Denver Water will be allowed to continue construction to raise the height of Gross Dam but will not be permitted to fill the expanded reservoir once the dam is complete, a federal judge ruled late Thursday.
Permanently halting construction of the controversial dam would create too much risk that the incomplete structure would fail and threaten habitat and lives downstream, U.S. District Judge Christine Arguello wrote in her decision.
“Although toleration of the potential of the risk of a catastrophic dam failure and flooding incident may have been necessary due to construction of a new dam, this Court will not gamble with human life, no matter the odds,” she wrote.
The ruling is the latest twist in years-long efforts by environmental groups to stop the expansion of Gross Reservoir. Denver Water plans to triple the size of the reservoir outside Nederland in Boulder County to increase its water storage and create a more resilient water delivery system.
The utility, which serves 1.5 million people in metro Denver, planned to raise the dam by 131 feet to an anticipated height of 417 feet and add capacity for enough water to serve approximately 156,000 additional households. Construction on the project began in 2022 and, by early May, the new dam was about 60% complete.
But environmental groups oppose the project because it requires the clear-cutting of half a million trees and will cause the utility to draw more water from the already-strained Colorado River system. Federal regulators failed to properly analyze the environmental impacts of the project or analyze other, less-harmful options for Denver Water to meet its water supply needs, the project’s opponents argued.
Arguello agreed with some of the environmental groups’ arguments and on April 3 ordered Denver Water to stop construction on the $531 million project because the U.S. Army Corps of Engineers violated federal environmental law in its permitting of the project. A few days later, she temporarily allowed construction to continue while she further considered the case.
Arguello’s decision Thursday follows a 10-hour hearing earlier this month during which experts from Denver Water and the environmental groups fighting the project explained to the judge the implications of not completing the dam.
Experts for Denver Water — the project’s manager, the engineer who designed the dam, and a federal official overseeing the project — testified that any lengthy pause in construction would increase the risk that a catastrophic weather or seismic event would overcome the incomplete structure.
Michael Rogers, the engineer who designed the dam, testified that it would take at least 18 months to design a new dam should Arguello force them to do so.
The environmental groups’ expert, Stephen Rigbey, told the court that there was little risk of catastrophic failure if the dam were left in its current state while new plans were created.
But Arguello found the environmental groups failed to prove that they would be irreparably harmed if the dam were completed. While Denver Water “regrettably” harmed the environment when it tore down the original dam, Arguello wrote that she did not find evidence that continuing construction would cause further environmental damage.
“In fact, the opposite is true,” she wrote. “There is a risk of environmental injury and loss of human life if dam construction is halted for another two years while Denver Water redesigns the structure of the dam and gets that redesign approved by (the Federal Energy Regulatory Commission).”
Her ruling finding the permitting process flawed still stands, however, and so the utility is banned from filling the reservoir to its new capacity once the dam is complete.
Gary Wockner, executive director of lead plaintiff Save the Colorado, said in a statement Friday that he respects the judge’s concern about public safety.
Denver Water’s Gross Reservoir expansion project in Boulder County on April 9, 2025. (Photo by RJ Sangosti/The Denver Post)Related Articles
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Denver Water in April appealed Arguello’s finding that the permitting process violated federal law, and the utility’s leaders on Friday said they will continue to pursue that case.
“Completing construction to raise Gross Dam on the current and mandated timeline by the Federal Energy Regulatory Commission is the safest path forward, as reflected by the judge’s order,” the utility said in a statement.
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