Adams Tunnel East Portal

Colorado-Big Thompson Project water travels 13.1 miles east from the West Portal of the Alva B. Adams Tunnel at Grand Lake and exits the mountains at the East Portal to feed the East Slope distribution system.

The Adams Tunnel is 9-feet 9-inches in diameter and runs beneath Rocky Mountain National Park. It cuts through the Continental Divide as much as 3,800 feet beneath the Earth’s surface.

East Portal of the Alva B. Adams Tunnel
East Portal of the Alva B. Adams Tunnel

The tunnel drops 109 feet in elevation between the west and east portals, enabling water to flow by gravity without the need to pump it. It takes about two hours for water to travel from the West Portal to the East Portal.

First Water Delivered in 1947 Construction began on the Adams Tunnel on June 23, 1940. Contractors bored the tunnel simultaneously from each portal. When the two crews met on June 10, 1944, the alignment was off by less than the width of a penny. Read a crewman's firsthand account of clearing post-blasting rubble out of the tunnel's east side in 1946.

The first C-BT Project water flowed from Grand Lake to the East Portal on June 23, 1947. The tunnel cost $12.8 million to construct.

Tunnel Delivers 213,000 Acre-Feet of Water
The tunnel has a capacity of 550 cubic feet of water per second, and delivers an average of 213,000 acre-feet of water annually to the East Slope.

The tunnel is named for Alva B. Adams, a U.S. senator from Colorado who played a key role in convincing Congress to fund and construct the C-BT Project. At the time it was built, the Adams Tunnel was the longest tunnel in the United States to provide water for irrigation.