Downstream view of Glen Canyon Dam in Page, Arizona. Image courtesy US Bureau of Reclamation

In response to growing concerns over the proliferation of nonnative fish, particularly smallmouth bass, downstream of the Glen Canyon Dam, the US Bureau of Reclamation (USBR) has unveiled a draft Supplemental Environmental Impact Statement (SEIS). This SEIS proposes alterations to the timing of water releases from the dam aimed at disrupting the establishment of these invasive species in the Colorado River.

The initiative comes as a supplement to the 2016 Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP) Final Environmental Impact Statement (FEIS) and Record of Decision. With Glen Canyon Dam forming Lake Powell, a primary storage unit of the Colorado River Storage Project, the proposed updates seek to address the rising numbers of warmwater predatory fish that pose a threat to native species protected under the Endangered Species Act.

The decline in Lake Powell's elevation has resulted in the epilimnion, the uppermost layer of the reservoir where warmwater predators thrive, moving closer to the dam's intakes. Consequently, there's an increased risk of these predatory fish passing through the dam into the Colorado River. The discharge of warmer water downstream creates optimal spawning conditions for nonnative fish, posing a significant threat to native species such as the federally threatened humpback chub.

The draft SEIS evaluates various reservoir releases with different temperature combinations to gauge their effectiveness in disrupting smallmouth bass spawning and survival. 

“If these predatory, nonnative fish become fully established downstream of Glen Canyon Dam, they may threaten the great progress we have made in recovering the humpback chub,” said Reclamation Upper Colorado Basin Regional Director Wayne Pullan. “Because eradicating an established invasive species is expensive and extraordinarily difficult, it is important that we have these tools – this range of reservoir releases – analyzed in this SEIS available to us before the 2024 spawning season.”

Additionally, the SEIS explores potential modifications to the protocol for conducting high-flow experiments (HFE) releases. These experiments are crucial for maintaining sandbars and beaches in the Grand Canyon region, essential for recreation, habitat preservation, and protection of cultural resources. By broadening the window for sediment assessment, the SEIS aims to facilitate more spring HFEs without altering annual release volumes from Glen Canyon Dam.

The initiative began with an Environmental Assessment (EA) in August 2022, followed by the release of the draft EA titled "Glen Canyon Dam/Smallmouth Bass Flow Options" for public comment in February 2023. Feedback from nearly 7,000 comments prompted USBR to undertake further analysis, culminating in the preparation of the draft SEIS since June 2023. Collaboration with federal agencies, states, tribes, stakeholders, and other interested parties has been integral in shaping the proposed revisions to the Colorado River management plan.