Workers for Denver Water started the latest expansion phase of the controversial Gross Reservoir expansion project in southwest Boulder County on Friday.

The multi-year project has sparked community opposition and multiple lawsuits. But Denver’s water utility says it needs the additional reservoir capacity to shore up its water resources and make the city less vulnerable to potential future droughts. Denver Water serves about 1.5 million people who live in Denver and most of the surrounding suburbs, according to its website

During this stage of the project, crews will raise the height of the dam from 340 feet to 471 feet, a 131-foot increase that’s expected to roughly triple the capacity of the reservoir from 42,000 acre-feet to 119,000 acre-feet. One acre-foot of water is an amount of water that covers one acre of land, one foot deep. An acre-foot of water generally can serve two families of four or five people for one year, according to various water utility calculations.

The dam, originally built in the 1950s, is set to become the tallest in the state with the expansion project. And the newly-expanded reservoir will become Denver Water’s second largest reservoir after the Dillon Reservoir in Summit County.Jeff Martin, a project manager for the Gross Reservoir expansion with Denver Water, said water utility officials are anxious to start this phase of work.

“We’re really excited about this next phase of the project, which will, in essence, change the structure of Gross Dam … and then ultimately do what we really need to do, which is put more water behind it, so we can have water security,” Martin told the Daily Camera.

Martin said the water utility will raise the dam using roller-compacted concrete, made from a blend of cement and fly ash, which occurs naturally but is also a by-product of coal burning.

Work on the Gross Reservoir expansion first started in 2022. Since then, crews have worked to excavate the abutments, or the mountain sides bookending the edges of the dam. Additionally, crews have roughened the surface of the dam in preparation for the work ahead, and they’ve also pumped a mixture of water and cement into the ground to create a less permeable area down below.

According to a project website, 260,000 cubic yards of rock were excavated from the site and 27,000 cubic yards of concrete were placed to prepare for the dam expansion. And it will take about 800,000 cubic yards of concrete to raise the dam.

“All of that was the preparation. Two years of extremely hard, difficult work — working on mountain sides, canyon sides, 500 vertical feet on two sides of the canyon — to get us to this phase … of raising Gross Dam,” said Martin.

But the origins of this project go farther back. According to Martin, Denver Water started the process of getting permits for the project two decades ago, in the wake of a severe drought and the devastating Hayman Fire in 2002.

Some in the community have opposed the project due to its potential environmental impacts. For one, Denver Water will need to remove 400 acres of trees. But environmental advocates also worry about the expanded reservoir diverting water from other waterways.

“(The Gross Reservoir Dam) would be the tallest dam in the history of Colorado, and it would be Colorado River water that is diverted through a tunnel in the Continental Divide to fill that reservoir,” Gary Wockner, executive director of Save the Colorado, told the Daily Camera. The Save the Colorado nonprofit group works on projects focused on trying to save the Colorado River.

“… Our beef against the project, and why we came out against it, was two things. One, it further drains and depletes the Colorado River and its headwaters in Grand County. And second, it’s a massive dam on South Boulder Creek, the tallest dam in the history of the state. And dams, of course, kill rivers, as they’re designed to do.”

Wockner also said he is skeptical the reservoir expansion is necessary, and he suggested that Denver Water should impose more stringent water conservation plans to make do with the water it already has.

Save the Colorado is one of several environmental groups that filed a federal lawsuit over the project in late 2018 against representatives of the U.S. Army Corps of Engineers, the U.S. Fish and Wildlife Service and the Secretary of the Interior.

The suit aims to invalidate the permits for the Gross Reservoir expansion and bring the project to a halt. The Denver Post reported the suit was dismissed in 2021, but the groups appealed the decision several months later, and the case was reinstated.

A decision on the suit could be months away, according to Wockner. But for now, Martin said, Denver Water plans to move ahead with the work that it has received permits to do. If the work continues as planned, the project is expected to finish in 2027.

Martin also said the possible impacts to the Colorado River had been extensively researched and considered. Ultimately, he said, the U.S. Army Corps of Engineers determined that expanding Gross Reservoir would be the least environmentally damaging of the available options for expanding Denver’s northern water system.

“I’ve been working on this for 13 years now, and I think it’s important that Denver Water has really endeavored, throughout this project, to do this the right way, to collaborate with different stakeholders,” Martin said.

In exchange for the 400 acres of forested land being lost, Denver Water has given 500 acres of its property upstream to the U.S. Forest Service for preservation, according to Martin.

Boulder County officials also previously opposed the project, questioning whether it was needed, given that Denver Water’s customers were not facing a water shortage. After county commissioners voted unanimously to require the project to go through the county’s review and permitting process, Denver Water sued the county, claiming the commissioners took too long to consider the expansion request, the Denver Post reported.

In late 2021, the county agreed to settle the lawsuit, forfeiting its right to review the reservoir expansion in return for $12.5 million in mitigation to help offset impacts on local residents and the environment.

The county is currently conducting a survey to learn how nearby residents want to see $5.1 million of that settlement money used.