Adams Tunnel East Portal, Colorado

A 13-mile long ten-foot wide water tunnel runs under Rocky Mountain National Park, from one end to the other, crossing 3,700 feet under the Continental Divide. It is the fulcrum of a network of trans-basin, trans-divide reservoirs and pipelines known as the Colorado-Big Thompson project, built by the U.S. Bureau of Reclamation, and completed in 1947, at a cost of $160 million. The system involves a number of reservoirs, pipelines, and pumping stations on the western slope to collect water that would otherwise flow into the Colorado River, and away to places like California. After passing through the tunnel, the water enters a pipeline and heads to a string of downstream reservoirs, including Lake Estes, a reservoir in Estes Park, made by damming the Big Thompson River. From there, Colorado River water, now joined with the eastern slope’s Big Thomson River water, flows off to other reservoirs that store water to be used by the booming population in cities north of Denver. When the tunnel opened in 1947, it was the longest irrigation water tunnel in the nation. Today the annual 220,000 acre-feet of water it delivers is consumed by a variety of end users, mostly urban.