About 200 fish were found dead on Aug. 18 on the banks of Grizzly Reservoir, a popular fishing and camping site near Aspen.
Colorado Parks and Wildlife officials determined that naturally occurring metals had become toxic for rainbow trout the agency had stocked in the reservoir.
CPW has been stocking Grizzly Reservoir for more than half a century without any problems, but scientists began closely monitoring water quality there after a similar fish mortality event in 2021.
Kendall Bakich, an aquatic biologist with CPW, is part of a team measuring the concentration of metals in the reservoir.
She said this new metal toxicity is part of a growing trend.
“I would probably say across the world, but certainly across North America, there's rivers that are becoming more impacted by heavy metals from natural sources, due to climate change,” Bakich said.
Human-caused climate change has led to warming temperatures and drought, increasing the concentration of naturally occurring metals in bodies of water and creating deadly conditions for fish.
Bakich said the main culprit in this case was copper, to which fish are especially sensitive.
That copper comes from a body of heavy metals at the top of Lincoln Creek, which feeds into Grizzly Reservoir and eventually into the Roaring Fork River.
It’s a compounding problem as a warming climate has also resulted in lower snowpack and less precipitation, resulting in less water to dilute the concentration of metals.
Since 2025 has been a dry year, water typically diverted into the reservoir was needed elsewhere, exacerbating the issue.
Bakich said CPW spent significant time and money hatching and raising the trout they stocked in Grizzly Reservoir, and she lamented the wasted resources. But the agency is also concerned about how this increased toxicity is impacting wild fish populations.
“What we're seeing in the last ten years is that there are fewer and fewer fish as you move downstream,” Bakich said. “Last year, in all of my monitoring sites in Lincoln Creek, we had no fish. That's the first time I've not detected any fish throughout the entire reach of Lincoln Creek.”
Bakich said the metals have also killed fish in the Roaring Fork River, with at least one mortality event near North Star Nature Preserve within the last few years.
“Even if it's not killing fish, a lower-level toxicity that's increasing will reduce the health of fish,” Bakich said. “It can reduce the growth of fish, and it can reduce the general abundance of fish.”
CPW is studying the issue with Pitkin County, local watershed organizations like Trout Unlimited and geologists from the University of Colorado.
Bakich said one idea is to suppress the metals coming from the historic Ruby Mine, but that only contributes to about 10% of the problem.
In the meantime, fishing opportunities in Grizzly Reservoir will likely be more limited.
Before stocking the lake in July and August, hatchery managers tested the water and deemed it healthy, but conditions changed quickly enough to kill the fish by mid-August.
