Federal regulators violated environmental laws when they improperly granted permits to Denver Water to expand Gross Reservoir, a judge ruled this week — although the impact of that finding on the ongoing project remains unclear.

The U.S. Army Corps of Engineers violated the Clean Water Act and the National Environmental Policy Act when it approved permits to build the dam, U.S. District Court Judge Christine Arguello found in the ruling issued Wednesday. The federal agency failed to adequately consider other options that could be less environmentally damaging than expanding the dam, Arguello wrote in her order.

Arguello did not order Denver Water to stop construction of the dam, in part because the utility already plans to halt construction in November for the winter season. An abrupt stop to the project could also compromise the integrity of the dam, she wrote.

The defendants and plaintiffs will now work to remedy the wrongful approvals. Each side must submit briefs proposing resolutions to Arguello by Nov. 15.

In a statement, Denver Water said it still hopes to “move the project closer to completion.”

The order was the latest decision in a protracted six-year legal battle over the controversial dam project. A coalition of environmental groups first filed a lawsuit in 2018 to stop the expansion of the reservoir because they said it would harm the health of the Colorado River system — where the reservoir’s water originates.

Denver Water began the permitting process for the project in 2002 and began construction on the $531 million dam project in 2022. Workers have already partially deconstructed the existing dam to prepare it for expansion, dumping fill material and pouring concrete for the expansion.

Once completed, the expanded dam will triple the reservoir’s capacity from 42,000 acre-feet to 120,000 acre-feet. One acre of water can supply two Colorado families for a year, allowing the expanded reservoir to store water for approximately 156,000 additional households.

Denver Water says the expanded reservoir will anchor Denver Water’s northern delivery system and help protect the utility’s ability to deliver water if its much larger southern delivery system is impacted by fire, mudslides or drought. The utility provides water to 1.5 million people in the Denver metropolitan area – about a quarter of the state’s population.

Denver Water argued in its filing that the issues raised were moot because construction had already begun and one of the permits in question had already been used.

However, Arguello rejected this argument because the reservoir had not yet been expanded and the 400 hectares of land and 500,000 trees it would drown still remained above water.

“To the extent that Denver Water disagrees, it has only itself to blame – because Denver Water has chosen to proceed with construction despite the obvious risk of pending federal litigation that could result in the revocation of permits for construction,” Arguello wrote and then referred to legal precedent: “Denver Water cannot ‘evade judicial review’ or ‘defeat a judgment’ through its own ‘questionable conduct.'”

One of the Army Corps of Engineers’ mistakes was the lack of analysis of the potential impacts of climate change on the project. As climate change reduces the amount of water available in the Colorado River system, Arguello asked: Is it practical and sensible to build a reservoir to store water that doesn’t exist?

“While the Corps acknowledged that future climate conditions could negate the value of Gross Dam as a water storage solution, it specifically declined to attempt to quantify the impacts of climate change—or even to make an educated guess for discussion purposes,” Arguello wrote.

The lack of analysis shows that the USACE did not fully analyze the practicality of the dam project, as required by law, she wrote.