Longest water supply and hydroelectric tunnel

What to remember

Under the Continental Divide in Colorado's Rockies, water crosses the mountains in a straight line. The Alva B. Adams Tunnel, opened in 1947, is 21.1 km long, the longest combined water supply and hydroelectric tunnel.

It is the principal component of the Colorado-Big Thompson Project, the largest transmountain water project in Colorado. It moves water from the western slope of the Colorado River drainage to the Front Range in the east. Lined with concrete, it is 2.97 m in diameter and drops 33 m in elevation along its length. It passes under Otis Peak in Rocky Mountain National Park and, at its deepest point, lies about 1,200 m below the mountain peaks.

Construction began on 15 June 1940. It was suspended from the end of 1942 to August 1943 because of World War II priorities, then resumed until the opening.