The Fight Against the Smallmouth Bass Invasion of the Grand Canyon

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 US 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.”

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