The refuge is about 10 miles from downtown Denver. Oborseth via Wikimedia
Bison make for a more diverse prairie. Eric Peterson
Rocky Mountain Arsenal plants manufactured chemical weapons and pesticides for 40 years. National Park Service
The bison's range on the refuge is slated to expand to 12,000 acres. Ryan Moehring
A recently reintroduced black-footed ferret peers out from a burrow. Ryan Moehring
Burrowing owls live in abandoned prairie dog burrows. Environmental Protection Agency
The lakes provide habitat for snapping turtles, northern pike and other wildlife. Eric Peterson
Bald eagles roosting led to the Rocky Mountain Arsenal's designation as a wildlife refuge. Environmental Protection Agency
The Rocky Mountain Arsenal National Wildlife Refuge is a haven for more than 300 species of animals. It's an emerging model for restoration of the plains and in the process of getting much more connected to Denver.
In 1961, the U.S. Army drilled a hole nearly two and a half miles straight down into the earth, a dozen miles from downtown Denver.
Then they poured 165 million gallons of toxic liquid waste into it. Nobody's sure of the exact recipe of the stuff, but the stock and trade of the Rocky Mountain Arsenal ran the gamut from napalm to asbestos to sarin nerve gas in its 40 years of operation, so it's safe to say it was extremely nasty. A 1985 Denver Post story called it "the most polluted piece of land in America."
![]()
Rocky Mountain Arsenal plants manufactured chemical weapons and pesticides for 40 years.
The 27-square-mile site served as one of the nation's top manufacturing centers for chemical and biological weapons starting in World War II, and the Army also leased space to Shell Oil and other companies to make pesticides and herbicides.
After manufacturing shut down in 1982, a 23-year, $2.1 billion cleanup commenced. Crews permanently sealed the toxic underground reservoir, and widespread contamination across the site's soil, ditches and sewers earned the site Superfund status.
Then the unthinkable happened. Bald eagles showed up and nested, spurring the Rocky Mountain Arsenal's designation as a national wildlife refuge.
A vast sea of grassland
It's absolutely huge, about 16,000 acres. That's basically the equivalent of a quarter of Denver city limits, say, the 40-block-by-80-block chunk from downtown to Baker to Cherry Creek to Park Hill, if you slid it southwest. And because of decades of almost zero human activity in most of that acreage, wild animals found it a good place to set up shop years before it was an official refuge.
It's also a rarity. "We have less than 2 percent of intact native prairie on this continent," says Ryan Moehring, public affairs specialist with the U.S. Fish and Wildlife Service. While the refuge doesn't qualify as intact, he adds," You can restore a prairie, which is what we've done at the Arsenal."
Moehring points to a broader trend of ecological restoration across the Great Plains. The U.S. Department of Agriculture's Conservation Reserve Program pays farmers and ranchers to restore prairie lands. "It's never going to be as diverse as it once was," he says. "You might get it close over time, over years and years and years."
The Army started transferring the former Rocky Mountain Arsenal to the U.S. Fish and Wildlife Service in 2004, and it's since emerged as one of the most noteworthy urban refuges in the U.S. A big part of that stems from the reintroduction of bison in 2007 and black-footed ferrets in fall 2015.
A herd of about 80 bison currently roams a fenced 1,400-acre area in the refuge. They were doing so well that a culling was necessary in 2013, but there are plans to open about 80 percent of the refuge -- roughly 12,000 acres -- to bison, an expansion that would support more than 200 animals. "We're basically at carrying capacity until we expand the acreage," says Moehring.
![]()
A recently reintroduced black-footed ferret peers out from a burrow.
Thought to be extinct in the wild before their rediscovery in Wyoming in the late 1980s, black-footed ferrets are one of the most endangered animals in Colorado and the U.S. After a concerted effort over the last two decades, the national population has hit about 1,000 animals in 18 distinct areas.
Conservationists released 32 of the obsidian-tootsied mustelids in the refuge in fall 2015. "They're doing well," says Moehring. A fall survey spotted "a greater percentage than average" for such efforts, but a spring follow-up was rained out. The plan is to introduce about 20 more ferrets in September.
Moehring says the ferret's near-extinction was a byproduct of prairie dog eradication programs that poisoned, shot, trapped and otherwise exterminated the animals, largely because ranchers saw them as hazardous to livestock due to the possibility of a horse or cow stepping into a burrow.
Beyond prairie dogs, bison and ferrets, the refuge is also home to snapping turtles and big northern pike, and deer and burrowing owls and bullfrogs, and of course the bald eagles. The refuge is now the winter roosting site for dozens of the majestic birds, including a nesting pair that's returned year after year since 2002.
A resilient prairie
It's all connected. Bison graze on the dominant grasses and make way for more diverse plant communities, which in turn provide habitat for a more diverse population of animals.
"Bison are amazing creatures, not only to behold, but for the prairie and its health," says Moehring, noting that their grazing, wallowing and wandering help deposit seeds, aerate soil and slow the spread of weeds. "The prairie ecosystem evolved with disturbance," he explains. "Without that disturbance, a lot of those things don't happen."
![]()
Bison make for a more diverse prairie.Suppressing prairie fires has had a similar effect to removing bison. "When you think about taking pieces out of the puzzle, things don't work as well," says Moehring.
Below ground, black-footed ferrets prey exclusively on prairie dogs and take over their burrows. It's very brutal (they attack while the prairie dogs are sleeping) and not at all wasteful (they consume every last part of the prairie dog). A broader reintroduction effort seems like a great check to balance the ever-expanding ring of prairie dog burrows around the Denver metro area.
Prairie dogs are "a keystone species," says Moehring. "They really are the backbone of the prairie ecosystem."
Burrowing owls, the smallest of owls, also live in prairie dogs' burrows, but it's a more amiable arrangement, and they prey upon the beetles and other insects that are attracted to the bison dung. The owls even bring dung into the burrows for olfactory camouflage and climate control.
Moehring says that these interrelationships all add up to a healthier ecosystem. "The more complex it gets, the more resilient it gets."
Expect more complexity in the future: Officials have discussed reintroducing beavers, greater prairie chickens and sharp-tailed grouses to the refuge.
Today and tomorrow
Not all of the refuge has opened for recreational use, but there is an increasing number of things to do. Beyond a self-guided nine-mile wildlife-watching drive, there are lakes to fish and about 10 miles of trails to hike at the refuge.
For photographers, the stark, untouched high desert plains provide a subtle foreground for views of the spires of downtown in the distance, with the Rockies dominating the western panorama. You can squint your eye and say to yourself, "This is what Denver looked like before the city took over."
Development is increasing on all sides. Home field for the Colorado Rapids soccer team and numerous Phish shows, Dick's Sporting Goods Park was built on former Army land spun off from the refuge in 2004, and the suburbs are sprouting along its borders.
But the history of the Rocky Mountain Arsenal means that human habitation isn't going to happen on its 16,000 acres. The toxic legacy means Denver has a permanent 27-square-mile wildlife refuge as a buffer to development along its northeastern fringe, and it's showing us new ways to manage our land, while providing an asset for recreation.
Case in point: The newly opened First Creek at DEN Open Space, the City and County of Denver's largest open space at 198 acres, will provide more access to the refuge's trails from the adjacent Montbello, Parkfield and Green Valley Ranch neighborhoods.
"There's a movement across Denver to have as much connectivity as possible," says Moehring, noting that a newly hired urban refuge coordinator will spearhead local outreach. "We have this amazing gem that people don't know about, especially people who are close to it."
Enjoy this story?
Sign up for free solutions-based reporting in your inbox each week.
Eric is a Denver-based tech writer and guidebook wiz. Contact him
here.