A river flowing under the mountains

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The rugged, craggy Rockies rise sharply in the middle of Colorado, dividing the state roughly in half between the western high country and the eastern plains. The extreme contrast of these landscapes also leads to an extreme disparity of the waters.

The West Slope receives 80% of the state’s precipitation as weather systems rising to cross the Continental Divide shed their rain and snow loads before moving east. Water that falls west of the divide flows to the Pacific Ocean, while water that falls to the east flows to the Gulf of Mexico and the Atlantic.

The eastern plains of Colorado, however, are semi-arid. In 1820, explorer Stephen Harriman Long, after whom Long’s Peak is named, called it a “great desert” unsuitable for agriculture. But sandy, loamy soil can make fertile farmland when irrigated.

In the mid to late 19th century, the Gold Rush and the coming of the railroad brought an influx of settlers to Colorado, including ranchers and farmers. Then, in the 1880s, the plains received above average rainfall. The new settlers plowed under drought-tolerant native grasses and used Eastern farming techniques to grow wheat and corn, practices that would later contribute to soil erosion and the Dust Bowl. When drier conditions returned, residents turned to the snowpack of the Rockies and the Colorado River, then known as the Grand River, as a reliable source of water for irrigation. One of the first efforts to exploit this supply was the Grand River Ditch. Beginning in 1900, the ditch diverted water from the Never Summer Mountains through Poudre Pass and into the Cache la Poudre River.

In the early 1930s, during the Great Depression and Dust Bowl drought, farmers and their representatives formed the Grand Lake Committee and devised a more ambitious plan to divert water from the western slopes of the Rockies and connect the Colorado Rivers and Big Thompson. After lengthy negotiations, construction of the Colorado-Big Thompson Project was initiated by the U.S. Bureau of Reclamation in 1938. By the time it was completed and declared fully operational in 1957, it included 18 dams, 12 reservoirs, six hydroelectric generating stations , 95 miles (150 kilometers) of canals and 35 miles (55 kilometers) of tunnels. The most critical of these is the tunnel that stretches 21 kilometers under Rocky Mountain National Park and is named after U.S. Senator Alva B. Adams, who championed the project in Congress.

In 1940, two teams of workers began digging tunnels on either side of Rocky Mountain National Park: one from the west portal in Grand Lake and one from the east portal southwest of Estes Park, Colorado. In 1944, when drilling crews met thousands of feet below the Continental Divide, the two sides of the tunnel were misaligned by barely the width of a dime. The complex task of lining the 9.75-foot (3-meter) diameter tunnel with concrete took a few more years before the first water flowed through the tunnel in 1947.

The portals are visible in the image above, which was acquired on September 2, 2021, with the Operational Land Imager (OLI) on Landsat 8 and overlaid with topographic data from the Shuttle Radar Topography Mission (SRTM).

Snowmelt and runoff collected in Lake Granby is pumped to a channel that empties into Shadow Mountain Reservoir and Grand Lake, where it enters the west portal of the Adams Tunnel. Exiting the eastern portal, water flows into the Wind River towards Mary’s Lake, then through other tunnels and channels to several Front Range reservoirs. Between the west and east portals, the elevation of the tunnel drops 109 feet (33 meters). Driven by the force of gravity, water flows through the tunnel at a rate of 550 cubic feet (15.5 cubic meters) per second, traveling the length of the tunnel in approximately two hours.

It was a $160 million (about the equivalent of $2 billion in today’s dollars) engineering feat. But it was not without controversy. Many West Slope residents felt they were not being adequately compensated for the water loss. Conservationists feared the project would destroy the natural beauty of Rocky Mountain National Park. The project continued after authorities reached an agreement to build the Green Mountain Dam and Reservoir to store water on the western slope and to move the tunnel portals outside the national park boundaries. .

Today, the Colorado-Big Thompson Project provides 200,000 acre-feet of water annually to northeast Colorado, quenches the thirst of one million residents, and irrigates more than 600,000 acres of farmland. Although the diversion project was originally built to irrigate farms and fields, it now also supplies water to towns and villages, industry, hydroelectric generation, recreation, fish and water. wildlife. In Colorado, where more than 80% of the population lives and where only 20% of the precipitation falls, these trans-basin water diversions have become part of life.

NASA Earth Observatory images by Joshua Stevens, using Landsat data from the US Geological Survey and topographic data from the Shuttle Radar Topography Mission (SRTM). Story by Sara E. Pratt.

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