A federal judge concluded Wednesday that the government violated several laws when it allowed Denver Water to undertake a massive expansion of its Boulder County reservoir, putting the project’s future in question.

In a detailed order, U.S. District Court Chief Judge Christine M. Arguello acknowledged that there is evidence that Denver Water needs a larger water supply. However, it concluded that the U.S. Army Corps of Engineers had wrongly eliminated alternatives that prevented wetland disturbance. Additionally, the Corps incorrectly combined two distinct objectives, resulting in the project being steered toward reservoir expansion.

The Corps’ “complete refusal to even consider alternatives that fail to thread the needle between Denver Water’s wish list items is not just an ‘ill-advised’ action by the agency – it is objectively ‘uninformed'” Arguello wrote in a strongly worded order dated Oct. 16.

While she determined that the project was based on an unlawful basis, Arguello held back from making a decision about what should happen next. She noted that Denver Water has already begun construction and the integrity of the dam could be at risk if all work were stopped. Therefore, Arguello instructed the parties to consult each other and to present to her further arguments for a decision if they could not agree on a way forward.

To the extent that Denver Water questioned whether a lawsuit was warranted given the work that had already been completed, Arguello responded that the utility had only itself to blame.

“Denver Water has decided to proceed with construction despite the obvious risk posed by a pending federal lawsuit,” 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’ approval. “We look forward to working with Denver Water to reach a good faith agreement on how to resolve the Corps’ noncompliance.”

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

“Throughout the permitting process, Denver Water was guided by a unique value: the need to do this expansion the right way by engaging the community; compliance with the highest environmental standards; providing our customers with a sustainable, high-quality water supply; and protecting and managing the water and landscapes that define Colorado,” the utility said.

Six environmental groups questioned Denver Water’s plan to store more water for the 1.5 million people it serves, representing 25% of the state’s population. The chosen alternative was to increase the gross reservoir and dam size by 131 feet, allowing Denver Water to capture 18,000 acre-feet of water annually – with a single acre-foot accounting for 325,851 gallons.

The Gross Reservoir and Dam near Boulder is connected to Denver Water’s north system, which is separate from the south system and supplies less water. Denver Water argued in court documents that in a single dry year, “the northern system is at risk of running out of water.” It also noted that a certain amount of water must be passed through the Northern System’s wastewater treatment plant to maintain a “minimum idle rate.”

Under the Clean Water Act, Denver Water required a permit from the Corps to discharge fill into wetlands during construction. The Corps issued its decision in 2017, concluding that the expansion project was the “more environmentally friendly alternative.”

However, Arguello agreed that there were numerous problems with the decision-making that violated 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 providing water specifically to the Northern System.

Arguello noted that, without elaborating, the Corps concluded that the purposes were “related to the water supply issues facing Denver Water.” As a result, 19 potential alternatives were eliminated without proper analysis. Arguello also did not find a clear answer to the question of why the purposes had to be fulfilled by a single project such as the expansion of the reservoir.

Arguello also expressed concern that the company had predicted that climate change could impact water storage in any way, but “explicitly declined to attempt to quantify the impacts of climate change – or even make an educated guess.”

Additionally, while the evidence supported the notion that Denver Water needed some additional supplies, a consultant’s 2012 report relied on data that was more than a decade old – dating back to 1971 – when more recent numbers were available .

Although it didn’t play a major role in her decision, Arguello parenthetically noted the interstate dependence on the Colorado River and the impact of water shortages throughout the West. She said the Corps should have addressed this regional issue more.

“It is clear from administrative records that a variety of Colorado River and Front Range water resources stakeholders are deeply concerned about any proposal that manipulates the region’s already precarious water management system and uncertain hydrological future. And rightly so,” she wrote.

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

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