Today, nonetheless, 4 of these 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 pure floods, erasing channels and backwaters the place chubs and suckers as soon as spawned and reared. And smallmouth bass and different invasive species devastated native fish in tributaries just like the Yampa River. (“Smallmouth” is a misnomer: Bass have maws so cavernous they’ll gulp down prey greater than half their very own dimension.) Bass arrived in Lake Powell in 1982, courtesy of a hatchery supervisor who, on a lark, dumped 500 spare smallmouth into the reservoir. The bass, he crowed decades later, “performed magnificently,” including, “Anglers have caught millions of smallmouth bass over the past 30 years.”

Through all of it, the Grand Canyon remained a bass-less sanctuary—thanks, paradoxically, to Glen Canyon Dam. Although smallmouth teemed in Lake Powell, they stayed within the reservoir’s heat, sunlit higher strata, properly above Glen Canyon Dam’s penstocks, the huge tubes that convey water by way of its hydropower generators and thence downriver. Bass by no means reached the Grand Canyon as a result of they by no means swam deep sufficient to go by way of the dam.

As Lake Powell withered, nonetheless, so did the Grand Canyon’s defenses. By the spring of 2022, 20 years of local weather change-fueled drought had lowered the lake’s floor by greater than 150 toes, drawing its tepid, bass-filled prime layer ever nearer to the penstocks. At the identical time, the hotter water flowing by way of the dam and downstream made the Grand Canyon extra hospitable to bass. “The temperature was ideal for them,” stated Charles Yackulic, a analysis statistician on the US Geological Survey.

Last summer season, after bass swam by way of Glen Canyon Dam’s penstocks, slipped previous its whirling generators, and apparently reproduced, managers hastened to regulate the incipient invasion, netting off the slough the place Arnold found the juveniles as if it have been against the law scene. The Park Service additionally doused the backwater with a fish-killing poison. When biologists electroshocked the river that fall and the next spring, although, they discovered a whole lot extra juveniles. The slough wasn’t an remoted beachhead; it was merely a battleground in a broader invasion.

If there’s a saving grace, it’s that the bass stay concentrated above the chilly, clear stretch of river referred to as Lees Ferry. Humpback chub, in contrast, have their stronghold deep within the Grand Canyon, some 75 miles downriver from the dam, the place bass haven’t proven up—at the least not but. “The worry is that you got them in Lees Ferry and they’re reproducing,” Yackulic stated. “And then suddenly, you’ve just got all these babies dispersing downstream.”