Sediment Release at Grizzly Reservoir Dam: Environmental Impact and Water Quality Assessment

Pitkin County, CO (8/29/24) - On July 16, 2024, draining of the Grizzly Reservoir to allow for maintenance of the dam caused a sediment release that discolored Lincoln Creek and the Roaring Fork River for approximately 36 hours. The Lincoln Creek Workgroup, comprised of local non-profits, citizens, and local, state, and federal agencies that have come together to assess the Lincoln Creek Watershed, is conducting various water quality studies, one of which is related to the dam renovation project. A team from Roaring Fork Conservancy and Pitkin County conducted planned water quality sampling during the reservoir drawdown and sediment release to assess downstream impacts. Analysis of the samples was provided by Colorado Parks and Wildlife's (CPW) River Watch program.

The sediment release, which turned the waters orange, had a visible impact but minimal biological effects-no fish kill was observed. The discoloration was due to larger particulate matter, mainly iron and aluminum, released during ongoing construction at the dam, and was anticipated as a possibility by the project team.

Twin Lakes Canal Company installed several sediment traps below the Grizzly Reservoir outlet to reduce sediment mobilization. On July 16, water quality results showed a significant reduction in metal concentrations from the reservoir outlet to downstream areas. For instance, total iron levels in Lincoln Creek from Grizzly Reservoir to the confluence with the Roaring Fork River decreased by 97%, and total aluminum decreased by 98%.

The released sediment was in particulate form, less bioavailable, and therefore less toxic to aquatic life. The event is not expected to have a significant long-term impact on aquatic ecosystems.

The release from the reservoir is unlikely to pose any ongoing risk to people using the lake or streams for recreation. As always, we encourage both people and pets to drink treated water to avoid the risk of waterborne illnesses like giardia. People can contact a healthcare provider or poison control at 303-389-1100 if they or their pets feel sick.

The sediment and metals originated from a naturally mineralized geologic formation near the headwaters of Lincoln Creek. These materials flow into Grizzly Reservoir during normal operations, but recent climatic changes may have increased metal contributions. The current dam renovation required a reservoir drawdown, leading to the sediment release.

Data from this event will be available on the Colorado River Watch website and the National Water Quality Data Portal after September 1. This data contributes to ongoing efforts to monitor water quality in Lincoln Creek, Grizzly Reservoir, and downstream areas.

While the likelihood of another release similar to the one on July 16 is low, Pitkin County and partnering agencies are working with Twin Lakes Canal Co. to expedite public communications in the event of an unexpected sediment release.

For more information, please contact Kurt Dahl [email protected]

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