In August, Kurt Dahl, environmental health manager for Pitkin County, made a field visit to Grizzly Reservoir following alarming reports of dead fish along the banks of the almost drained lake.
On his August 19 trip, Dahl and his team covered three-quarters of the lake, discovering around 150 dead fish.
He estimated approximately another 50 fish were in an area they did not examine.
This incident occurred shortly after Colorado Parks and Wildlife (CPW) stocked the lake with rainbow trout just a week earlier, believing the conditions were suitable for the fish.
However, unbeknownst to them, a significant concentration of naturally occurring copper had accumulated in the reservoir due to the warm temperatures and reduced water inflow typical of this year, ultimately reaching lethal levels for aquatic life.
Local environmental groups are deeply concerned about the implications of climate change, fearing that such fish kills and heightened pollution levels may become more frequent.
Lisa Tasker from the Pitkin County Healthy Rivers Program expressed worries about future water flow management, reflecting on the potential for encountering similar issues like those observed this summer.
Grizzly Reservoir sits at an elevation of 10,542 feet in a secluded alpine region of Pitkin County.
Accessing the site requires caretakers to travel through the Twin Lakes Tunnel beneath Independence Pass.
As a part of a larger diversion system, the reservoir, which is owned by the Twin Lakes Reservoir and Canal Company, facilitates the transfer of water from the Western Slope to the Arkansas River Basin and Front Range water users.
CPW has been stocking Grizzly Reservoir since the 1970s.
However, a previous fish kill investigation in 2021 raised concerns about the watershed’s conditions, particularly regarding the Ruby Mine’s influence on contamination levels.
The Environmental Protection Agency concluded that Ruby Mine contributed less than 5% of the contaminants affecting Lincoln Creek and Grizzly Reservoir, with the remaining pollutants arising from the natural geology of the area.
Yet, fish kills reoccurred in both 2023 and again in August, pointing to a troubling pattern.
The August event shocked local environmental experts, especially since the state had previously restocked the lake without incident.
In preparation for the stocking, CPW staff had conducted measurements to ensure that water temperature and acidity were within state criteria.
However, they did not conduct tests for copper, which proved to be a critical oversight.
On August 19, the pH level of water in Lincoln Creek above Grizzly Reservoir fell to approximately 4.5, while the reservoir itself displayed a variation between about 6.4 and 7.8.
According to Chad Rudow from Roaring Fork Conservancy, the state’s acceptable aquatic life standards for streams and rivers are between 6.5 and 9 on the scale.
By August 18, Dahl had noted that around 200 rainbow trout had already perished along the reservoir’s shore.
He suspected something was wrong when he noticed a more pronounced turquoise color in the lake’s water.
Tasker corroborated this observation, linking the color change to elevated copper levels.
Upstream from the reservoir, a mineral-rich tributary feeds into Lincoln Creek.
Research from the University of Colorado’s Institute of Arctic and Alpine Research indicated that the highest copper levels from this tributary measured more than 2,000 times the acute aquatic life standard in September 2024.
Approximately 7 river miles downstream, where Lincoln Creek flows into Grizzly Reservoir, copper concentrations exceeded 100 times the acute aquatic life standard during the same period.
Experts noted that under normal conditions, the water flow would typically dilute copper levels.
However, in August, the flow of Lincoln Creek dropped to about 18 cubic feet per second (cfs), plummeting further to 5 cfs around August 10.
The diminished water flow led to higher copper concentrations entering the reservoir, exacerbated by reduced drainage from other creeks feeding into Grizzly.
As flows in Lost Man Creek and the Roaring Fork River dipped, diversions into the reservoir ceased, culminating in heightened contamination levels.
CPW staff acknowledged the need to reevaluate their fish stocking protocols while emphasizing that they are in the midst of ongoing data collection and do not currently have a concrete plan.
The situation at Grizzly Reservoir could have far-reaching consequences, potentially impacting the Roaring Fork River and businesses reliant on water activities, like local flyfishing companies, as cautioned by both McKnight and Tasker.
Tasker articulated a concern for the Roaring Fork River, suggesting that the limited water exiting Grizzly Reservoir could lead to detrimental outcomes such as fish kills downstream.
With the ongoing data collection efforts, local experts hope to clarify the situation and comprehend the evolving challenges they face.
The implications of heat, drought, and diminishing snowpack present serious concerns for the water quality in Colorado’s streams and rivers.
Early summer snowmelt typically helps in diluting metal concentrations like copper in Lincoln Creek; however, this snowmelt has shifted roughly three weeks earlier over the past 30 years, according to McKnight.
Consequently, reduced snowmelt provides more opportunity for shallow soils to dry in the warm summer months.
This cycle leads to essential shifts: exposure of pyrite in the soil to oxygen results in the release of sulfuric acid and iron.
These conditions set off a series of chemical processes, accelerated by microorganisms that can further contaminate the soil with acidity.
Precipitation and groundwater wash these chemicals into local waterways, with Lincoln Creek being particularly vulnerable.
According to Colorado’s Water Conditions Monitoring Committee, temperatures in August were notably warmer than average statewide.
From October to July, the temperatures ranked as the 10th warmest observed in the last 130 years.
Over the past three decades, Colorado’s average yearly temperature has escalated by 2 degrees F, as documented in the 2023 Colorado Water Plan.
The current year also marked a low-water scenario, with streamflows significantly reduced across the state.
August streamflows fell in the bottom half compared to the 30-year average, with several areas even recording unprecedented lows.
Research indicates that other stream systems within the Colorado Mineral Belt, from southwestern Colorado to the Front Range, have encountered worsening levels of sulfuric acid, zinc, and copper during the past 30 years.
While much remains to be studied, experts emphasize the importance of testing copper levels before stocking Grizzly Reservoir in the future.
Community members are hopeful about preventing contaminated water from flowing into the Roaring Fork River.
“Ideally, there will be enough water in the Roaring Fork to adequately dilute these metals,” McKnight stated, adding the vital hope that locals have for their waterways.
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