DENVER (CN) – A 10th Circuit panel on Monday heard arguments on whether the Army Corps of Engineers incorrectly applied Clean Water Act standards only to mitigating environmental damages rather than to the entirety of a project to expand a Colorado reservoir.

The 5,378-acre Chatfield State Park, located 25 miles south of Denver where the South Platte River meets Deer and Plum creeks in Littleton, Colorado, attracts 1.6 million annual visitors. The wildlife oasis is home to the infamously threatened Preble’s jumping mouse as well as 375 species of birds – 15 of which are also protected.

Chatfield’s dual roles as reservoir and nature reserve clash with a $180 million plan to expand the lake by 587 acres, which opponents say is not worth losing important habitat over.

The Audubon Society of Greater Denver sued the Army Corps of Engineers in 2014, claiming the corps failed to choose a less environmentally damaging alternative.

“The record and the state show that the most environmentally damaging plan was chosen,” said attorney Kevin Lynch with the University of Denver Sturm College of Law, representing the Audubon Society during oral arguments at the 10th Circuit on Monday.

In order to meet federal regulations, Lynch said the corps evaluated the full reallocation project under the National Environmental Policy Act (NEPA), and then misapplied the Clean Water Act to assess only alternative plans for environmental mitigation. Lynch said both NEPA and the Clean Water Act need to be applied to the full project.

U.S. Attorney Sommer Eagle confirmed the Clean Water Act was applied only to consider discharge from the mitigation efforts rather than the full project. When U.S. District Judge Philip A. Brimmer issued final judgment in favor of the corps in December 2017, he found no issue with this reading of the law.

On Monday, U.S. Circuit Judge Scott Matheson seemed to address both parties when he asked, “How much of your argument is based on your interpretation of ‘overall project purpose?’”

Matheson said the panel would comb through the law to determine whether “project” under the Clean Water Act means the full construction or can refer to parts of it.

Other project alternatives passed over by the Army Corps of Engineers included expanding other water reserves, increasing groundwater storage, using readily available offsite gravel pits directly adjacent to the park, and a “no-action alternative relying on Penley Reservoir” coupled with increased water conversation efforts.

Chief Circuit Judge Mary Beck Briscoe was not impressed by the suggestion that better water conservation would solve metropolitan Denver’s increasing demand.

“Would that have been something that would have been effective? You hope that this week people do better, but next week maybe they won’t?” Briscoe challenged.

Construction on the Chatfield Storage Reallocation Project is expected to be completed by 2020. Although construction has commenced on the western edge of the lake, activists still hope for a decision that would protect the Plum Creek Nature Area.

U.S. Circuit Judge Carlos Lucero also sat on the panel. The judges did not indicate when they would reach a decision.