PITKIN COUNTY, Colo. (August 25, 2025) — On Monday, Aug. 18, Colorado Parks and Wildlife (CPW) was alerted to a potential fish kill after approximately 200 fish were discovered dead along the shoreline at Grizzly Reservoir in Pitkin County.
In collaboration with Pitkin County Public Health, Pitkin County Healthy Rivers, the Roaring Fork Conservancy, the U.S. Forest Service, and the Twin Lakes Reservoir and Canal Company, CPW has made a preliminary determination on the cause of recent fish mortality. After conducting an investigation that involved measuring water temperature and pH levels, CPW staff concluded that the fish deaths were the result of several contributing factors. These include deteriorating water quality entering Grizzly Reservoir from Lincoln Creek, rising water temperatures, and the loss of a clean water source from Lost Man Diversion Canal due to naturally declining flows.
High Temperatures:
Recent warm weather and low water flow have caused temperatures in Grizzly Reservoir to rise. As a result of this increase in water temperature, the toxicity of various metals, including copper, has also increased. CPW believes that while water temperatures were within the acceptable range for rainbow trout, the combination of rising temperatures and increased copper toxicity likely contributed to the fish mortality observed during this event.
Lincoln Creek water quality:
Water Quality in Lincoln Creek has been monitored for several years, including collecting monthly water quality samples throughout the summer. Recently, members of the Lincoln Creek Workgroup observed a decrease in the pH of water flowing into Grizzly Reservoir. In the past, lower pH has been associated with higher metal concentrations, particularly copper.
On August 19, Pitkin County and Roaring Fork Conservancy collected additional water quality samples in response to this event. Results from these samples are still pending.
Loss of dilution water source:
By Aug. 10, flows in Lost Man Creek and the Roaring Fork River had naturally declined to a level that diversions to Grizzly Reservoir were no longer being made. This caused a reduction in fresh water entering Grizzly Reservoir, which diminished the dilution of already poor-quality water from Lincoln Creek.
CPW will continue to work closely with our partners to monitor conditions at Grizzly Reservoir.
In July and August, CPW stocked Grizzly Reservoir with “catchable” rainbow trout, as part of the reservoir's fishery management strategy. Since 1973, CPW has used a put-and-take management strategy to maintain angling opportunities in the reservoir. Before stocking fish, hatchery staff take water temperature readings and test for pH levels to ensure they fall within acceptable ranges for the trout. During both stocking events in July and August, these tests were conducted, and the results indicated that both the water temperature and pH levels were appropriate for the fish.