The tunnel drops 109 feet (33 m) in elevation along its length and runs in a straight line under the Continental Divide from west to east, passing under Otis Peak in Rocky Mountain National Park.

Construction began on 15 June 1940, but was suspended as a result of World War II priorities, from the end of 1942 to August 1943.

The tunnel was holed through on 31 March 1944, an event that was broadcast throughout the United States by NBC Radio.

[1] West of the Continental Divide, the input of the tunnel is water pumped from Windy Gap Reservoir, Willow Creek Reservoir and other North Fork of the Colorado River drainages to Lake Granby.

Lake Granby is the second largest reservoir in the state of Colorado, with a nominal capacity of 470,000 acre-feet (0.58 km3).