A federal judge on Tuesday ordered Denver Water to share information with the environmental groups who successfully challenged a reservoir expansion project in Boulder County, as both sides prepare for a hearing to determine how much additional construction is necessary to stabilize the structure.

Earlier this month, U.S. District Court Senior Judge Christine M. Arguello found that, as a result of federal law violations, the expansion of Gross Reservoir and Dam should cease permanently and any further construction on the ongoing project would stop temporarily. The pause on construction, Arguello wrote, would give her time to hear from engineers and determine what work would need to occur to make the dam safe.

Days later, Arguello allowed for necessary construction to temporarily resume, and the U.S. Court of Appeals for the 10th Circuit has since extended that window while it reviews Arguello’s order. However, last Wednesday, the groups that challenged the project’s legality asked Arguello to intervene on another issue related to the upcoming hearing about how much stabilizing work is warranted.

“Despite Petitioners’ repeated, good-faith efforts to obtain information bearing on the subject matter of the hearing, Denver Water has refused to provide all parties with pertinent materials and information that is essential to prepare,” wrote attorneys for the environmental groups.

In response to the groups’ questions about risk management plans, spillway capacity and failure modes — plus a request for project documents — Denver Water told the petitioners that disclosure “poses Dam security risks.”

“The fact remains that Denver Water is the only party that currently has available to it extensive documentation that bears directly on the specific safety issues that this Court ordered all parties to address at the hearing,” the environmental groups added in their court filing.

Denver Water responded that an exchange of documents and information ahead of the hearing was “unnecessary.”

“Plaintiffs appear to view the hearing as an opportunity for them to provide an alternative design or plan for Dam construction. That is not what the Court’s Order contemplated,” wrote the utility’s lawyers.

Arguello disagreed with Denver Water.

The information being requested “is neither substantial nor burdensome,” she wrote in an April 22 order. It is “precisely the type of information that this Court believes needs to be presented” at the hearing and that the environmental groups need for their own preparation.

With the exception of the petitioners’ request for documentation of “general arrangements,” Arguello directed Denver Water to share blueprints, construction diagrams and safety materials, and to answer the petitioners’ specific questions. She further directed that the documents remain confidential.

Arguello previously found the U.S. Army Corps of Engineers wrongfully approved a discharge permit for the project after it improperly 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 actions violated both the Clean Water Act and the National Environmental Policy Act.

Denver Water said it has already incurred $314 million in construction costs on the project, and warned of safety risks if the deconstructed-and-reconstructed dam is left in its current state.

Arguello will hear from experts on May 6 about what work is necessary to stabilize the dam as it exists now.

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