A federal judge concluded on Wednesday that the government violated multiple laws when it permitted Denver Water to undertake a massive expansion of its Boulder County reservoir, throwing the project’s future into question.

In a detailed order, U.S. District Court Senior Judge Christine M. Arguello acknowledged there was evidence of Denver Water’s need for a greater water supply. However, she concluded the U.S. Army Corps of Engineers wrongfully eliminated alternatives that avoided disturbing wetlands. Further, the Corps improperly combined two distinct goals that had the effect of steering the project toward reservoir expansion.

The Corps’ “downright refusal to even consider alternatives incapable of threading the needle between Denver Water’s wish list items is not just ‘unwise’ agency action — it is objectively ‘uninformed,'” Arguello wrote in a sternly worded Oct. 16 order.

At the same time she found the project rested on an unlawful foundation, Arguello held off on a decision about what should happen next. She noted Denver Water has already begun construction and the dam’s integrity could be compromised if all work were to cease. Therefore, Arguello directed the parties to consult with each other and, if they cannot agree on a path forward, submit further arguments to her for a decision.

To the extent Denver Water disputed that legal proceedings were warranted given the work already completed, Arguello shot back that the utility had only itself to blame.

“Denver Water chose to proceed with construction despite the obvious risk posed by pending federal litigation,” she wrote.

“This is a stunning victory for the Colorado River, the people of Boulder County, and the rule of law,” said Gary Wockner of Save the Colorado, one of the plaintiffs challenging the Corps’ permit. “We look forward to engaging with Denver Water to reach a good faith agreement about how to remedy the Corps non-compliance.”

In a statement, Denver Water said it was still reviewing Arguello’s decision, but defended the 20-year effort to seek more water for its system.

“Throughout the permitting process, Denver Water has been driven by a singular value: the need to do this expansion the right way, by involving the community; upholding the highest environmental standards; providing a sustainable, high-quality water supply to our customers; and protecting and managing the water and landscapes that define Colorado,” the utility said.

Six environmental groups challenged Denver Water’s plan to store more water for the 1.5 million people it serves, equating to 25% of the state’s population. The selected alternative was to raise Gross Reservoir and Dam by 131 feet and enable Denver Water to capture 18,000 more acre-feet of water per year — with a single acre-foot amounting to 325,851 gallons.

Gross Reservoir and Dam, close to Boulder, is linked to Denver Water’s North System, which is separate from and supplies less water than the South System. Denver Water argued in court documents that in a single dry year, “the North System risks running out of water.” It also noted a certain amount of water must be piped through the North System’s treatment plant to maintain a “minimum idle rate.”

Under the Clean Water Act, Denver Water needed a permit from the Corps to discharge fill material into wetlands during construction. The Corps issued its decision in 2017 and concluded the enlargement project was the “environmentally preferable alternative.”

However, Arguello agreed numerous problems plagued the decision-making, violating both the Clean Water Act and the National Environmental Policy Act.

First, the U.S. Environmental Protection Agency had warned the Corps against combining multiple purposes into a single project, which included more water to serve customers and having water placed specifically in the North System.

Arguello noted the Corps, without elaboration, concluded the purposes were “interconnected in the water supply issues that Denver Water is facing.” As a result, 19 potential alternatives were eliminated without proper analysis. Arguello also did not find a clear answer as to why the purposes had to be fulfilled through a single project like the reservoir expansion.

Arguello was further concerned the Corps had projected climate change might affect the water storage in some way, but “expressly declined to attempt to quantify the impacts of climate change — or even provide an educated guess.”

Moreover, while the evidence supported the idea that Denver Water needed some additional supplies, a consultant’s report from 2012 relied on data that was more than a decade old — stretching back to 1971 — when more recent numbers were available.

Although it did not play prominently in her decision, Arguello parenthetically noted the interstate dependence on the Colorado River and the impacts of water scarcity across the West more broadly. She suggested the Corps should have dwelled to a greater extent on that regional concern.

“The administrative record makes abundantly clear that a plethora of stakeholders in the Colorado River and Front Range’s water resources are deeply concerned about any proposal that tampers with the region’s already precarious water management system and uncertain hydrological future. And rightfully so,” she wrote.

The Army Corps of Engineers did not respond to an email seeking comment on Arguello’s findings.

The case is Save the Colorado et al. v. Semonite et al.