How a 'cold shock' of water from Lake Powell could thwart invasive Grand Canyon bass

Federal dam managers are preparing a springtime assault against smallmouth bass on the Colorado River, possibly using cool water from deep in Lake Powell to keep the non-native fish from getting entrenched in Grand Canyon.

Environmental and river recreation advocates hope the U.S. Bureau of Reclamation will pair cold water with a rush of water from deep behind Glen Canyon Dam, both disrupting bass reproduction around Lees Ferry and restoring eroded sandbars farther downstream. That option, officially under study, is politically sensitive because it would cost hydropower production and move water out of a reservoir that has dropped to about a quarter of its storage capacity.

The stakes for humpback chub and other native fishes are high. Young smallmouth bass, which grow into voracious warm-water predators, were found between the dam and Lees Ferry last year. The discovery led to a trapping and poisoning campaign then, and to consideration of using a pulse of cold water to disrupt any spawning this year. Getting that cold water would require drawing water through the dam’s bypass tunnels, which are deeper than the hydropower intakes that usually supply the river’s flow.

Grand Canyon advocates are hoping that the combination of the healthy snowpack now amassing in the Rocky Mountains and the ecological threat of a bass invasion will give the U.S. Bureau of Reclamation the political cover it needs to release a restorative flood after years of drought prevented such releases.

Paul McNabb, The Republic's tour guide, holds a smallmouth bass on June 9, 2022, on Lake Powell, near Page.
Paul McNabb, The Republic's tour guide, holds a smallmouth bass on June 9, 2022, on Lake Powell, near Page.

“It’s really critical this year,” said Kelly Burke, who directs the Grand Canyon Wildlands Council and serves on a work group advising Reclamation on dam operations. That’s not just because of the imperative of thwarting bass reproduction, but also because the meltwater set to flow toward Lake Powell is currently forecast to be well above average, a condition that may not soon return in this century of megadrought.The Bureau of Reclamation must walk a tightrope this year, attempting to balance the legal requirements of the Endangered Species Act and Grand Canyon Protection Act against its core responsibility to store and provide water for the region’s farms and cities. Flushing water downstream from the dam could slow the bass invasion in Grand Canyon, but emptying too much of Lake Powell could have the opposite effect in future years. That’s because as the water level sinks against the dam, the water passing through is closer to the surface and therefore warmer.


The agency’s call for action explains the problem this way: “Water levels in Lake Powell continue to decline to historically low levels, which has contributed to record high water temperature releases from (Glen Canyon Dam). Below the dam, these warmwater releases are creating ideal spawning conditions for smallmouth bass, a predatory invasive fish species.”

The bass were introduced upstream and are a favored sport fish in Lake Powell. Having slipped through the dam’s turbines and begun to show signs of reproduction in the river, though, they are now a pariah.

Grand Canyon National Park has seen a resurgence of humpback chubs pre-bass invasion, enough that they were downgraded from endangered to threatened. Some of them spawn near the mouth of the Little Colorado River, where they would be unaffected by cold-water dam releases.

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A 'cold shock' or a 'cool mix'?

Reclamation is considering what it calls a “cold shock” option that draws heavily on the bypass tubes from May to July, and a “cool mix” option that blends the cold bypass water with warmer water from penstocks nearer the surface. The goal is to keep the Lees Ferry and upper Grand Canyon stretches cooler than about 60 degrees, which the water exceeded last year.

The agency also is studying either option with the addition of three “flow spikes,” in which it would blast up to twice as much water — perhaps 30,000 cubic feet per second — for 36 hours. The spikes could both cool the water below the comfort level for bass spawning, and push young bass away from their parents’ protection.

The U.S. Fish and Wildlife Service wrote to Reclamation in December saying that such flow alterations in spring and early summer offer the best chance of preventing bass establishment and complying with Endangered Species Act requirements.

Water is released from Glen Canyon Dam through a bypass tube on Nov. 5, 2018, during a high-flow experiment. The flood will help move sand and sediment down the Colorado River the way the river's natural flows did before construction of Glen Canyon Dam.
Water is released from Glen Canyon Dam through a bypass tube on Nov. 5, 2018, during a high-flow experiment. The flood will help move sand and sediment down the Colorado River the way the river's natural flows did before construction of Glen Canyon Dam.

The National Park Service said it views the option of a cool mix with spike flows as most likely to prevent bass establishment without harming native fish, but added that it wants to see further analysis of how the various options would affect beaches and sandbars.

The Park Service also noted that canyon-affiliated tribes such as the Zuni have opposed mechanical and chemical killings of non-native fish, and would find altered flows more acceptable. The Navajo Nation supported last year's poisoning on a limited basis to prevent bass establishment, though the tribe's delegate to Reclamation's work group told The Arizona Republic that nonlethal measures such as a net blocking further movement through the dam are preferred in the long term.

The spikes could also build canyon beaches. After two good monsoon seasons that washed sediment from the Paria River into the Colorado at Lees Ferry, there’s ample sand waiting for high water to shape into new sandbars and beaches.

“This is the prime time,” Burke said.

For subscribers:As the Colorado River is stretched thin by drought, can the 100-year-old rules that divide it still work?

Rafters and campers would benefit, but what about power?

The U.S. Interior Department, which oversees Reclamation as well as the Fish and Wildlife and National Park services, periodically unleashes floods from Glen Canyon Dam when there’s sufficient sand below it to rebuild beaches, which provide campsites for river runners while also approximating Grand Canyon’s natural conditions. The last one occurred in the fall of 2018. Since then, while the government held back water to protect the reservoir and its hydropower production, the beaches have eroded.

Absent a flood this spring, the result will be a tough year for river rafters and the companies that support them, river guide David Brown told federal officials at a work group meeting last month.

“These are your constituents,” said Brown, representing the Grand Canyon River Guides.

Others at the meeting said there’s more than either beaches or bass to consider. Among them was Leslie James, who heads the Colorado River Energy Distributors Association. She submitted written comments reflecting fears of lost potential to generate hydroelectricity that flows from Glen Canyon Dam to customers across the West.

“The Western grid faces increasing shortages and reliability risks, particularly during the summer months,” James wrote in a letter asking Reclamation officials to analyze how bypassing water past the dam would affect customers that include dozens of Native American tribal utilities. “The reduction of available hydropower in the Colorado River system during those months further exacerbates those risks and adds related challenges.”

One way or another, the water needed for a Grand Canyon flush would ultimately flow downstream and into the Southwest’s big holding tank: Lake Mead. The question before federal decision-makers, and ultimately Interior Secretary Deb Haaland, is whether protecting Grand Canyon and its native fish merits bypassing some power production this spring while potentially speeding losses in Lake Powell’s storage if recent drought patterns return after this wet winter.

Brandon Loomis covers environmental and climate issues for The Arizona Republic and Reach him at or follow on Twitter @brandonloomis.

Environmental coverage on and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at and @azcenvironment on Facebook, Twitter and Instagram.

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This article originally appeared on Arizona Republic: Cold water releases could thwart invasive bass in Grand Canyon