Corps of Engineers Erred in Denver's Gross Reservoir Expansion Permit, Judge Finds

Crews roughen the surface of Gross Dam in Colorado to improve its adherence to new roller-compacted concrete as part of a project to raise it 131 ft and expand the Gross Reservoir.
*Photo courtesy Denver Water*

The future of Denver Water’s $531-million project to raise the height of Gross Dam by 131 ft may be in question after a federal judge in Colorado found the U.S. Army Corps of Engineers violated the Clean Water Act and National Environmental Policy Act when it issued a dredge-and-fill permit for the work. But the judge deferred on a final ruling and is allowing construction to continue ahead of a planned pause for the winter.

Denver Water is raising the dam from 340 ft to 471 ft to nearly triple the capacity of Gross Reservoir from 42,000 acre-ft of water to 119,000 acre-ft. Officials of the city-owned utility say the project is needed to improve the system’s ability to meet growing demand and withstand drought, as well as to address an imbalance between the water utiliry's south system, which delivers 80% of its supply, and its north system, which includes the reservoir.

The expanded structure would affect 2.2 acres of wetlands and 3.5 acres of other waters of the United States. Following Corps permitting in 2017, environmental groups led by the nonprofit Save the Colorado filed a lawsuit in 2018, alleging the agency had rejected alternatives without “clearly” showing they were not available options as required by U.S. Environmental Protection Agency guidelines, and that it failed to prove the selected option is the least environmentally damaging practical alternative.

U.S. District Court Judge Christine Arguello in Denver wrote in an Oct. 16 order that she agreed with the groups, noting that EPA had even advised the Corps in a 2003 comment on its notice of intent to prepare an environmental impact statement that the scope of the project did not need to resolve both the need for more water and the imbalance between the north and south systems simultaneously, because that could lead to the Corps ignoring alternatives that could resolve them independently.

The order came after Arguello had previously dismissed the case in 2021 over lack of jurisdiction related to a Federal Energy Regulatory Commission permit, but an appeals court later reversed the decision.

“This is a stunning victory for the Colorado River, the people of Boulder County and the rule of law,” said Gary Wockner of Save the Colorado in a statement following Arguello’s latest order.

However, she did not order a halt in construction, noting that interrupting concrete placement could compromise the dam’s structural integrity and that the reservoir has not yet impounded additional water and the shoreline remains unchanged for now. Work is expected to pause in November until spring. The judge ordered the parties to try to reach an agreement to remedy the issues ahead of a briefing in mid-November.

Denver Water said in a statement that it is reviewing the judge’s decision, and it “remains focused on maintaining the safety and forward progress of the project.” The utility emphasized the importance of the project to help secure water supply for 1.5 million people against climate change, and added that it has a legally mandated federal deadline to complete the project in 2027.

“It’s critical we continue with construction on schedule to ensure the integrity and safety of both the current project configuration and future dam and to meet the federally required completion deadline from [the commission]” Denver Water said.

Representatives of the Corps Omaha District, which issued the permit, did not immediately respond to inquiries.

Gross Dam was originally built in the 1950s. Work on the expansion began in 2022, led by a Kiewit-Barnard contracting joint venture. As planned, the enlarged dam would be the tallest in Colorado.

James Leggate is an government news correspondent at ENR. He has reported on a variety of issues for more than 10 years and his work has contributed to several regional Associated Press Media Editors and Murrow award wins.