• Bureau of Reclamation released a supplemental environmental impact statement to address the growing presence of smallmouth bass in the Colorado River below Glen Canyon Dam.
• Warmer water temperatures in Lake Powell have created ideal spawning conditions for these nonnative fish, posing a threat to the endangered humpback chub.
• The preferred alternative, known as the Cool Mix option, would be activated if the average water temperature exceeds 60°F for three consecutive days.
• The plan also includes potential modifications to the protocol for conducting high-flow experiment releases.

June 3, 2024 — Last week, the Bureau of Reclamation released an updated plan for Glen Canyon Dam operations designed to protect native fish populations in the Colorado River. The plan focuses on addressing the growing presence of smallmouth bass, a nonnative fish species that poses a threat to the endangered humpback chub.

The problem stems from rising water temperatures in Lake Powell, the reservoir impounded by Glen Canyon Dam. As the reservoir’s water level has declined, warmer water from the surface has been drawn closer to the dam’s water intakes. This warmer water not only provides ideal conditions for smallmouth bass to pass through the dam and into the Colorado River, but also creates suitable spawning conditions for them downstream.

The Proposed Solution.

To address this issue, the Bureau of Reclamation has proposed a series of flow options designed to disrupt smallmouth bass spawning and prevent their population from expanding. The preferred alternative, known as the Cool Mix option, would be activated only if the average water temperature in the river exceeds 60°F for three consecutive days.

This option involves adjusting the timing and other features of water releases from the dam to create conditions that are less favorable for smallmouth bass spawning. The goal is to protect the humpback chub, a federally threatened fish species that is particularly vulnerable to predation by smallmouth bass.

Additional Measures.

In addition to the flow options, the plan also includes potential modifications to the protocol for conducting high-flow experiment releases. These releases involve releasing large volumes of water from the dam to mimic natural flood events, which can help restore natural sediment flows and improve habitat for native fish.

The plan also addresses concerns about the capacity of the dam’s river outlet works, which are used to release water from the reservoir. Recent projections suggest that Lake Powell’s water level will remain above a critical threshold for the next few years, ensuring the continued operation of the outlet works.

Public Review and Next Steps.

The final supplemental environmental impact statement was published in the Federal Register on May 31, 2024, initiating a 30-day public review period. After this period, the Bureau of Reclamation will select a final alternative for implementation.

The updated plan for Glen Canyon Dam represents an important step in protecting native fish populations in the Colorado River. By addressing the threat of smallmouth bass and improving the management of water releases, the Bureau of Reclamation is working to ensure a healthier and more sustainable ecosystem for future generations.

Image:

Smallmouth bass. Photo by Brett Billings, U.S. Fish & Wildlife Service, via Bureau of Reclamation News Release.