A dam engineer who designed a major expansion of Gross Reservoir Dam in Boulder County told a federal judge Tuesday that the raising of the dam, facing a potential halt due to an April federal court ruling, needs to proceed to protect public safety.
Mike Rogers, the civil engineer who designed the $531 million expansion of the dam, said bad weather could create flood conditions that would lead to a catastrophic failure similar to what occurred with the Oroville Dam failure in California in 2017.
But Stephen Rigbey, a Canadian dam safety expert testifying for Save The Colorado, said any issues with putting the construction project on hold, even in its partially-complete state, could be addressed, and that the risk of a catastrophic failure was “negligible.”
Visitors approach the base of the dam during a site visit April 9 at Gross Reservoir in Boulder County. (Alyte Katilius, Special to The Colorado Sun)Rogers’ and Rigbey’s testimony Tuesday came during a federal hearing in Denver, after which U.S. District Court Judge Christine Arguello will determine whether to allow construction to move forward on the Denver Water project or whether the construction will be paused until new federal reviews she has ordered are completed and legal questions are answered.
But at the end of Tuesday’s hearing, Arguello said the parties to the case had not provided enough information for her to make a decision and ordered them to submit more data later this month.
The massive construction project has raised fierce opposition in Boulder County and prompted several legal challenges from Save The Colorado, a group that advocates on behalf of rivers. Though its early lawsuits failed, in 2022 the river defenders won an appeal that put the legal battle back in play. Despite months of settlement talks, no agreement was reached.
Boulder County Commissioner Ashley Stolzmann was unmoved by Rogers’ testimony, saying she hopes the judge halts the work to prevent further environmental damage in Boulder County and to protect the Fraser River, a tributary to the Upper Colorado River. The Fraser has served as the source of water for Gross Reservoir since the 1950s, when it was built.
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10:02 AM MDT on May 6, 20253:05 PM MDT on May 6, 2025“It’s incredibly disappointing that Denver has chosen to move forward,” Stolzmann said. “With climate change, it really is a time for different entities to work together to repair the climate. I want to see Denver seek alternative solutions.”
Denver Water first moved to raise Gross Dam more than 20 years ago when the water provider began designing the expansion and seeking the necessary federal and state permits. Denver Water has said raising the dam and expanding the reservoir is necessary to ensure it has enough water throughout its delivery system and to help with future water supplies as climate change continues to reduce stream flows.
After years of engineering, environmental studies and federal and state analyses, Denver received a permit from the U.S. Army Corps of Engineers and construction began in 2022. It has involved taking apart a portion of the original dam and raising its height by 131 feet to nearly triple the reservoir’s storage capacity to 119,000 acre-feet from 42,000 acre-feet.
The case took center stage again April 3, when Judge Arguello put a temporary halt to construction of the higher dam, at Save The Colorado’s request.
In that high-profile ruling, Arguello said, in part, that 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.
At the same time, she ordered a permanent injunction prohibiting enlargement of the reservoir, including tree removal and water diversion, and impacts to wildlife.
Almost immediately, Denver Water filed for temporary relief from the order, saying, in part, that it would be unsafe to stop work as the incomplete concrete walls towered above Gross Reservoir.
Arguello granted that request, too, allowing Denver to continue work on the dam considered necessary for safety.
Denver Water has also filed an appeal with the U.S. 10th Circuit Court of appeals, seeking to permanently protect its right to continue building the dam. The appeals court is expected to wait for the lower court to rule, before considering Denver Water’s request.
Colorado Sun reporter Michael Booth contributed to this report.
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