It’s not too late to stop a bass invasion in the Colorado River

This article was originally featured on High Country News.

On July 1, 2022, a National Park Service biologist named Jeff Arnold was hauling nets through a slough off the Colorado River, several miles downstream from Glen Canyon Dam, when he captured three greenish fish lined with vertical black stripes. He texted photos of his catch to colleagues, who confirmed his fears: The fish were smallmouth bass, voracious predators that have invaded waters around the West. Worse, they were juveniles. Smallmouth weren’t just living below the dam—they’d likely begun to breed. 

It was a grim discovery. Smallmouth bass, whose native range encompasses rivers and lakes in much of the Eastern United States and Great Lakes, have long plagued the Colorado River. State agencies and anglers probably began stocking them in the watershed in the mid-1900s, and they’ve since conquered much of the basin, including Lake Powell, the reservoir that sloshes above Glen Canyon Dam. Downriver from the dam, however, lies the Grand Canyon, whose sandstone depths have historically provided a bass-free haven for native fish—most of all, the humpback chub, a federally threatened species endowed with an odd dorsal bulge. Now, biologists realized, neither the canyon nor its chub were safe.

Scientists have long dreaded this development. As Lake Powell has shrunk over the past two decades, drained by overallocation and chronic drought, its diminishment has created prime conditions for bass to infiltrate the Grand Canyon. But Brian Healy, a postdoctoral researcher at the U.S. Geological Survey and Grand Canyon National Park’s former fish biologist, said that even though he and his colleagues expected the species to eventually become a problem, “we didn’t realize it would be an issue so quickly.”

Preventing a bass takeover won’t be simple, biologically or politically. The Colorado’s users expect it to simultaneously serve as a pipeline for water conveyance, a source of cheap electrons, a recreational playground, and, not least, suitable habitat for native fish. For decades, the river’s human managers have uneasily balanced these often contradictory purposes—and now they must also work to exclude smallmouth bass, an immense challenge that may well compete with the river’s many other functions. “The best way to think about this is that everything in the Colorado River is connected to everything else,” said Jack Schmidt, a watershed scientist and emeritus professor at Utah State University’s Center for Colorado River Studies. “Everything has a ramification.” 

FORTY MILLION PEOPLE rely on the Colorado River’s largesse, from Wyoming ranchers to the residents of sprawling Arizona subdivisions to the lettuce farmers in California’s Imperial Valley. Less visibly, the river is also a lifeline for 14 native species of fish. They are rarely seen by humans—the river they inhabit is as turbid as coffee and they’re rarely fished for sport—yet they require a healthy Colorado as much as any Angeleno or Tucsonan. 

Today, however, four of those fish—the humpback chub, the Colorado pikeminnow, the razorback sucker and the bonytail—are federally listed as threatened or endangered. Lake Powell commandeered the Colorado’s payloads of silt and stymied natural floods, erasing channels and backwaters where chubs and suckers once spawned and reared. And smallmouth bass and other invasive species devastated native fish in tributaries like the Yampa River. (“Smallmouth” is a misnomer: Bass have maws so cavernous they can gulp down prey more than half their own size.) Bass arrived in Lake Powell in 1982, courtesy of a hatchery manager who, on a lark, dumped 500 spare smallmouth into the reservoir. The bass, he crowed decades later, “performed magnificently,” adding, “Anglers have caught millions of smallmouth bass over the past 30 years.”

Through it all, the Grand Canyon remained a bass-less sanctuary—thanks, paradoxically, to Glen Canyon Dam. Although smallmouth teemed in Lake Powell, they stayed in the reservoir’s warm, sunlit upper strata, well above Glen Canyon Dam’s penstocks, the massive tubes that convey water through its hydropower turbines and thence downriver. Bass never reached the Grand Canyon because they never swam deep enough to pass through the dam.

As Lake Powell withered, however, so did the Grand Canyon’s defenses. By the spring of 2022, two decades of climate change-fueled drought had lowered the lake’s surface by more than 150 feet, drawing its tepid, bass-filled top layer ever closer to the penstocks. At the same time, the warmer water flowing through the dam and downstream made the Grand Canyon more hospitable to bass. “The temperature was ideal for them,” said Charles Yackulic, a research statistician at the U.S. Geological Survey.

Last summer, after bass swam through Glen Canyon Dam’s penstocks, slipped past its whirling turbines, and apparently reproduced, managers hastened to control the incipient invasion, netting off the slough where Arnold discovered the juveniles as though it were a crime scene. The Park Service also doused the backwater with a fish-killing poison. When biologists electroshocked the river that fall and the following spring, though, they found hundreds more juveniles. The slough wasn’t an isolated beachhead; it was merely a battleground in a broader invasion.

If there is a saving grace, it is that the bass remain concentrated above the cold, clear stretch of river known as Lees Ferry. Humpback chub, by contrast, have their stronghold deep in the Grand Canyon, some 75 miles downriver from the dam, where bass haven’t shown up—at least not yet. “The worry is that you got them in Lees Ferry and they’re reproducing,” Yackulic said. “And then suddenly, you’ve just got all these babies dispersing downstream.”

THE COLORADO RIVER is at once in a state of crisis and rebirth. The decline of Lake Powell has revealed Glen Canyon, the gorgeous red-rock labyrinth that the reservoir drowned in the 1960s. Ironically, the forces behind this restoration are also imperiling native fish. “Last year was the closest we’ve had to a natural thermal regime in more than 50 years,” Yackulic noted. But for the humpback chub, it was a catastrophe.

River managers thus face a conundrum: How do you preserve native species in a broken ecosystem? In February 2023, the Bureau of Reclamation, the federal agency that controls Glen Canyon Dam, released a draft environmental assessment evaluating four options for manipulating river flows to deter smallmouth bass. The plans are variations on a theme: When the Colorado gets dangerously warm, the agency  releases cold water to lower its temperature below the threshold where bass spawn. Two options—favored by conservation groups like the Center for Biological Diversity—include high-intensity “flow spikes” designed to freeze bass out of sloughs and backwaters. “We need flows that are cold enough for long enough that it prevents smallmouth bass from spawning,” said Taylor McKinnon, the center’s Southwest director. “Not disrupt reproduction—prevent reproduction.”

Managing the Colorado River to thwart bass, however, could conflict with Reclamation’s other goals. For one thing, all four options would release water through Glen Canyon Dam’s “bypass tubes,” outlets closer to Lake Powell’s frigid bottom. But the bypass tubes, as their name suggests, don’t pump water through the dam’s hydroelectric turbines — which, as the agency acknowledges, could lead to “a reduction in the revenue generated from power proceeds.” That possibility doesn’t thrill the Colorado River Energy Distributors Association, which represents electric utilities and co-ops and has warned of “measurable financial impacts” to ratepayers.

Some environmentalists may find themselves at odds with bass deterrence, too. For years, the Glen Canyon Institute has called on river managers to “Fill Mead First,” letting Lake Powell shrivel while sending Colorado’s water downstream to Lake Mead, the river’s other massive reservoir. As scientists pointed out in a 2020 paper, however, this strategy could “lead to warmer water temperatures throughout Grand Canyon” and render invasive fish control “especially problematic.” Indeed, if your sole goal were to protect humpback chub in the immediate term, Lake Powell—whose deep, chilly waters staved off bass for 40 years—might be the first reservoir you’d fill. “The decisions of where you store water in the system are going to determine the fate of native fish,” said Utah State’s Schmidt.

Although last winter’s strong snowpack should ultimately raise Lake Powell’s surface by around 70 feet, the invasion continues. Scientists have so far pulled 667 bass from the slough this year, along with thousands of carp and sunfish, two other warm-water nonnatives. The Park Service poisoned the slough again in late August, but that fix is clearly neither complete nor lasting. In February 2023, a group of researchers convened to study the bass problem by the Bureau of Reclamation and U.S. Geological Survey recommended outfitting Glen Canyon Dam with “fish exclusionary devices”—basically fancy nets—to keep bass from swimming through the penstocks. That’s hardly a new idea—biologists first recommended that the Bureau “pursue means” of preventing invasive fish from passing through the dam in 2016 —but, at an August meeting of federal managers and researchers, one Reclamation official claimed that an effective screen design is still at least five years away.

Ultimately, staving off the bass crisis may call for even more ambitious fixes. In one paper, Schmidt and his colleagues raised the idea of drilling colossal diversion tunnels that would funnel water and sediment around Glen Canyon Dam and thus restore the silty, flood-prone conditions that favor native fish. Re-engineering the Colorado would be neither simple nor cheap, but, in recent comments to the Bureau, McKinnon and other conservationists claimed that the “climate-inevitable obsolescence” of Glen Canyon Dam calls for drastic measures. If bass take over an ever-warmer river, McKinnon said, “it’s game over.”

Ben Goldfarb is a High Country News correspondent and the author of Eager: The Surprising, Secret Life of Beavers and Why They Matter. His next book, on the science of road ecology, will be published by W.W. Norton in 2023.