Federal judge orders Denver Water to halt Gross Dam construction; Army Corps must rewrite permits 

A federal judge late Thursday ordered Denver Water to halt construction on the massive expansion of Gross Reservoir’s dam in western Boulder County and sent three key environmental permits back to the Army Corps of Engineers for a rewrite. 

The order hands a major, if temporary, victory to environmental and neighborhood opponents fighting the half-finished, $531 million project to nearly triple the storage capacity of the reservoir on South Boulder Creek. 

Senior U.S. District Court Judge Christine Arguello put a halt to further construction nearly four months after Denver Water and the river-defending nonprofit Save the Colorado failed to negotiate a settlement that would further mitigate damage from the project. When settlement talks stalled, Save the Colorado asked for an injunction and Denver Water argued it should go forward pending more talks.

Arguello had been weighing their positions until Thursday afternoon. 

“Denver Water rolled the dice with ratepayers’ money, which was a mistake,” said Save the Colorado founder Gary Wockner, whose group has financed and led litigation against Denver Water and many other agencies seeking new dams or river diversions. “We remain open to negotiations to find a mutually agreeable path forward.” 

Wockner noted that the judge not only put a preliminary injunction on raising the concrete dam, but appeared to come down even harder on the tree-clearing and other construction work underway to raise the water pool itself. 

“Finally, the Court orders a permanent injunction prohibiting enlargement of the

Gross Reservoir, including tree removal, water diversion, and impacts to wildlife,” Arguello wrote. 

“Denver Water is reviewing the order issued this evening by the court,” said agency spokesperson Todd Hartman. “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.”

Arguello first ruled in October that bolstering Gross Reservoir capacity with a heightened dam violates the Clean Water Act and National Environmental Protection Act. The Army Corps failed to properly consider other water-producing alternatives to the bigger dam, Arguello ruled. 

The judge also said the Army Corps should have considered whether ongoing climate change and drought would leave the Colorado River and Western Slope waterways too depleted to safely allow transfer of Denver Water’s rights into a larger Gross Reservoir for Front Range water users. 

Boulder County and environmental groups have long opposed Denver Water’s expansion plan, but in the end decided they didn’t have the power to stop it before construction began in 2022. Save the Colorado later filed suit against the Army Corps for issuing the permit, and was joined by Sierra Club, The Waterkeeper Alliance and others. Along the way, they negotiated previous rounds of environmental and construction-impact mitigations for the project, including enhanced wetlands. 

Now Denver Water will have to consider more substantial mitigations or modifications in order to restart construction. The agency, the largest server of water in Colorado with 1.5 million users, says it needs more storage in Gross Reservoir to give a northern balance to its sprawling system that draws heavily on South Platte River Basin water from the southwest.

The judge’s Thursday order also calls for a hearing in April or May on “what is reasonable and necessary to make the currently existing structure safe” during a construction halt.

Denver Water argued against the injunction by saying the permit fixes could be simple. But the judge’s injunction said Denver Water, the Army Corps and the Department of Justice “have had ample opportunity to explain how the Corps could provide such justification but have failed to do so. 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 dismissed Denver Water arguments that an injunction and re-permitting would increase Gross project costs, calling the agency’s problems largely “self-inflicted” because of how it handled opposition.

“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 wrote.

Denver Water poured masses of concrete against the existing dam during 2024 in a stair-step process that thickened the base and provided structure for the higher dam wall. With an eventual target of 471 feet in height, Denver Water’s November end-of-season update said, “Workers started building the new steps in May and completed the first 269 feet by the end of November. Work on the steps will resume in spring 2025.”

When negotiations failed in December, Denver Water said the expansion was 60% complete, and that halting construction risked stressing of temporary bolting and other measures that hold back rock while the stronger and larger dam is completed.

Environmental attention turned to the fate of Gross Dam in late February after a $100 million mitigation settlement was announced in another long-running dispute between Save the Colorado and Northern Water. Northern Water wants to begin building a $2 billion complex of diversions, dams and pipelines on the Cache La Poudre and South Platte Rivers to supply growing northern communities. 

Save the Poudre sued in January 2024 to block the Northern Integrated Supply Project, saying the Poudre is often severely depleted by the time it reaches Fort Collins, among other arguments. The lawsuit targeted the federal permit issued and alleged that the Army Corps of Engineers had not adequately weighed the environmental impacts and considered less harmful ecological alternatives to the project.