As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion

GLEN CANYON DAM — The day’s first gillnet haul of nonnative fish on lower Lake Powell was already alarming: three striped bass, three gizzard shad and a channel catfish.

Any one of them or their offspring would be unwelcome intruders were they to slip through the massive concrete dam’s hydropower tubes and turbines to swim a few dozen miles downstream into the heart of Grand Canyon.

Above Glen Canyon Dam, state fisheries managers in decades past would introduce alien species to support recreational angling on the lake. But below the dam, the fish could drift downstream to eat or outcompete Grand Canyon’s threatened humpback chub population, swelling the ranks of nonnatives that biologists are already battling.

The threat grows each day that the reservoir’s surface slips down the orange-and-white sandstone cliffs and closer to the dam’s intakes, a decline linked to the region’s aridification and overuse of the Colorado River. The Southwest's thirst for the drying river is pushing a challenged aquatic environment further out of whack.

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It was the second net that Utah State University researchers pulled aboard their boat that June morning that pointed to the most feared horde massing above the dam for invasion. Two smallmouth bass — one 9 ½ inches and the other 13 — dangled among four other fish snared in the webbing. These are the voracious fish eaters that have sent federal biologists scrambling to protect the native chubs that only last fall were upgraded from endangered to threatened because of their surging numbers in the Grand Canyon.

Each net measured 80 feet long and 6 feet deep and was left out overnight, tied to an anchored buoy or to a bolt in the canyon walls and weighted at bottom so it sloped downward to avoid entanglements with anglers or the personal watercraft jetting past. The third haul of the day yielded 16 nonnatives, three of them smallmouths.

They represented a preternatural danger lurking just above one of the so-called Seven Natural Wonders of the World.

"Smallmouth bass will completely change not just chub, but also the ecology," said Charles Yackulic, a researcher with the U.S. Geological Survey's Southwest Biological Science Center. "Part of the heritage of Grand Canyon is the ecosystem."

Utah State's netting study, to continue next year, is supported by the U.S. Bureau of Reclamation, which manages the dam and seeks to learn which species are poised to pour through it if the lake keeps dropping. Biologists and others have observed a few smallmouth bass, stripers and other warm-water invasive species slipping past the dam over the last decade. Fear of the fish multiplying there has never been greater than this summer, when the river's temperatures are rising in tandem with the fall of its supply.

Paul McNabb, The Republic's tour guide, holds a smallmouth bass, while Utah State University researchers Barrett Friesen (right) and Justin Furby (left) record fish species during a survey, June 9, 2022, above Glen Canyon Dam (back left) on Lake Powell, near Page. With lake levels dropping, biologists fear bass and other nonnative fish will slip through the dam, swim downstream toward the Grand Canyon and eat or outcompete Grand Canyon's threatened humpback chub population.

Changing an already-changed environment

The growing menace is a reminder that the Colorado’s accelerating decline will take bites out of more than just urban water supplies and irrigated farmlands, but from the river itself.

The warming Rocky Mountain West’s yearly surge of snowmelt has shrunken over the last two decades, driving down Lake Powell’s surface by 143 feet since July of 2000, and 70 feet in the last two years alone. The plunge has driven the warm band of surface water that the introduced sportfish like bass favor to within a few dozen feet of the dam’s hydropower intakes, setting up what could become a steady stream of bass to mar Grand Canyon’s already dam-warped environment.

The humpback chub is one of three chubs that evolved in the warmer and muddier water that coursed through Grand Canyon before the dam brought cooler and clearer water, and is the only one still swimming there. It once faced frigid waters only in winter and spring, then bathed in 80 degrees or more during spawning season. The dam's year-round chill stressed the species, but it has persisted.

The Colorado pikeminnow, a salmon-sized predator once it reaches age 7 or so, disappeared below the dam, though it and the other chub species still swim waters above Lake Powell. Other Grand Canyon dwellers at risk from an invasion include native bluehead and flannelmouth suckers and tiny speckled daces.

While rumors of anglers spotting or hooking smallies below the dam were already swirling on the day Utah State grad student Barrett Friesen pulled his nets from the reservoir, he said it would take more than that — sustained bass reproduction downstream — to set off an emergency. The relatively cold water gushing through the dam and into the river could inhibit the warm-water species’ fertility. If and when the full-scale invasion came, though, it could be unstoppable.

“It’d be pretty terrible,” Friesen said. “It would likely entail a costly suppression campaign.”

Three weeks later, beginning on July 1, the National Park Service scooped three juvenile smallmouth bass from a riverside slough several miles below the dam, confirming at least their presence and suggesting their local reproduction. The agency announced that it was working with partners to develop a rapid response in hopes of preventing permanent establishment.

“That’s the best chance you have,” said Kevin Bestgen, a Colorado State University aquatic ecologist who has studied and battled bass throughout the Colorado and Green rivers upstream of Lake Powell. Once the invaders become entrenched, the battle bogs down into permanent suppression rather than eradication.

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Invasive fish threaten a recovering chub population

Bass that spilled out of smaller reservoirs have chomped through native species for years above Lake Powell on the Colorado and its largest tributary, the Green River. The damage might have kept federal officials from removing humpback chub from the endangered species list if Glen Canyon Dam had not, until now, shielded a growing Grand Canyon population from predators. In Grand Canyon National Park’s Little Colorado River, for instance, a chub population that numbered 2,000 to 3,000 in the early 1990s grew to 12,000 by last year.

The Little Colorado meets the Colorado less than 60 miles downstream from where young bass were confirmed this month. Its water, spilling from Arizona’s Painted Desert without welling up behind a dam, provides a warm breeding ground. Chubs, like bass, historically bred in warmer water than what has flowed down the Colorado from Glen Canyon Dam year-round since the dam's 1963 completion.

The mainstem Colorado in the western end of the Grand Canyon, farther downstream from the dam, also has seen a rebounding chub population. While the river closer to the dam may be too cold for bass breeding, it could easily push live bass to these and other warmer backwaters and tributaries along the way. After all, Bestgen said, bass evolved to endure waters that ice over in winter.

Justin Furby (left) and Barrett Friesen (right, both Utah State University researchers) pull a gillnet while working on a survey above Glen Canyon Dam, June 9, 2022, on Lake Powell, near Page.
Justin Furby (left) and Barrett Friesen (right, both Utah State University researchers) pull a gillnet while working on a survey above Glen Canyon Dam, June 9, 2022, on Lake Powell, near Page.

Officials already were battling invasive species below the dam, suppressing green sunfish that apparently swept through the turbines, and rewarding anglers with a bounty on brown trout. The appearance of a few young bass in the slough was not surprising, Yackulic said, especially since the river's warming has created a more welcoming environment. He'll be more alarmed, he said, if and when more are found in other locations around Lees Ferry, indicating breeding success by more than one couple.

This July the river below Glen Canyon is running about 64 degrees, he said, compared to the season's normal reading of about 50. Unless a prodigious snow year props up Lake Powell, he said, further declines in the reservoir will produce still warmer water and likely enable a larger invasion in future summers.

Nonnative predators also swim downstream of Grand Canyon in the river's largest reservoir, Lake Mead. The relatively silty waters that flow from the lower canyon into the lake may have impeded their upstream progress. In recent years, Lake Mead's own plunge in response to drought and overuse has left a waterfall as a fish barrier at Pearce Ferry.

A 'forever war' against non-native fish

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.

On the Green and the upper Colorado, upstream of Powell, biologists are fighting a forever war against smallmouth bass. From boats, they pulse electric shocks into the water to stun fish that then float to the top, allowing the biologists to net and remove the bass while leaving the others to recover. This method captures a fraction of the bass, helping to keep their population from exploding.

They’ve also reported success timing cold bursts of water from dams such as Flaming Gorge to flush bass eggs and young from their nests and protective parents. That method, though, requires abundant water, something that is increasingly scarce and coveted by other interests as Lake Powell’s surface edges closer to the level where the power plant will have to stop operating. The reservoir, essentially full in 2000, now holds only a quarter of its capacity.

Bestgen participated in a 2008 study of nonnative fish on the Yampa River, a Green River tributary, that suggested bass had the potential to hammer native fish more than two other invaders that biologists were already trying to remove there: northern pike and channel catfish.

By examining the predators’ stomachs, the researchers found that pike at that time consumed the most bonytail chub, Colorado pikeminnow and other natives, while the catfish ate few. The smallmouth bass, newer to the river, favored crayfish half of the time, though the team concluded that was likely because fish small enough for them to fit in their mouths were already badly reduced in number.

Justin Furby (left) and Barrett Friesen (right, both Utah State University researchers) pull a gillnet while working on a survey above Glen Canyon Dam, June 9, 2022, on Lake Powell, near Page.
Justin Furby (left) and Barrett Friesen (right, both Utah State University researchers) pull a gillnet while working on a survey above Glen Canyon Dam, June 9, 2022, on Lake Powell, near Page.

Elsewhere in the upper Colorado River basin, the researchers found, the availability of smaller fish led the bass to switch their diets to fish. When appropriately sized prey is available, the researchers’ modeling predicted, smallmouth bass could consume 10 times the fish of either pike or catfish, because of their own relative abundance.

The “smallmouth” moniker offers little comfort to those worried about Grand Canyon’s humpback chub, which take two years to reach an adult length of 8 inches or more.

“We found adult bass can consume up to 68% of their length, so a 12” bass could eat an 8” chub,” Bestgen said in an email describing another study, this one in the Green River. “This is possible because bass can swallow a fish and have the tail protruding from its throat or mouth and be just fine.”

In the week following confirmation of juvenile smallmouth bass below Glen Canyon Dam, the National Park Service and partners placed a 100-foot-long net across the slough where the fish swam to keep any others from escaping into the main river channel. They also installed hoop nets and minnow traps there in hopes of gathering any others, according to a Grand Canyon National Park spokesperson.

Park biologists were conferring with interagency peers to consider other options for removal, and for keeping more bass from spilling through the dam. The latter could include deterrents such as air bubblers or acoustic devices intended to haze bass back up-lake from the dam.

Park officials considered the three young bass, each less than an inch long, to be recently hatched and likely evidence of reproduction below the dam. The river immediately downstream, sourced from deep in Lake Powell, is generally considered too cold for bass reproduction, though backwaters and shallow sloughs allow some of the water to pool and warm in the sun.

Countermeasures are time-consuming and costly

Before confirmation of juvenile bass, National Park Service biologists addressing a technical work group of the Glen Canyon Dam Adaptive Management Program this spring said they anticipated the movement of the fish based on declining water levels and rising water temperatures.

Besides nets, they said, one option could be electrofishing to shock and then scoop bass from the river. Another would be to dump chemical fish killers in sloughs and side channels, though that would require consulting Native American tribes with ties to the river. The biologists said the parks on their own did not have sufficient funding for those options this year, but would seek partners’ help.

Installing bubblers or other deterrents above the dam could require a couple of years of review, Yackulic said. He suggested a quicker option could be to start drawing water through the dam's bypass tunnels, which are deeper in Lake Powell than the hydropower intakes and would shoot colder water downstream to disrupt further bass reproduction. Such an option would reduce the dam's ability to crank out power, though, costing the government revenue and some Westerners higher electric bills.

Friesen, the Utah State biology student netting fish above the dam, said his catches so far this year suggest the threat of more bass passing through may increase as summer drags on.

In March, he was pulling bass from shallower waters near Wahweap Marina. But near the dam, 3 miles away, the canyon walls that encase a deeper water column were cold and bass-free.

By June, as summer began to warm the dam zone, he caught bass as deep as 30 feet there, roughly 40 feet above the level of the hydropower intakes, known as penstocks. As mountain meltwater begins to subside and summer irrigation demand ramps up, the warm surface layer will both expand and drop closer to those portals.

By August?

Justin Furby (Utah State University researcher) weighs a striped bass during a fish survey, June 9, 2022, on Lake Powell, near Page.
Justin Furby (Utah State University researcher) weighs a striped bass during a fish survey, June 9, 2022, on Lake Powell, near Page.

“These fish could be at the level of the penstocks,” queued to slip through, Friesen said.

Whatever happens to any invading bass currently in the slough downstream, reinforcements are on their way. Joining them might be the walleye, another nonnative sportfish that in recent years has been spreading and eating native Colorado River fish above Lake Powell.

Like smallmouth bass, adult walleyes have been spotted below the dam, though they are not yet known to have spawned there. If they do, Bestgen's experience with the walleyes spreading upstream from Lake Powell suggests they could eat just as many native fish as smallmouths.

"It's alarming because they have now invaded the very best (upper basin) habitat where very small pikeminnows hang out." Young chubs would also fit easily in the average walleye's mouth.

Friesen’s third net near the dam on that June day contained one 15-inch walleye. It weighed 1.2 pounds on an empty stomach. That's a keeper for some anglers, but almost 9 pounds lighter than Lake Powell's record walleye.

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: Smallmouth bass: The newest threat to Grand Canyon's endangered fish