In August, Kurt Dahl, Pitkin County’s environmental health manager, headed up to Grizzly Reservoir to investigate reports of dead fish along the banks of the mostly drained lake.
“We walked three-quarters of the lake, and there were approximately 150 dead fish,” Dahl said of the Aug. 19 trip, estimating there were an additional 50 fish in the part of the shore they didn’t cover.
Colorado Park and Wildlife had just stocked the lake with rainbow trout a week prior. The agency said it tested the reservoir, and conditions were fine to release the fish. But within the turquoise waters lurked a “remarkably high” concentration of naturally occurring copper, experts said. This year, warm temperatures and less water flowing into Grizzly Reservoir allowed the metal’s concentration to reach a deadly threshold.
With a changing climate in the mix, local groups are worried more fish kills, and higher pollution levels, are in the future.
“It’s definitely a huge concern as to how these flows are going to be managed in the future,” said Lisa Tasker of the Pitkin County Healthy Rivers Program. “How many issues are we going to bump into, like this summer?”
Grizzly Reservoir, located at 10,542 feet, is in a remote, alpine area of Pitkin County. Its onsite caretakers travel through the Twin Lakes Tunnel beneath Independence Pass to get to work.
The reservoir, owned by the Twin Lakes Reservoir and Canal Company, is part of a larger diversion system that moves water from the Western Slope to the Arkansas River Basin and Front Range water users.
Colorado Parks and Wildlife has been stocking Grizzly Reservoir since the 1970s. In 2021, a fish kill launched an investigation into the watershed’s conditions and whether the nearby Ruby Mine was the cause of the contamination.
The Environmental Protection Agency determined Ruby Mine contributed less than 5% of the contaminants in Lincoln Creek and Grizzly Reservoir. The rest came from natural geology.
Then another fish kill happened in 2023. And again in August.
“What may be happening due to climate, warmer summers and drier summers, in the Lincoln Creek watershed is also happening across the Colorado mineral belt,” said Diane McKnight, a University of Colorado professor.
What led to the fish deaths?
The agency restocked the lake in July without issue. In August, CPW staff measured the water’s temperature and acidity to make sure it met state criteria for stocking.
The department declined to share the exact level of acidity, saying it was within normal limits, in a written statement to The Colorado Sun. The staff did not test for copper.
The water was around 4.5 on the pH scale, which measures acidity levels, in Lincoln Creek above Grizzly Reservoir on Aug. 19. The reservoir’s water varied between about 6.4 and 7.8.
The state’s aquatic life standards for streams and rivers are between 6.5 and 9 on the scale, said Chad Rudow of Roaring Fork Conservancy. He and other members of the Lincoln Creek Workgroup are collecting data to analyze contamination in the watershed.
By Aug. 18, about 200 rainbow trout were dead on the reservoir’s shore.
Dahl could tell something was wrong by the color of the lake, which looked a little more turquoise than usual, he said.
“That’s evidence of the levels of copper in there,” Tasker added.
Upstream from the reservoir, a naturally mineralized tributary flows into Lincoln Creek. The highest copper level from that tributary, measured in September 2024, was more than 2,000 times the acute aquatic life standard, according to research from the University of Colorado’s Institute of Arctic and Alpine Research.
Around 7 river miles downstream, where Lincoln Creek enters Grizzly Reservoir, the highest dissolved copper concentration was more than 100 times the acute aquatic life standard in September 2024.
Typically, there is enough water flowing through the system to dilute the copper levels, experts said. A common mantra among water professionals is “the solution to pollution is dilution.”
This August, Lincoln Creek’s flow hovered around 18 cubic feet per second, but dropped to 5 cfs around Aug. 10. With less water in the creek, copper levels were higher heading into the reservoir.
Other creeks that drain into Grizzly Reservoir were also low, which meant losing another way to dilute the copper concentrations. By Aug. 10, flows in Lost Man Creek and the Roaring Fork River had naturally declined, halting diversions into the reservoir, according to a CPW news release Aug. 25.
“We are looking to reevaluate our stocking procedures; however, we are in the middle stages of data collection and currently don’t have a plan,” CPW staff said in a written statement.
What happens at Grizzly could have larger impacts, potentially on the Roaring Fork River and local water-based businesses, like flyfishing companies, McKnight and Tasker said.
“The little bit of water coming out of Grizzly Reservoir can really tip the Roaring Fork into a space where we’re going to see fish kills,” Tasker said. “That’s a future worry, and this is why we’re trying to collect this data and understand what’s happening.”
A sign of something larger
Heat, drought and snowpack are impacting the water quality of streams and rivers across Colorado.
Early summer snowmelt helps dilute the amount of copper in Lincoln Creek. But that snowmelt has shifted earlier in the year by about three weeks over the last 30 years, McKnight said.
Earlier snowmelt leaves more time for shallow soils to dry during the warm summers.
That sets off a ripple effect: Pyrite in the soil is exposed to oxygen, releasing sulfuric acid and a form of iron. From there, chemical processes and hungry bacteria can accelerate acidic contamination in the soil. Rain and groundwater wash it into waterways, like Lincoln Creek.
Hotter conditions help accelerate these processes, while drying soils, heating waters and impacting fish habitat.
In August, Colorado’s temperatures were warmer than average across most of the state. The heat from October to July ranked 10th warmest in the 130-year record, according to Colorado’s Water Conditions Monitoring Committee.
The state’s average yearly temperature has increased 2 degrees F in the last 30 years, according to the 2023 Colorado Water Plan.
This year was also a low-water year, and streamflows were down across the state. August streamflows were in the bottom 50% compared with the 30-year average, including some record lows.
Other stream systems in the Colorado Mineral Belt — from southwestern Colorado to the Front Range — have seen higher concentrations of sulfuric acid, zinc and copper over the past three decades, McKnight said, referring to a 2024 study.
There’s still a lot to study, she said, but for Grizzly Reservoir, one place to start is by testing for copper before stocking the lake — if that’s feasible for the state. Right now, locals want to keep the contaminated water out of the Roaring Fork River.
“Hopefully, there’s enough water in the Roaring Fork that these metals will be diluted,” McKnight said. “Hopefully.”