The United States Army Corps of Engineers have given a water agency in Colorado’s Front Range the green light to build a large dam and reservoir to divert and store water from the Colorado River – the first such project that has been permitted in decades.

The $400 million Chimney Hollow dam and reservoir is designed to “firm” water supply to around a dozen quickly growing communities in Colorado’s Front Range communities, north of Denver. In water parlance, firming refers to making a variable water supply secure.

Today, the Northern Colorado Water Conservancy District pulls water from the Windy Gap Reservoir, on the western slope of the Continental Divide, through a tunnel and sends it to Broomfield, Greeley, Longmont and Louisville, among other towns. Instead of going straight to those communities, that water will be stored in Chimney Hollow Reservoir, which will be formed by the construction of a 300ft-tall dam. The project will divert 30,000 acre-feet from the Colorado River, and the reservoir will provide a steady supply, regardless of whether the Colorado is running low or high in a particular year or season, to meet growing demand.

A map of the proposed new reservoir at Chimney Hollow.

Brian Werner, spokesman for Northern Colorado Water Conservancy District, said the population of roughly 925,000 that the agency serves today is expected to grow up to 1.5 million by midcentury. “We’re the bullseye for growth in Colorado,” he said. “We’re close to Denver, to the Denver International Airport and we’ve got the [Rocky] Mountains in our backyard.”

But concern over the health of the riparian ecosystem in those mountains makes diverting water from the headwaters of the Colorado River controversial. Gary Wockner, director of Save the Colorado River, said he has a team of lawyers poring over the Army Corps permit right now. He said in the coming month or two the group, which has long opposed the project, is likely to file suit against the federal government over the issuance of the permit.

Without delays from legal action, the construction of the dam could start in early 2019. The project has been moving through the permitting process since 2003, according to Werner, but its germination dates back to the mid-1990s.

Wockner called adding a new dam to the Colorado River “absolutely insane” because “it is already the most dammed, drained, depleted river on the planet – with every drop drained before it reaches the Gulf of California.” He added that climate change models indicate the Colorado River will have less water in years to come, so a new dam near its headwaters will only exacerbate water scarcity in cities and states downriver, and leave less water in the rivers for fish.

Wockner asserts that agencies need to look at other methods of securing water, such as arranging leases from farmers with water rights, and that Front Range communities need to consume much less water than they currently do. He pointed to Las Vegas as an example of where strict water use regulations significantly cut per capita consumption.

Werner countered that tiered pricing for water rates does induce Front Range residents to conserve, since the more they use, the more they pay. And that communities promote reductions in non-essential water use through the use of drought-tolerant plants. As for seeking supply from agriculture, Werner said it is difficult to strike deals with farmers, and Front Range cities would rather purchase water outright.

Trout Unlimited, which advocates for healthy rivers and streams across the U.S., is also concerned with impacts that new diversions could have on the health of the river, but it has taken a different approach than other opponents.

Following years of negotiation, Trout Unlimited forged an agreement with Northern Water. Contingent on receiving all permits to build Chimney Hollow Reservoir, the agency said it will pay $2 million toward a $12–13 million project to add a channel to the Windy Gap Reservoir, which will provide a means for fish and other aquatic organisms, as well as sediment, to bypass the reservoir and continue moving downstream. This will also help regulate water temperature during low-flow summer months. (Other funding for the bypass construction is coming from state and federal sources, but Trout Unlimited still needs to raise an additional $3 million.)

Mely Whiting, a lawyer with Trout Unlimited, said 65 percent of the flows of the Colorado River headwaters are already diverted by Front Range communities. After the Windy Gap Firming Project and another planned diversion called the Moffat Collection System Project, it is expected that 80 percent of flows will be pulled from the headwaters.

Whiting said Trout Unlimited’s approach is to tell water agencies that if they want to continue diverting, they need to pay for mitigating the harms that diversion have already caused in the river.

“The bigger picture is that we’ve got to figure out how to make the limited amount of water we have available work for everybody, including the environment and including the fish,” Whiting said. “If we don’t work together on this stuff, we’re not going to survive.”