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Reclamation finalizes environmental process to combat nonnative fish below Glen Canyon Dam

Completed environmental process allows Reclamation to use water from Lake Powell to prevent the establishment of smallmouth bass below the dam and revises the protocol for high-flow experiment releases

Media Contact: Upper Colorado Basin Public Affairs
For Release: Jul 3, 2024
Illustration showing the lake Powell elevations and the associated water temperatures. Illustration showing the lake Powell elevations and the associated water temperatures.

PAGE, Ariz. – The Bureau of Reclamation today finalized its process to protect the humpback chub and other federally protected fish species with the signing of the Supplemental Environmental Impact Statement for the 2016 Long-Term Experimental and Management Plan Final Environmental Impact Statement Record of Decision.

Reclamation initiated the environmental review process in response to the increasing numbers of smallmouth bass in the Colorado River below Glen Canyon Dam.

As Lake Powell’s elevation has declined and water released from Glen Canyon Dam has warmed in recent years, warmwater invasive fish such as smallmouth bass residing in the upper layer of Lake Powell can pass through the dam and successfully spawn downstream in the Grand Canyon. These warmwater predatory fish can prey on Federally protected native fish species in the Grand Canyon.

With the completion of the environmental process, Reclamation can now use water releases from the dam to disrupt smallmouth bass spawning.

Preferred Alternative

The Preferred Alternative allows Reclamation, if needed, to adjust Glen Canyon Dam water flows to cool the river to disrupt the establishment of smallmouth bass and other warmwater predators. Reclamation identified the ‘cool mix’ as the preferred alternative for potential flow actions to address smallmouth bass this summer. The other cold water alternatives would be further considered in 2025 through 2027.

For 2024, triggering of the cool mix would occur if the average observed daily water temperatures reach smallmouth bass reproduction thresholds, above 15.5 degrees Celsius (60 degrees Fahrenheit). All alternatives strategically release water from both the penstocks and river outlet works to cool the Colorado River downstream of the dam. The intakes for the river outlet works are located approximately 100 feet below those for the penstocks and therefore can send much cooler water downstream.

“The Service endorses this action because the science indicates that the risk of smallmouth bass establishment is reduced through cold water discharges intended to disrupt their spawning. Under conditions where smallmouth bass or other warmwater nonnative predatory species become established in the Grand Canyon, the predation threats to humpback chub become greater,” stated the U.S. Fish and Wildlife Service in a response letter to the biological opinion.

The Record of Decision also modifies the protocol for conducting high-flow experiment releases, specifically adjusting sediment accounting periods and implementation windows to incorporate the latest scientific information on Grand Canyon sediment. A high-flow experiment involves water releases from Glen Canyon Dam through the penstocks and the river outlet works to create high flows and water velocities to transport sediment downstream to be deposited as sand beaches for riparian habitat, recreation campsites, and protection of cultural resources.

Colorado River Operations

The 2016 Environmental Impact Statement and Record of Decision and this supplement involve the timing of hourly, daily, monthly, and experimental releases from Glen Canyon Dam. Annual releases are currently controlled by the 2007 Colorado River Interim Guidelines, which has been supplemented by the Supplemental Environmental Impact Statement for Near-term Colorado River Operations Record of Decision. Those guidelines, along with the 2019 Drought Contingency Plans, expire at the end of 2026. A separate, ongoing planning process to develop post-2026 guidelines is underway.

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