Colorado’s largest water provider must stop construction on a $531 million dam expansion already underway in Boulder County after a federal judge found that assessments of how the project would impact the environment were flawed. U.S. District Court Judge Christine Arguello in an order late Thursd ay blocked Denver Water from enlarging Gross Reservoir east of Nederland until major federal environmental permitting processes are redone. The judge found that allowing the reservoir expansion to continue without redoing the permits would cause irreparable environmental damage that cannot be compensated for by monetary payments. That harm would outweigh any financial costs Denver Water would incur from halting construction, she wrote. “Environmental injury is often the very definition of irreparable harm — often permanent or at least of long duration,” Arguello wrote. “All parties agree that there will be environmental harm resulting from completion of the Moffat Collection System Project, including the destruction of 500,000 trees, water diversion from several creeks, and impacts to wildlife by the sudden loss of land.” She issued a preliminary injunction ordering Denver Water to halt construction on the dam until a further hearing when engineers can explain how much further construction is needed to make the partially built dam safe and structurally sound. Denver Water planned to raise the height of the dam by 131 feet, allowing the utility to store more water. She will then issue a permanent injunction on how much more construction will be allowed. The order is a huge victory for environmental groups that for years have opposed the controversial project. A coalition of environmental groups first filed suit in 2018 to stop the expansion of the reservoir, which they say would harm the health of the Colorado River system — where the reservoir’s water is sourced. “Denver Water rolled the dice with ratepayers’ money, which was a mistake,” Gary Wockner of Save the Colorado, the lawsuit’s lead plaintiff, said in a statement. “We remain open to negotiations to find a mutually agreeable path forward.” Denver Water officials are reviewing the order, spokesman Todd Hartman said in an emailed statement. “We have grave concerns regarding the ruling,” he said. “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.” Denver Water — which provides water to 1.5 million people in metro Denver — began the permitting process for the project in 2002 and began construction in 2022. When completed, the expanded dam would triple the reservoir’s capacity from 42,000 acre-feet to 120,000 acre-feet — enough water to serve approximately 156,000 additional households. Already, workers have partially deconstructed the existing dam, dumped fill material and poured concrete for the expansion. Denver Water temporarily halted construction for the winter season in November but planned to resume in the spring. Arguello in October found that the U.S. Army Corps of Engineers violated the Clean Water Act and the National Environmental Policy Act when approving permits for the construction of the dam. Arguello said the Corps failed to sufficiently consider other options that could be less environmentally damaging than dam expansion and, in her ruling Thursday, she said the Corps’ deficiencies were “extensive and serious.” After the October ruling, Arguello asked defendants and plaintiffs to try to find a resolution outside of court, but they failed to do so. The judge also criticized the Corps for failing to quantify how climate change will impact precipitation and, therefore, how that change could impact the need for more water storage. It’s unclear whether it’s practical and reasonable to build a reservoir to store Colorado River water that may not exist or be available, she previously said. Related Articles Feds’ approval of Gross Reservoir dam expansion violated environmental law, judge rules Environmental groups renew legal challenge of massive Denver Water reservoir expansion Denver Water was warned about unanswered questions on Gross Reservoir permit request Denver Water argued that delaying or halting construction would be expensive and dangerous, but Arguello was not persuaded. She also criticized Denver Water for starting construction on the dam while the project’s legality was being challenged. The water provider cannot then argue the project should continue simply because it is already underway, she said. “The Court does not find Respondents’ and Intervenor’s arguments about alleged cost, delay and the alleged urgent need to address droughts persuasive because these alleged hardships are largely self-inflicted,” she wrote. Denver Water has said the expanded reservoir would anchor Denver Water’s northern supply system and help protect the utility’s ability to deliver water if its much larger southern supply system is impacted by fire, mudslides or drought. Get more Colorado news by signing up for our Mile High Roundup email newsletter. ...read more read less