Non-native smallmouth bass have been found in the Colorado River below Glen Canyon Dam/NPS

A scenario scientists feared has arrived at Grand Canyon National Park, where smallmouth bass have shown up, a non-native fish that somehow has passed through the Glen Canyon Dam to continue on down the Colorado River where it poses a serious threat to native humpback chub, a threatened fish.

Reaching the lower Colorado River was not an easy task for the bass. But as the level of Lake Powell behind the dam continues to fall due to reduced runoff, warmer water levels that the bass navigate brought them closer to the dam's penstocks, which funnel water to the turbines that generate hydropower. Apparently some bass survived being flushed through the turbines and were spotted by Jeff Arnold, Grand Canyon's fisheries biologist, while he was sampling the river's waters for fish species. 

Now there are fears that smallmouth bass might be breeding in the river below the dam, and that's bad for the native humpback chub that fisheries experts have been working to recover from low numbers. A native of the Colorado River, the chub last fall was "uplisted" from endangered to threatened under the Endangered Species Act.

The chub, named for the distinctive hump on its back, was first documented in the lower Colorado River Basin in the Grand Canyon in the 1940s and the upper Colorado River Basin in the 1970s. It was placed on the list of endangered species in 1967 due to impacts from the alteration of river habitats by large mainstem dams. This fish is uniquely adapted to live in the swift and turbulent whitewater found in the river's canyon-bound areas. The fleshy hump behind its head evolved to make it harder to be eaten by predators, and its large, curved fins allow the humpback chub to maintain its position in the swiftly moving current, according to the Fish and Wildlife Service.

Recovery efforts in the Lower Basin population have raised the estimated population to more than 12,000 individuals in the Little Colorado River and the Colorado River at their confluence. There also have been increasing densities in the Grand Canyon's western end due to the receding Lake Mead exposing river habitat. 

However, smallmouth bass eat humpback chub.

Dr. Jack Schmidt, professor of Watershed Sciences at Utah State University and head of the Center for Colorado River Studies, told the Traveler back in 2020 that climate change would open the door for invasive species in Lake Powell. 

“The most obvious invasives, that are a direct function of climate change and water supply management, are nonnative fish. There are two kinds: cold water, mainly trout, and warm water, mainly smallmouth bass, gizzard chad (Dorosoma). As things get warmer nonnative reservoir fish might come up and swamp the system," he said at the time.