In a major ruling, a federal judge has ordered Denver Water to stop construction on the expansion of Gross Reservoir’s dam in Boulder County, citing permit deficiencies. The Army Corps of Engineers must now rewrite three key environmental permits, delivering a temporary but significant victory to environmental and neighborhood opponents.

The $531 million project, aimed at nearly tripling the reservoir’s capacity, had been the subject of ongoing legal challenges. Senior U.S. District Court Judge Christine Arguello issued the injunction late Thursday, halting construction months after negotiations between Denver Water and the nonprofit Save the Colorado failed to yield an agreement.

“Denver Water rolled the dice with ratepayers’ money, which was a mistake,” said Gary Wockner, founder of Save the Colorado, which has been leading litigation against the project. “We remain open to negotiations to find a mutually agreeable path forward.”

The ruling not only stops work on raising the dam but also blocks tree-clearing and other activities that would expand the water pool.

“Finally, the Court orders a permanent injunction prohibiting enlargement of the Gross Reservoir, including tree removal, water diversion, and impacts to wildlife,” Arguello stated.

Denver Water expressed concerns about the decision and signaled a potential appeal.

“Denver Water is reviewing the order issued this evening by the court,” spokesperson Todd Hartman said. “We have grave concerns regarding the ruling. We are prepared to appeal the decision so that we can continue to meet the water supply needs of the 1.5 million people we serve. We will have more to say after we further evaluate the order.”

The judge had previously ruled in October that the project violated the Clean Water Act and the National Environmental Protection Act. The Army Corps, she found, failed to adequately assess alternative water supply solutions and did not fully consider the long-term impacts of climate change and drought on the Colorado River and Western Slope waterways.

Boulder County and environmental groups had opposed the project for years but ultimately lacked the authority to prevent construction when it began in 2022. However, Save the Colorado, along with the Sierra Club and Waterkeeper Alliance, filed suit against the Army Corps, challenging the legality of the permits.

Now, with the judge’s order, Denver Water will have to reassess its approach to mitigate environmental concerns before construction can continue.

Additionally, Arguello has scheduled a hearing in the coming weeks to determine “what is reasonable and necessary to make the currently existing structure safe” while work is paused.

Denver Water had argued that fixing the permit issues would be straightforward, but the judge disagreed, stating that the deficiencies in the Army Corps’ justification were too severe for a simple revision.

“The deficiencies found by the Court are extensive and serious. They are not of the type that can simply be supported by better reasoning,” Arguello wrote.

The ruling also dismissed Denver Water’s claims that an injunction would lead to significant cost increases, saying the agency had created its own problems.

“This Court will not reward Denver Water for starting construction on the Project despite being aware of the seriousness of the environmental law challenges,” Arguello stated.

With construction about 60% complete as of December, the ruling raises questions about the future of the project. Environmental groups have pointed to this decision as a major step toward stronger oversight of water infrastructure projects across Colorado.

Originally reported by Micheal Booth in The Colorado Sun.