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Gross Reservoir Dam outside Boulder, Colo., on Monday, July 1, 2019. Denver Water is planning a major expansion of the reservoir that will increase height of the Gross Reservoir Dam by 131 feet and water storage by 77,000 acre-feet. Local residents and environmental groups are fighting against the expansion due to concerns about environmental impacts and the lack of conservation efforts. (Chris Schneider/Colorado Sun)

Denver Water is cutting down too many trees on federal land at the major expansion of Gross Dam and Reservoir in Boulder County, and must conduct more environmental impact work if it wants to expand a quarry used for the project onto U.S. Forest Service property, according to a notice from the agency.

The Forest Service letter dated Tuesday faults Denver Water for “unauthorized timber removal,” and said the utility’s proposed onsite quarry expansion “would require a new and separate National Environmental Policy Act analysis.” 

Denver Water hasn’t been giving the Forest Service, whose land surrounds Gross Reservoir in western Boulder County, enough lead time to evaluate change requests on cooperative agreements, the letter said. 

“The Forest Service has requested at least a three year plan for the previously authorized activities at the reservoir expansion project site that require agency review,” according to the letter signed by Arapaho-Roosevelt forest supervisor Monte Williams. 

It is unclear if the Forest Service demands would slow down the project, which was previously embattled in litigation and disputes with environmental groups and Boulder County commissioners. Boulder County reluctantly approved a mitigation agreement with Denver Water requiring funds to limit air and noise pollution for residents and granting the county more open space. 

The primary environmental opponent of the dam expansion, which would pool more resources for Denver Water on South Boulder Creek, said the mistakes pointed out by the Forest Service call more of the big earth-moving project into question. 

“The construction plan for the Gross Dam expansion appears to have been somewhat half-baked,” said Gary Wockner of Save the Colorado, which has worked with other groups to fight local and federal permits for the dam expansion. “If Denver Water could not correctly estimate how much rock material they needed to build a massive new dam, what else have they estimated incorrectly?” 

Denver Water officials said they are reviewing the letter and would not have further comment Wednesday. 

A Forest Service spokesperson said in a statement, “This letter from Forest Supervisor Monte Williams to the Federal Energy Regulatory Commission is part of the normal back and forth correspondence when multiple agencies are involved in managing a complex project like the Gross Reservoir expansion.”

The Forest Service said Denver Water asked for permission to build a temporary road at least 1,000 feet long to handle a drilling rig, removal of boulders and 50 to 60 trees, room for nine test borings, and spreading 1,800 gallons of drilling slurry over the area. 

But the final environmental assessment from 2019 that was part of the permitting analyzed a site solely on Denver Water property, not extending into federal land, the Forest Service said. 

In addition, Forest Service law enforcement officers confirmed Denver Water’s self-reports of “unauthorized overcutting of timber” on federal land that is part of the project, the letter said. The Forest Service said it is receiving a number of “rush requests” for the project, in addition to all the other land uses it oversees. 

“The Forest Service has limited personnel and time capacity to process requests from the multitude of external parties that request use and occupancy of NFS lands and therefore would like to reiterate its need for advanced notice for proposed activities,” the letter said.

Save the Colorado asked in a public statement that the Army Corps of Engineers and the Federal Energy Regulatory Commission, which oversees dams with large hydropower elements embedded, halt Denver Water’s construction until the Forest Service questions are answered. Denver Water, serving about 1.5 million people in the city and some suburbs, brings Colorado River tributary water from the Winter Park/Fraser area underneath the Continental Divide to pool at Gross Reservoir and continue down South Boulder Creek. 


When the Boulder County commissioners approved the expansion deal in 2021, they said they “hated” the destruction that would occur and called the project “20th century, Hoover Dam thinking.” But, they added, county attorneys advised them that federal laws preempt county planning authority because the existing dam includes hydroelectric power and is therefore controlled by federal energy regulators, not local interests.

The attorneys said Boulder County would lose a federal lawsuit filed by Denver Water and that the agency would withdraw its mitigation offer if they delayed a vote. 

Denver Water had other federal approvals it needed to begin raising the dam on South Boulder Creek by 131 feet, and inundate the surrounding forest for 77,000 more acre-feet of storage, nearly tripling capacity. 

The $10 million mitigation deal was sweetened at the last minute by $2.5 million to soften construction impacts for neighbors.

The mitigation funds cover: 

  • $5 million for the construction impacts on immediate neighbors of the reservoir.
  • $5.1 million to Boulder County open space funding to acquire new land or repair and maintain trails and facilities under extra strain from visitors who can’t use Gross Reservoir spaces.
  • $1.5 million to mitigate greenhouse gas emissions from construction.
  • $1 million for South St. Vrain Creek restoration. 
  • A transfer of 70 acres of Denver Water land near Gross Reservoir to Boulder County to expand Walker Ranch Open Space.

Save the Colorado and other coalition members continue to fight the dam expansion in other venues. 

Federal courts allowed a lawsuit to go forward in U.S. District Court, claiming the Army Corps should never have approved construction permits. The lawsuit claims the approvals don’t account for how climate change will reduce water available on the Western Slope to transfer to Boulder Creek. Save the Colorado also says Denver Water’s claimed needs for a bigger water supply for the Front Range are contradicted by the agency’s recent claims about successful water conservation.

A coalition brief in the lawsuit says “Denver Water will clear-cut more than 500,000 trees” and “blast and mine 1.6 million tons of rock” for the Gross Dam project. 

Michael Booth is The Sun’s environment writer, and co-author of The Sun’s weekly climate and health newsletter The Temperature. He and John Ingold host the weekly Sun-Up podcast on The Temperature topics every Thursday. He is co-author with...