Bass are about to conquer the Grand Canyon. Cold water could stop them.

Glen Canyon Dam managers will release cold water from the depths of Lake Powell into the Colorado River to help prevent invasive smallmouth bass from colonizing the Grand Canyon, federal officials announced Wednesday.

The hope is that cooling the river below ideal temperatures for bass reproduction could thwart a feared explosion in predator numbers, buying time for bass culling efforts to save federally protected humpback minnows throughout the canyon.

When biologists call for cold water, the U.S. Bureau of Reclamation releases it through the dam’s diversion tunnels, mixing it with warmer surface water from the hydroelectric plant to create a “cold mix” in the river below.

Water restoration officials said they will use the cold mix to minimize bass spawning this summer while keeping other options available for future years. The plan is to add cold water to the river when it reaches 60 degrees Fahrenheit, a plan favored by the U.S. Fish and Wildlife Service.

“The Service approves this action because scientific data indicate that the risk of smallmouth bass establishment is reduced by cold-water releases to disrupt their spawning,” the agency, which is responsible for restoring endangered native fish, concluded in its biological opinion on the cold-mixing plan. “Under conditions where smallmouth bass or other warm-water nonnative predator species become established in the Grand Canyon, predation threats to the humpback shiner become greater.”

The plan does not increase the amount of water the government will send downstream and into Lake Mead during the year, even though that will result in a loss of hydroelectric generation.

The stretch of river that flows from Glen Canyon Dam through the Grand Canyon and into Lake Mead is a haven for native fish that has allowed minnows to recover to the point of being removed from the endangered species list. Then, in 2022, biologists discovered hundreds of juvenile smallmouth bass in the stretch of river between the dam and Lees Ferry, suggesting a potentially catastrophic invasion if these fish were able to reproduce and populate the entire canyon with bass.

The fish are believed to have descended from parents that once swam in Lake Powell and then survived a trip through the dam’s turbines before spawning in a warm swamp a few miles downstream.

Non-native species: As Lake Powell Shrinks, Ravenous Smallmouth Bass Prepare for Grand Canyon Invasion

The U.S. Interior Department is currently studying possible modifications to the river and dam to block further migration from Powell and discourage spawning in the swamp, including installing a net anchored above the dam and building a new channel to allow cooler river water to flow into the swamp. In the meantime, federal biologists have worked to remove fish and, occasionally, spray the swamp with a fish-killing agent.

The cold-mix discharges will draw water through the dam’s diversion tunnels, which are 100 feet below the hydroelectric intakes and, therefore, draw from the coldest part of the lake.

Smallmouth bass typically reach an adult length of 200 millimeters and begin spawning around age 3, biologist Drew Eppehimer of the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center told members of the Glen Canyon Dam Adaptive Management Program Advisory Working Group this spring. That could mean that young fish that emerged into the river in 2022 could spawn and trigger a much larger invasion next year.

Bass growth also depends on warmer temperatures, so it’s possible all the fish won’t reach maturity anytime soon. Last year’s snowmelt and the influx of water into Lake Powell could prove fortuitous. That naturally lowered the temperature of the water flowing past the dam, Eppehimer said.

“But we have the marsh,” he said. “We have various habitats in still water that can warm up more quickly.”

Humpback minnows have evolved to survive both the cold spring meltwater and the warm summers of lower flows. The current need to cool summer flows is a symptom of the dam’s profound alteration of the ecosystem. Unusually cold summer water releases were once thought to be a threat to native fish, but they also appeared to prevent warm-water predators like bass from establishing populations.

After two decades of drought, Lake Powell’s decline has brought warmer surface waters closer to the intakes and contributed to the warming of the river. Managers are now seeking to reverse some of the warming by drawing water from deeper into the reservoir.

Brandon Loomis covers environmental and climate issues for The Arizona Republic and azcentral.com. Contact him at [email protected].

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust.

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