Environmental

Reclamation advances options to counter nonnative fish below Glen Canyon Dam

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Reclamation advances options to counter nonnative fish below Glen Canyon Dam

The Bureau of Reclamation has released a draft Supplemental Environmental Impact Statement (SEIS) that analyzes varying the timing of water released from Glen Canyon Dam to disrupt the downstream establishment of nonnative fish, primarily smallmouth bass.

On June 6, the Glen Canyon Dam Adaptive Management Work Group directed Reclamation to prepare an SEIS to the December 2016 Glen Canyon Dam Long Term Experimental and Management Plan (LTEMP) Record of Decision. This document supplements the 2016 LTEMP Final Environmental Impact Statement (FEIS) and Record of Decision. Glen Canyon Dam impounds Lake Powell, the largest storage unit of the Colorado River Storage Project, and provides water for a 1,320 MW hydroelectric powerhouse.

Proposed updates to the 2016 LTEMP FEIS are in response to increasing numbers of warm water predatory fish below Glen Canyon Dam, which can prey on native fish threatened under the Endangered Species Act. As Lake Powell’s elevation has declined, the epilimnion (warmest, top-most layer of the reservoir), where these nonnative, warm water predators reside, has become closer to the dam’s water intakes. As a result, warm water predatory fish are more likely to pass through the dam into the Colorado River.

When water from this layer is discharged downstream, it increases the temperature of the river, creating ideal spawning conditions for these nonnative, warm water fish that pose a threat to native species, including the federally threatened humpback chub.

A range of reservoir releases with varying combinations of temperature are analyzed in the draft SEIS to assess their effectiveness in disrupting and preventing further 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.”

The SEIS also includes potential modification of the protocol for conducting high-flow experiment (HFE) releases. The natural condition of the Colorado River, especially through the Grand Canyon, includes sandbars and beaches formed by river-transported sediment that are important for recreation, habitat and cultural resource protection. Because the river’s natural sediment drops out in Lake Powell, the water released from Glen Canyon Dam is largely sediment-free. To build and maintain sandbars and beaches, the LTEMP includes HFEs to transport sediment below the dam. The SEIS examines a broader window for assessing sediment available for beach-building, which may facilitate more spring HFEs. HFE releases do not change annual volumes released from Glen Canyon Dam.