This article originally appeared in High Country News.
In July 2022, National Park Service biologist Jeff Arnold made a disturbing discovery while hauling nets through a slough off the Colorado River. He captured three juvenile smallmouth bass, voracious predators that have invaded western waters. It was a grim realization that these bass, typically found in the eastern United States, had likely begun breeding in the Colorado River, including the Grand Canyon.
This development had long been feared by scientists. Over the past two decades, as Lake Powell has shrunk due to overallocation and drought, smallmouth bass have found prime conditions to infiltrate the Grand Canyon. The effects of this invasion will be difficult to prevent and could compete with the many other functions of the Colorado River.
The Colorado River supports around 40 million people, including ranchers, residents, and farmers. It is also crucial for 14 native species of fish, four of which are federally listed as threatened or endangered. Lake Powell, created by the Glen Canyon Dam, has disrupted natural flooding and erased spawning grounds for native fish. Invasive species like smallmouth bass have devastated fish populations in tributaries such as the Yampa River.
Although smallmouth bass have been present in Lake Powell since 1982, Glen Canyon Dam prevented them from reaching the Grand Canyon. However, as Lake Powell has dwindled, warmer water and closer proximity to the dam have made the Grand Canyon more hospitable for bass. Researchers discovered bass in the Grand Canyon below the dam, and their numbers continue to increase.
Preserving native species in this broken ecosystem poses a challenge for river managers. The Bureau of Reclamation has released a draft environmental assessment proposing options to deter smallmouth bass, including manipulating river flows and releasing cold water to prevent spawning. However, these options could conflict with the bureau’s other goals and have financial implications for hydroelectric power generation and ratepayers.
It has become clear that decisions about water storage in the system will determine the fate of native fish. Filling Lake Mead while allowing Lake Powell to shrink could lead to warmer water temperatures throughout the Grand Canyon and make invasive fish control more difficult. Scientists and environmentalists must grapple with these complexities to find solutions that protect both native fish and the diverse interests and needs of the Colorado River’s users.
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