Chimney Hollow is the largest dam to be built in the US in decades.
Source: Northern Water
As the West Dries Out, a New Generation of Dams Rise
Environmentalists may hate dams, but the cities of Colorado’s Front Range are counting on them to support their booming population in a deepening megadrought.
The Chimney Hollow Dam looks ancient in its simplicity, like a Neolithic mound. The largest dam built in the US in a quarter century, it is not a monumental wall of concrete like the Hoover Dam. Instead, it’s a triangular prism of granite boulders, 350 feet high and 3,700 feet long, stretching across a dry valley in northern Colorado. Two white pipes snake up the far side of the valley and out of sight, heading toward the Colorado River, about 30 miles away. On April 20 engineers turned a valve and began pumping water from the river and putting it behind the dam. Filling the reservoir completely will take years.
A new dam of this size is very rare in the US. By turning rivers into lakes, dams destroy riverine ecosystems, and environmentalists have long fought to stop new dams and remove existing ones. More than 100 were torn down last year alone, according to American Rivers, an advocacy group. In 2024 the last of four large dams on the Klamath River in California and Oregon were removed, completing what the group calls “the world’s largest dam removal.”