GRAND CANYON NATIONAL PARK — A shot of cold water from Glen Canyon Dam appears to have stalled a smallmouth bass invasion of the Grand Canyon and protected rare Colorado River fish there, federal officials say.
In early July, two years after first finding the predatory bass spawning below the dam and in threatened humpback chub territory, the U.S. Bureau of Reclamation began releasing cold water from deep in Lake Powell in an effort to chill the river past the temperature at which bass are known to reproduce.
So far this summer, numerous netting, snorkeling and electrofishing trips on the river have turned up no newly hatched bass, biologists reported to an advisory committee meeting on Grand Canyon’s South Rim on Thursday.
“That’s huge,” said Kelly Burke, executive director at Wild Arizona and its Grand Canyon Wildlands Council, which had pushed for flow alterations from the dam to disrupt the bass invasion.
Cooler water was a must for preventing possible biological disaster this summer in particular, she said. “It couldn’t be better timed. We’re having an extraordinarily hot summer.”
The initial success also means the National Park Service will not dump a fish-killing chemical into spawning grounds a few miles downstream of the dam this year as it did last summer. Last year’s effort drew a rebuke from some tribal officials associated with Grand Canyon, who prefer nonlethal controls.
Federal officials considered the bass invasion an emergency requiring quick action to prevent a population explosion that could devastate humpback chubs, 90% or more of which live in the Canyon. Cooling the river below 60 degrees Fahrenheit has at least stalled that explosion.
With potentially hundreds of adult or year-old bass still swimming upstream of or within the Canyon, though, Reclamation and the National Park Service are contemplating more lasting changes, such as dredging a river channel through the prime spawning habitat of a backwater slough and draping a screen across lower Lake Powell to prevent more fish from spilling through the dam’s power turbines, as the initial invaders are thought to have done.
At this point, it may be impossible to remove the bass already living in the Lees Ferry zone above Grand Canyon, committee members acknowledge. Instead, the goal is to keep their numbers from swelling to the point where they consume large numbers of native fish downstream, or ideally to let them eventually die out without replacing themselves.
Protecting native fish cuts into power production
The cooling program’s apparent initial success comes at a cost, as the Western Area Power Administration is forced to spend several million dollars a month to replace power that Glen Canyon could have generated if the water had poured through the hydropower turbines as usual, instead of into the dam’s deeper and consequently colder bypass intakes.
Hydroelectricity’s value is not just measured in dollars but in life-preserving summer cooling, Sheri Farag of Phoenix-area Salt River Project reminded fellow members of the Glen Canyon Adaptive Management Work Group, who advise the U.S. Interior Department on such programs.
“People do need it, especially in the desert Southwest, to stay alive,” she said.
The committee has endorsed the river-cooling experiment, which required an environmental assessment that federal officials signed off on just days before water temperatures required action to prevent bass spawning.
“It truly has been a race against time,” said Wayne Pullan, Reclamation’s Upper Colorado regional director who leads the advisory committee that includes representatives from federal, state, tribal, energy, environmental and recreational interests.
A legal requirement to study the experiment’s possible effects before getting Interior Secretary Deb Haaland’s approval caused enough delay to give the bass a short head start.
Studies show that smallmouth bass spawn when water reaches 16 degrees Celsius. The approved experiment in cooling calls for the dam to release water through the bypass tubes when temperatures at the Colorado’s confluence with the Little Colorado — some 76 miles downstream, and a major humpback chub rearing area — reaches 15.5 degrees Celsius for three days.
A water temperature model had suggested that wouldn’t happen until mid-August, but this year it happened during the week of June 23, according the Bureau of Reclamation. The secretary OK’d the program on July 3, and the cold water started flowing six days later.
So far, various agency biologists report, there’s no evidence that the bass were able to capitalize. Fish-trapping trips will continue through fall, and the committee will then consider whether to recommend a second year of cooling flows for next year.
Non-native species: As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion
How non-native fish found their way downstream
Water temperature has been a point of worry for protecting Grand Canyon’s native fish, including the humpback chub, ever since Glen Canyon Dam began impounding river water in Lake Powell in 1963. For years, the fear was that by drawing relatively deep and cool water through its power plant, the dam harmed native fish by year-round chilling of a river that had previously been flashy, flush with cold snowmelt in springtime but warm during the summer and fall.
Some native fish disappeared from the Canyon, including the salmon-length Colorado pikeminnow. But the chub, a smaller, silvery eater of insects and small fish, found refuge and spawning habitat in the Little Colorado.
More recently it has proliferated in the Colorado mainstem, where it warms farther from the dam in western Grand Canyon, enough so that the U.S. Fish and Wildlife Service upgraded the species’ status from endangered to threatened.
As the warming climate dried the mountain streams that supply the Colorado and Lake Powell, though, it seems the river may have warmed too much for the chub’s own good. As more than two decades of drought drained much of Lake Powell’s capacity, the water line plunged closer to the hydropower intakes. That brought both the warm surface layer and the warm-water invaders swimming in it closer to those intakes.
Non-native bass, stocked in Lake Powell for decades as sport fish, were known to have occasionally made it through the dam before, but until 2022 they were not known to have subsequently spawned in the river below. Now the dam’s warming outflows created welcoming conditions for them and their young.
To cool the river and restore a buffer for the chubs downriver, Reclamation is mixing deeper outlet tube water with some of the warmer water now passing through the power turbines. To minimize the economic harm, that only happens outside of peak power demands in the late afternoon and evening hours.
So far, according to Brian Sadler with the Western Area Power Administration, it’s costing $5 million or more per month in power that the organization must purchase elsewhere on the grid. If the experiment lasts as expected into October, he said, it will have cost $15 million to $20 million, not counting potentially higher prices that individual utilities may pay when bidding for the same power the Western Area Power Administration distributes to its customers.
The long-term costs worry Leslie James, who represents the Colorado River Energy Distributors Association on the committee. Losing potential hydropower year after year to cool the river for fish would cost utilities that rely on the power, including rural and tribal providers. Better to make physical changes such as the slough alteration that the government is currently considering, she said.
The slough in question, about 3 miles downstream from the dam, provides a warm-water nursery that connects to the river but holds still water behind a sandbar. The Interior Department is studying the possibility of dredging a river channel through that sandbar as early as this winter to allow the Colorado’s flow to better disrupt spawning beds there.
The cost, estimated at $26 million, sounds high except when compared with continuing bypass flows that reduce power production, James said.
“Flow options cannot be the only alternative,” she said.
Besides, James said, she believes it’s too early to call the so-called “cool-mix” flows a success.
“I’m very glad they haven’t found evidence of spawning,” she said. “Can the scientists say for sure that’s the result of the experiment? No.”
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An ‘existential threat’ to native species
Others on the committee say it will take multiple measures, including flow changes, to protect native fish and ultimately prevent even costlier changes that the Endangered Species Act could trigger.
“This is an existential threat to the native fish,” Grand Canyon National Park Superintendent Ed Keable said.
Some Native tribal officials remain troubled by some components of the bass-control program, such as the electrofishing rigs with which biologists stun and then remove and kill non-native fish. The cooling program, though, has broad support.
“It’s the more ethical approach,” said Erik Stanfield, the Navajo Nation’s representative on the committee.
Serendipitously for sport anglers, the cooling program may aid in recovery of the one-time trophy rainbow trout fishery that gained notoriety after Glen Canyon Dam cooled and cleared the river around Lees Ferry. That fishery has suffered in recent years as the river warmed and consequently lost oxygen. The cool mix is reversing that damage this summer.
“It’s providing good, cool water and dissolved oxygen that trout like,” said Jim Strogen of Trout Unlimited.
Rainbow trout are, like the bass, non-native in the Colorado, but they are considered less of a threat to native fish. (Brown trout are more of a threat to eat chubs, and a separate program pays cash incentives to anglers who catch and keep them.)
While officials and advocates hope the cold water continues to block a larger bass invasion into Grand Canyon, other threats loom ominously in Lake Powell. The reservoir holds other non-native predators that have devoured chubs and other native fishes on the Upper Colorado, upstream of Powell. Some of them, like the walleye, have been spotted below Glen Canyon Dam but are not known to have reproduced there.
Such threats may warrant a costlier barrier above the dam in coming years, and Interior has begun studying the options. Meantime, Grand Canyon Wildlands Council ecologist Larry Stevens said, diverting cold water downstream is the river’s best medicine, despite the cost.
Stevens described floating on the river more than 50 miles downstream of the dam this year and glancing down to see an adult walleye swimming there.
“To randomly see one out of the blue, just looking at the river, tells me we’re well on the way to a walleye invasion,” he said. “It’s hugely concerning.”
Brandon Loomis covers environmental and climate issues for The Arizona Republic and azcentral.com. Reach him at brandon.loomis@arizonarepublic.com.
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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This article originally appeared on Arizona Republic: Federal experiment appears to block Grand Canyon bass spawn for now
