- A warmwater slough near Glen Canyon Dam was reshaped to discourage invasive fish.
- The project reduces smallmouth bass spawning, which threatens native species.
- Flows from Glen Canyon Dam were managed to protect wildlife during construction.
- The effort supports humpback chub recovery and other native fish.
- Further strategies are underway to control warmwater fish below the dam.
Tuesday, June 10, 2025 — Last week, the Bureau of Reclamation and the National Park Service announced
the completion of a habitat modification project targeting invasive smallmouth bass in the Colorado River near Glen Canyon Dam. The work reshaped a warmwater slough located about 3.5 miles below the dam, in an area known for encouraging the reproduction of nonnative fish species that endanger native aquatic life.
Warmwater Conditions Fuel Invasive Threats.
The shallow slough, situated within the Glen Canyon National Recreation Area, had developed into a favorable spawning ground for smallmouth bass due to its warmer, slow-moving water. These conditions posed an increasing risk to native species such as the humpback chub, a federally protected fish classified as threatened. In recent years, prolonged drought has lowered Lake Powell’s water levels, allowing warmer surface water to flow into the Colorado River system. This shift has intensified the spread of warmwater invasive species downstream.
Engineering a Cooler, Swifter Channel.
To address this threat, Reclamation’s in-house construction team based in Provo, Utah, removed portions of the riverbank to reconnect the main river channel to the slough’s upper end and narrowed its lower section. This modification transformed the area from a stagnant slough into a flowing channel with average water velocities reaching approximately one foot per second. These changes help cool the water and discourage smallmouth bass from reproducing there.
Interagency Collaboration and Tribal Input.
“The establishment of warmwater nonnative predatory fish—particularly smallmouth bass—in the Colorado River below Glen Canyon Dam is an imminent threat to the recovery of the humpback chub, a federally protected, threatened species,” said Wayne Pullan, Regional Director for Reclamation’s Upper Colorado Basin. “This channelization effort demonstrates the power of partnerships and adaptive management in protecting the Colorado River ecosystem.”
Project managers worked closely with tribal representatives to preserve sensitive natural features. In response to tribal input, the National Park Service modified the project’s design to protect springs located within the site. Reclamation also coordinated Glen Canyon Dam flows to support construction activities while minimizing environmental disruption. Care was taken to avoid harming native wildlife, including rare canyon salamanders.
Part of a Broader Strategy Against Invasives.
This recent effort is part of a broader multi-pronged strategy to manage invasive species. Other tactics include chemical and mechanical fish removal, along with the use of “cool mix” flows—drawing colder water from deeper in Lake Powell to lower temperatures downstream. These cooler waters reduce the survival chances of warmwater invaders.
Jake Ohlson, Acting Superintendent of Glen Canyon National Recreation Area and Rainbow Bridge National Monument, explained, “A multi-faceted approach is necessary to address the invasive smallmouth bass and protect native species. The channelization is one component to address the invasion, and other measures will be needed to achieve success.”
Benefits Extend to Multiple Native Fish.
Beyond humpback chub, the reshaped channel may also benefit other native species like the endangered razorback sucker and the flannelmouth sucker. Observations have even recorded rainbow trout using the newly modified habitat near Lees Ferry.
Monitoring and Future Management Plans.
The Bureau of Reclamation will continue monitoring river temperatures to guide flow operations, while the National Park Service remains focused on fish surveillance and removal below the dam. The project underscores how interagency cooperation can support ecological restoration in the Colorado River Basin.
Image and Description from the Bureau of Reclamation’s News Release:
Photos of before (left photo) and after (right photo) the channelization of the slough area. In the bottom left of the “before” photo, you can see a ponded slough and wider channel; a narrowed channel through the project site replaced these features and can be seen in the “after” photo. National Park Service crews salvaged riparian vegetation during the project and will replant it, along with additional native plants, in the coming months. Photos courtesy Jake Ohlson, National Park Service.
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