938 and 1957, the C-BT Project provides supplemental water to 30 cities and towns. The water is used to help irrigate approximately 640,000 acres of Northeastern Colorado farmland.

Twelve reservoirs, 35 miles of tunnels, 95 miles of canals and 700 miles of transmission lines comprise the complex collection, distribution and power systems. The C-BT system spans 150 miles east to west and 65 from north to south.

Tunnel Transports Water Under Continental Divide West of the Continental Divide, Willow Creek and Shadow Mountain reservoirs, Grand Lake and Lake Granby collect and store runoff from the upper Colorado River. The water is pumped into Shadow Mountain Reservoir where it flows by gravity into Grand Lake. From there, the 13.1-mile Alva B. Adams Tunnel transports the water under the divide to the East Slope.

Once the water reaches the East Slope, it is used to generate electricity as it passes through five power plants on its way to Colorado's Front Range. Carter Lake, Horsetooth Reservoir and Boulder Reservoir store C-BT water before it is released to supplement native water supplies in the South Platte River basin.

The C-BT Project annually delivers 213,000 acre feet of water to northeastern Colorado for agricultural, municipal and industrial uses.

For a year-by-year historical account of the C-BT, see the Northern Water Timeline. 

Adams Tunnel construction

Construction of the Adams Tunnel began on June 23, 1940 and was completed on June 10, 1944. The tunnel is 13.1 miles long and 9 feet 9 inches in diameter.

For a more detailed history and photos of the C-BT Project and Northern Water, download the Colorado-Big Thompson Project and Northern Water brochure.

Northern Water’s online videos have detailed historical accounts of Northern Water and the C-BT Project.