The Colorado River has been called the American West’s hardest-working river. Lately, it’s been earning overtime.
The river supplies water to some of the country’s largest cities, including Los Angeles and Las Vegas, as well as its most fertile swath of farmland, California’s Imperial Valley. Forty million people in seven states rely on the Colorado every day, and each year six million more visit its most magnificent stretch, the Grand Canyon.
But many non-human creatures also depend on the Colorado watershed, most of all the strange, hardy fish that prowl its turbid depths. This is the domain of the bonytail chub, the razorback sucker, and the Colorado pikeminnow, a six-foot-long predator that early anglers caught by tying fishing lines to their truck bumpers. The lower Colorado has the highest ratio of endemic fish in North America—meaning that six of its eight native species exist nowhere else on Earth.
The best-studied member of the river’s distinctive ecosystem is the humpback chub, a creature as bizarre as fish come. Gila cypha is a silvery, foot-long member of the minnow family that has big orange fins and a fleshy, ridge-like protuberance along its back—its mysterious hump. If you crossed a carp with a bison, the humpback chub is what you’d get.
“They’re a unique part of the Grand Canyon, just like the rocks and the springs and everything else that makes this place special,” says Brian Healy, lead fish biologist for Grand Canyon National Park. “They evolved here over millions of years.” During the 1800s, one gold prospector in the canyon reported that you “could pull them out by the tail two at a time.”
The 20th century, however, wasn’t kind to the humpback chub. Fisheries managers stocked non-native trout and bass and encouraged fishermen to kill chub and other “trash fish,” so-called because of their perceived lack of value as sportfish. Gargantuan federal dams, such as the Hoover Dam, the country’s largest, drowned many of the fast, rocky river stretches that chub need to survive. In 1967 the government declared the chub an endangered species, a measure that failed to halt its spiral toward extinction.
But in recent years, chub numbers have begun to tick upward again, thanks partly to conservation efforts, such as translocating fish into productive tributaries unblemished by dams. Some 12,000 adult chub now live in the Grand Canyon stretch of the Colorado, and several thousand more are scattered across four populations upriver.
In a 2018 study, the Fish & Wildlife Service found that the humpback chub had improved enough to qualify as a threatened species, meaning it was no longer at imminent risk of extinction. (Read how the Grand Canyon stretch of the Colorado River is one of the most endangered in the U.S.)
After three years of review, the agency finalized that rule on October 15. It has also proposed taking the same action for the razorback sucker, listed as endangered since 1991. Together these rulings suggest that the Colorado River’s beleaguered ecosystem is heading in the right direction, validating decades of research and conservation.
“To see its population status improving in a culturally significant place is a real positive,” says Gloria Tom, director of the Navajo Nation Department of Fish and Wildlife, which manages land on which the humpback chub spawns.
Yet the chub’s future isn’t secure. The federal government recently declared the Colorado Basin’s first-ever water shortage, triggering cuts for certain states, such as Arizona—even as others, such as Utah, explore the possibility of using more. According to one projection, climate change could cut the Colorado’s flows in half by century’s end. (Read more about the megadrought that has hit the U.S. water supply.)
“It’s still a precarious picture for the humpback chub,” says Taylor McKinnon, senior public lands campaigner at the Center for Biological Diversity, a nonprofit based in Arizona. “I’m far from convinced that it’s out of the woods.”
How the humpback chub was dammed
Before humans intervened, the Colorado River was prone to wild fluctuations, from a raging torrent during spring run-off to a trickle in summer. Some 65 million tons of sediment—the liquefied stuff of the West’s mountains and deserts—flowed through the Grand Canyon each year. The humpback chub evolved to thrive in this volatile environment, in part, many scientists believe, because of its namesake hump. Some suggest that the bulge acts as a keel-like stabilizer, steadying chub during spring floods. Others hypothesize that it makes the fish harder for predatory pikeminnows to gulp down.
Beginning in the 1930s, dams altered the river. A series of massive concrete walls—designed to control floods, store water, and churn out hydroelectricity—cleaved the Colorado into a chain of sluggish reservoirs unfit for chub, which prefer swift currents and spawn in rocky-bottomed stretches of river. The Hoover Dam, which straddles the border between Nevada and Arizona, and the Flaming Gorge Dam, in Utah, wiped out nearby populations. (Read how scientists are saving the Colorado, one habitat at a time.)
In the protected confines of Grand Canyon National Park, the chub endured. But even there, dams afflicted the fish. In 1963, the government completed construction on Arizona’s Glen Canyon Dam, upstream from the Grand Canyon, creating massive Lake Powell. The lake transformed the canyon below, to the chub’s detriment. Humpback chub need warm water (at least 60 degrees Fahrenheit) to grow and spawn, but the water that passed through Glen Canyon Dam drew from the chilly depths of Lake Powell—around 20 degrees colder than the natural river. The frigid flows delayed spawning and stunted the chub’s growth, leaving young fish vulnerable to hungry trout.
The dam also starved the river of sediment. Chub once sheltered in calm backwaters behind the canyon’s sandbars, which formed and shifted as the river deposited silt during floods. But Lake Powell stymied floods and intercepted silt, wiping away beaches and sandbars—and depriving chub of crucial habitat. (Can the Colorado River keep on running?)
Over time the humpback chub split into five small, isolated populations. The largest group congregated around the Little Colorado River, a tributary that meets the mainstem Colorado in the Grand Canyon. Four smaller populations also hung on in the upper Colorado and Utah’s Green River. But these lonely clusters continued to dwindle. By 2002 just 10,000 or so humpback chub remained in the wild.
Regaining a finhold
During the 2000s, biologists with the U.S. Fish and Wildlife Service and other agencies began to take a more active role in the fish’s recovery. In 2003, the service started moving chub from the Little Colorado River into Grand Canyon tributaries by helicopter, hoping to establish new populations as insurance against extinction. Relocated chub now are breeding in the upper Little Colorado River, Havasu Creek, and Bright Angel Creek—warm, undammed tributaries that offer prime growing conditions for the fish.
“If they can grow to adulthood faster, they’re going to theoretically survive predation better and contribute at a higher rate to the population,” says Mike Pillow, a biologist with the wildlife service in Arizona. “We look at translocation as a head start.”
The government also tweaked operations at Glen Canyon Dam. Beginning in 1996, the Bureau of Reclamation, the agency that manages the dam, began to unleash sporadic “high flow experiments”—pulses of water intended to mimic historic floods. The sudden flows, and the sediment they deposited, were intended to build more camping beaches for rafters. But chub also may have benefited from the protection provided by new sandbars and backwaters.
Perhaps the biggest boost came from an unexpected ally: climate change. Since 2000, the Colorado River has been stricken by drought, a climate-fueled disaster that has cut flows by nearly 20 percent—and, luckily for chub, warmed the water from Lake Powell. What’s more, Lake Mead, the reservoir behind Hoover Dam, has shriveled to just 34 percent of its capacity. Chub have taken advantage of reemerging sandbars and eddies, and exposed rapids have blocked invasive predators from swimming upstream, creating a refuge for native fish.
More threats loom
Though the chub’s downlisting from endangered to threatened seems unambiguously good news, signs of trouble linger.
For one thing, humpback chub continue to struggle north of the Grand Canyon. One cluster of fish that lives in Black Rocks, a deep gorge in Colorado, consists of only 450 adult fish—perhaps half as many as two decades ago. Other populations have stabilized, but failed to increase.
Voracious invasive fish such as walleye, northern pike, and smallmouth bass dominate upriver, preying on juvenile chub. And these invasives could become more widespread as climate change further warms the river. (Learn how freshwater fish populations have declined worldwide.)
In the Grand Canyon, the chub’s historic stronghold, non-native brown trout and green sunfish are already on the rise. The same warm flows that are helping chub may eventually assist the arrival of more invasive predators. “If we have more dry years, there’s a higher probability that warm-water non-native fish squeak through the dam and establish spawning populations in the park,” says Grand Canyon National Park’s Healy.
The Colorado’s warm, dry future makes the picture especially murky.
To ensure that humpback chub have a future, lawyers, engineers, and politicians must broker solutions in collaboration with fish biologists and other researchers, says Jack Schmidt, a river scientist at Utah State University, in Logan.
Although the well-being of the Colorado River’s finned denizens has been an afterthought in the past, measures to promote their survival must be included in future water deals, Schmidt says.
“All of the major policy decisions and heavy negotiations that lie ahead will have major implications for the watershed’s ecosystems,” Schmidt says.
That could mean manipulating dam releases to maintain suitable water temperatures, or engineering more stable flows to promote aquatic insects. But these actions may be a tough sell as climate change heightens competition for increasingly scarce water in the hardworking Colorado.
“We’re trying to have a balance between providing power, water, recreation, and fish habitat,” says Mark McKinstry, a biologist at the Bureau of Reclamation. “When you start tilting the balance, then the other uses may suffer.”