A warming Colorado River grapples with invasive species

As climate change bakes the U.S. West and dries up key Colorado River reservoirs, a slew of invasive species is flourishing in warmer waters at the expense of the artery’s native inhabitants.

Among the most disruptive of this wide range of invaders is the smallmouth bass, which scientists fear could pose a possible danger to the native ecology of the Grand Canyon region should the fish continue making their way downstream.

Of particular concern is the fate of the humpback chub — a federally threatened species that has long inhabited the area.

Federal officials and scientific researchers are seeking ways to stop the invaders and stave off the harm they could bring. But the potential solutions come with their own concerns — and are provoking controversy throughout the region.

Warming waters fuel invaders

Until a few years ago, the Grand Canyon was largely dominated by native species, Brian Healy, a research biologist at the U.S. Geological Survey, told The Hill.

“That’s a really rare and unique thing in the Colorado River system,” added Healy, who previously served as a fisheries biologist for Grand Canyon National Park.

The Grand Canyon area, Healy explained, enjoyed “a little bit of a sweet spot” until conditions began to change in recent years. Colder temperatures first fueled the increase of non-native fish such as brown and rainbow trout, and a subsequent warmup helped the humpback chub regain strength.

“Now we’re getting to a point where it’s warm enough, which is good for native species, but it’s also good for a non-native species,” added David Rogowski, a Colorado River research biologist for the Arizona Game and Fish Department.

Smallmouth bass, which are aggressive predators, were first introduced to the Colorado River system when Utah began stocking them in the Lake Powell reservoir in 1982, according to a 2021 report from the state’s Division of Wildlife Resources.

The report touts the smallmouth bass’s propagation in Lake Powell as “an unequivocal success” that resulted in the rapid development of “a fantastic fishery.”

But that success, combined with the impacts of climate change, has also fueled an unforeseen domino effect that is now jeopardizing species down the line.

As Lake Powell’s levels have plunged, the reservoir’s upper layer — where the fish reside — has edged closer to the hydropower intakes of the Glen Canyon Dam, according to the Bureau of Reclamation. As a result, warmer waters, and the fish that populate them, have been able to pass through the system.

“I always had picked up a couple of fish every now and then, a smallmouth bass just below the dam,” Rogowski told The Hill. “But the water was consistently cool enough that smallmouth bass or warmwater fish species could not reproduce within that system.”

Today, however, Rogowski said that “the water coming through the dam is warm enough to support growth as well as reproduction.”

Efforts to combat invaders arouse controversy

Both this summer and lastlast, the National Park Service has taken preventative action to remove both smallmouth bass and green sunfish from a backwater area below the Glen Canyon Dam called the Colorado River Slough — by deploying a piscicide, or substance poisonous to fish, called rotenone.

The National Park Service stressed in an August statement that the treatment would occur in a manner that minimizes exposure of surrounding species to rotenone. In addition to being toxic to fish, the substance can also be a threat to people and other animals — though the World Health Organization only considers it a direct threat to humans in high concentrations.

“Should any rotenone enter the main channel, it will immediately be diluted to concentrations that are insignificant to wildlife or humans, due to the volume of flow in the Colorado River,” the agency stated.

Although such treatment would be ineffective in combating an entire population, targeting a spot where the invasive species are known to reproduce on a local level could be productive, according to Healy.

And the Colorado River Slough area is “pretty much the only place we know that they have been reproduced,” Rogowski added.

The Bureau of Reclamation, meanwhile, announced this week that it would be initiating a formal process to evaluate proposals that would shift flow velocity and temperatures from the Glen Canyon Dam itself with the aim of disrupting the invaders’ downstream reproduction.

The Glen Canyon Dam, a federal hydroelectric power facility, is located just downstream from Lake Powell in Arizona, across the border from Utah.

The warm but plunging upper layer of Lake Powell is now raising river temperatures, creating what federal officials describe as “ideal spawning conditions specifically for smallmouth bass.”

The bureau’s efforts to evaluate the problem and potential solutions involving the dam have proved controversial, and raised conflicting viewpoints from environmental groups and utilities.

An initial August 2022 environmental assessment from the agency, in which it investigated four different options to release cold water flows, generated nearly 7,000 public comments.

Among the responses were letters from environmental groups advocating for an option that would involve regular cool water releases, supplemented by high-flow cold water “spikes” in the summer.

The group Trout Unlimited described this plan as “the most viable and cost-effective solution for mitigating the impacts of smallmouth bass,” while minimizing harm to other dam operations.

Other environmental organizations have advocated for more drastic action.

Following the bureau’s formal launch of a much more comprehensive evaluation than the assessment last week, the Center for Biological Diversity said that this “generally good news” is coming “too little too late.”

“In the long run, the Grand Canyon’s endangered fish need river conditions similar to those that they evolved in,” Taylor McKinnon, southwest director for the center, said in a statement.

“The Bureau should plan a phased decommissioning of Glen Canyon Dam,” he added, accusing the bureau of “sleepwalking into climate-inevitable deadpool” — a situation in which a reservoir no longer has enough water. 

But the options detailed in the environmental assessment — let alone a full decommissioning of the dam — have garnered significant pushback from utilities that rely on its hydroelectric power.

The Colorado River Commission of Nevada raised concerns that the proposed options could decrease electricity production, because they require a partial bypass of the turbines. The group also expressed worry about knock-on effects to region-wide power prices.

Similar sentiments came from the Colorado River Energy Distributors Association (CREDA), which represents 155 consumer-owned electric systems that purchase federal hydropower.

A letter from CREDA, which serves 4.1 million consumers, criticized the assessment for failing “to meaningfully identify or analyze the affordability of replacement power.”

“Sleeper species” are biding their time

Smallmouth bass are hardly the only invasive species that have emerged across the length of the Colorado River Basin.

Also among the invaders are green sunfish and walleye, whose potential impacts Healy said remain unclear.

And about 600 miles northeast of the Glen Canyon Dam, Colorado Parks and Wildlife recently announced the discovery of an arthropod known as rusty crayfish, which had never been seen in the Upper Colorado River Basin.

The officials found the rusty crayfish in Lake Granby, a reservoir located near the Colorado River headwaters.

Rusty crayfish, native to the Ohio River Basin, are more aggressive than freshwater crayfish and are believed to have been illegally introduced into Colorado by anglers as bait, according to Colorado Parks and Wildlife.

A sampling and monitoring team, as well as area aquatic biologists, have since set multiple crayfish traps around the lake and in other waters nearby, per the agency.

“While finding any invasive species is detrimental to our state’s aquatic ecosystems, finding rusty crayfish in Lake Granby, which feeds into the Colorado River, poses an even greater threat to the entire Colorado River Basin,” Robert Walters, Colorado Parks and Wildlife invasive species program manager, said in a statement.

Other fish species of concern across the Colorado River system include the flannelmouth sucker, the bluehead sucker, the speckled dace and some razorback suckers, according to Rogowski.

Among plants, he identified the tamarisk, the Russian olive and a phragmite reed as non-native species that have become prolific. Rogowski also mentioned certain “chronic invertebrates,” such as the New Zealand mud snail, as well as the algae didymo — also known as “rock snot.”

As to when and where the invaders will appear, Healy said he believes that the species of most concern have probably already been introduced into the system.

“It’s just that the changing conditions might allow them to proliferate,” he said.

Describing these invaders as “sleeper species,” he explained that these organisms are likely present in low numbers and are biding their time.

“Shifts in the conditions could cause them to explode and become a real problem,” Healy said. “We just don’t really know yet, the extent of the issue.”