The U.S. Army Corps of Engineers’ approval of Denver Water’s ongoing expansion of Gross Reservoir violates the Clean Water Act and the National Environmental Policy Act, according to a ruling Wednesday by a U.S. District Court judge.

Senior federal judge Christine Arguello did not order Denver Water to halt construction work underway in Boulder County since 2022, but said the environmental plaintiffs would be entitled to compensation for any damage that would occur to surrounding properties and forests once the dam is closed the expanded pelvis rises.

“To the extent that Denver Water disagrees, it has only itself to blame – because Denver Water chose to proceed with construction despite the obvious risk posed by a pending federal lawsuit,” the U.S. District Court decision said for Colorado.

The court accused the Army Corps of approving the construction without adequately considering all alternatives that could avoid dredging and filling “waters of the United States” like South Boulder Creek, where Gross sits. The judge said the Corps also should have more thoroughly analyzed whether climate change and reduced rainfall in the Western Slope watershed will mean that Denver Water will not have the right to release additional water into Gross Reservoir in several years.

A lawful permit analysis under federal environmental laws “requires confirmation that additional water will even be available for impoundment,” the judge wrote.

The ruling rejected one of the plaintiffs’ claims that the U.S. Fish and Wildlife Service violated the Endangered Species Act’s requirements to review the permit for possible harm to protected wildlife.

Arguello ordered Denver Water to begin discussions with plaintiffs, including Save the Colorado, the Sierra Club, WildEarth Guardians and others, about how to “remediate” the company’s violations of two major environmental laws.

Denver Water said Thursday afternoon, “As we review the judge’s decision regarding the U.S. Army Corps of Engineers’ 2017 Record of Decision, Denver Water remains focused on maintaining safety and moving the project forward.”

Because the dam includes a hydroelectric power plant that will also be constructed as part of the expansion, portions of the project are subject to regulation by the Federal Energy Regulatory Commission. Denver Water noted in its statement that FERC requires the dam to be completed by 2027.

“It is critical that we continue construction on schedule to ensure the integrity and safety of both the current project configuration and the future dam and to meet the federal FERC-required completion deadline,” Denver Water said.

The Army Corps of Engineers regional office in Omaha said late Thursday that any comments on the permit lawsuit or decisions would have to come from the U.S. Department of Justice.

The environmental coalition was quickly able to claim a major victory from the ruling.

“It’s huge. Write that in all caps,” Gary Wockner, founder of Save the Colorado, said in an interview. “It’s a stunning victory for the Colorado River, for the people of Boulder County and Grand County,” Wockner said. “Boulder County because this huge project was being built there, and in Grand County because their rivers were going to be further drained. And it is a victory for the rule of law.”

“The court’s ruling in our favor is a monumental victory that will help protect the waterways and communities of Boulder County and beyond from this irresponsible and destructive project,” said Daniel E. Estrin, general counsel and legal director for the coalition partner Waterkeeper Alliance. “The Army Corps’ consideration of reasonable alternatives to Denver Water’s preferred project and its determination of the least environmentally damaging viable alternative were fatally flawed.”

The plaintiffs were still reading the 86-page ruling and had no time to discuss whether they should seek an injunction against further construction at Gross Reservoir or what environmental protections they might seek in discussions with Denver Water, Wockner said.

Work crews in western Boulder County are pouring large amounts of concrete daily to reinforce the lower walls of the existing Gross Dam, raising the height of the 340-foot-tall dam by 131 feet. The taller dam will more than double the holding capacity of the Gross Reservoir while simultaneously flooding the surrounding forest areas.

Denver Water has worked for decades to authorize construction, arguing that new storage was needed in Boulder County to serve northern communities and balance the rest of Denver’s system, which relies heavily on supply and delivery south of the metropolitan area concentrated. Denver Water serves a total of 1.5 million customers, many in the city of Denver but many more serving water in suburbs from Wheat Ridge and Lakewood to Littleton and Centennial.

Denver Water’s statement Thursday said the gross expansion “has been analyzed and approved in various forms by no fewer than seven state and federal environmental agencies and that extensive consultations have been conducted with environmental organizations, nonprofits, the public and other stakeholders to achieve this “Identify improvement measures.” Adequately restore resources in both the West Slope and Front Range.

“Denver Water looks forward to working through the legal process to take possible remedial action and bring this important project to completion,” the statement said.

Save the Colorado and various coalition partners have successfully slowed or stopped dam and river restoration projects as part of their mission to stop further diversions of west-flowing Colorado River water to Front Range water authorities and leave more water in the rivers for wildlife. Wockner noted Thursday that they have also sued over permits for the sprawling $2 billion Northern Water project called the Northern Integrated Supply Project.

“Court precedent is important,” Wockner said, noting the potentially far-reaching impact of the Gross Reservoir ruling. Environmental lawsuits against Army Corps dam permits are often based on allegations that the Corps and the relevant water agencies did not seriously consider cheaper and more damaging water supply alternatives, including increased conservation.

The judge said the parties should file briefs on their positions if they fail to reach an agreement by Nov. 15. The judge noted that winter concrete work will end in November, resulting in a break in work during which remedial measures can be discussed.

“It is ordered that counsel for each party consult and attempt in good faith to reach agreement on remedial action with respect to the issues on which the Corps has failed to comply,” Arguello wrote.