The dispute over whether Denver Water can ever fill its expanded Gross Reservoir in Boulder County will head back to U.S. appellate court in July, after settlement talks between the frustrated water agency and environmental opponents failed to reach a mitigation deal.
Denver Water is asking the 10th Circuit Court of Appeals to nullify rulings by a U.S. District Court judge against the $600 million Gross Dam project.
Federal judge Christine Arguello first ruled in October 2024 that the U.S. Army Corps of Engineers unlawfully issued permits the expansion that had been planned by Denver Water for more than two decades.
Arguello said the Corps had not properly considered alternatives that would have been less damaging to wetlands. She also said the Corps should have more thoroughly analyzed whether climate change and lower precipitation in the Western Slope catchment area mean that Denver Water won’t have the right in some years to transfer water from the Colorado River Basin into the expanded reservoir.
After court-ordered negotiations between Denver Water and opponents Save the Colorado couldn’t reach a deal on new environmental mitigation for the bigger dam, Arguello issued an injunction in April 2025. Denver Water successfully protested that for safety reasons it needed to keep building and reinforcing the higher dam. But Arguello left in place an injunction against Denver Water clearing more trees to create a bigger basin or ever filling the additional capacity in the larger reservoir.
There is still no mitigation agreement that would settle the case, said Save the Colorado’s Gary Wockner, and so now, Court of Appeals oral arguments are set for July 31 in Santa Fe.
While Denver Water’s appeal briefs say the injunction destroys decades of careful work to balance the northern and southern ends of its system serving 1.5 million metro residents, Save the Colorado says the ongoing drought bolsters its case against the expansion.
“Given the extreme drought and political chaos on the Colorado River, we believe district court Judge Arguello’s ruling is now more important than ever,” Wockner said. “She cited how the new depletion would further drain the river and how climate change was intensifying and reducing flows. The Gross Dam project was ill-conceived from the get-go, and we believe we have a very strong case in the court of appeals.”
Denver Water wouldn’t comment about the pending arguments, but does vent its frustrations in long briefs supporting the injunction appeal.
The district judge should never have dismissed the key permits, because the filled wetlands at issue at the base of the dam amounted to only an acre of ground, Denver Water says. The Corps properly concluded that Denver Water’s extensive mitigation efforts there and in other parts of South Boulder Creek would more than offset the damage and be a benefit for the watershed, the water agency says.
No dam permitting case has ever required the Corps to consider climate change in deciding whether a dam can be authorized and filled, Denver Water adds. The Corps already concluded that it couldn’t be determined whether climate change would provide more or less water over time for Denver, the city brief says.
Besides, Denver Water adds, demanding any more relief through an injunction is moot because the dam expansion is nearly finished.
“Construction on the project has progressed such that there are no feasible alternatives, and a new alternatives analysis would not provide meaningful relief,” Denver Water’s attorneys write.The plaintiffs who first brought the case against Gross expansion include Save the Colorado, the Sierra Club, WildEarth Guardians and others.