Smallmouth bass expansion downstream of Glen Canyon Dam
In the Upper Colorado River Basin, smallmouth bass (Micropterus dolomieu) are considered the greatest threat to native fishes and have been linked to declines in federally protected humpback chub (Gila cypha), including one population that was rapidly extirpated in the past. Long-term management efforts have been underway to remove smallmouth bass from rivers in the Upper Basin, but smallmouth bass persist in these areas.
Dropping the Bass: A video about stopping the spread of smallmouth bass in Grand Canyon
Smallmouth bass are a warm water, nonnative fish species that have been stocked throughout the western United States for recreational fishing. This adaptable species reproduces easily, allowing them to proliferate and spread from their initial points of introduction and predate on native fishes.
Smallmouth bass were introduced into Lake Powell in the 1980s and now are one of the most abundant fishes in the reservoir. Until recently, smallmouth bass were extremely rare in Grand Canyon, downriver of Lake Powell. During much of the 21st century, natural runoff in the Colorado River basin has declined, while water use has remained relatively constant, leading to historically low reservoir storage. These low reservoir elevations:
- Increase rates of fish passage through dams due to decreasing depth to dam intakes, and
- Increase the temperature of dam releases, improving suitability for warm water nonnatives.
Smallmouth bass live in the surface waters of Lake Powell. Lower water levels mean that the surface is closer to Glen Canyon Dam’s turbines. In 2022, due to lower water levels, water from Lake Powell’s upper layer was released downstream into the Grand Canyon segment of the Colorado River, dramatically warming river temperatures and carrying smallmouth bass through the dam into the Colorado River. This led to the first records of reproduction of smallmouth bass below the dam.
The Grand Canyon segment of the Colorado River, located downstream of Lake Powell, is the stronghold for threatened humpback chub, which are recently experiencing some recovery success. However, if reservoir levels remain low for an extended period of time, smallmouth bass are likely to pose a continued threat to the recovery of humpback chub. To assist managers concerned about this invasion, researchers have developed models that
- Predict immigration rates at different reservoir elevations, and
- Link reservoir storage/operations, water temperatures, and smallmouth bass population dynamics to forecast population growth rates under different climate and management scenarios.
Researchers also assist managers to assess the effectiveness of various mitigation efforts to help humpback chub continue their path to recovery.
The USGS Grand Canyon Monitoring and Research Center models used by the Bureau of Reclamation for NEPA analyses can be accessed at:
In the News:
How a ‘cold shock’ of water from Lake Powell could thwart invasive Grand Canyon bass - Arizona Republic, March 2023
Plunging water levels push invasive bass toward Grand Canyon - Arizona Republic, March 2023
What’s the best strategy to keep this invasive fish upstream of the Glen Canyon Dam? - St. George News, March 2023
Officials fear ‘complete doomsday scenario’ for drought-stricken Colorado River - The Washington Post, December 2022
A shock of cold water could help control exotic fish in Grand Canyon - NPR KNAU Arizona Public Radio, October 2022
Electrofishing begins below Glen Canyon Dam to kill smallmouth bass - NPR KNAU Arizona Public Radio, October 2022
As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion - Arizona Republic, July 2022
Invasive smallmouth bass make it through Glen Canyon Dam into mainstem Colorado River, threatening native fish - NPR KNAU Arizona Public Radio, July 2022
‘Worst fears confirmed' in biologists' fight to save ancient Colorado River fish - USA Today, July 2022
Declining levels at Lake Powell increase risk to humpback chub downstream - The Aspen Times, June 2022
Biologists try to save ancient fish as Colorado River fades - USA Today, June 2022
Humpback Chub in the Colorado River, Grand Canyon
GCMRC modeling used in decision-making process for new Glen Canyon Dam Environmental Impact Statements
Grand Canyon Monitoring and Research Projects
The Impacts of Climate Change and Water Supply Management on Fish in the Colorado River
In the Upper Colorado River Basin, smallmouth bass (Micropterus dolomieu) are considered the greatest threat to native fishes and have been linked to declines in federally protected humpback chub (Gila cypha), including one population that was rapidly extirpated in the past. Long-term management efforts have been underway to remove smallmouth bass from rivers in the Upper Basin, but smallmouth bass persist in these areas.
Dropping the Bass: A video about stopping the spread of smallmouth bass in Grand Canyon
Smallmouth bass are a warm water, nonnative fish species that have been stocked throughout the western United States for recreational fishing. This adaptable species reproduces easily, allowing them to proliferate and spread from their initial points of introduction and predate on native fishes.
Smallmouth bass were introduced into Lake Powell in the 1980s and now are one of the most abundant fishes in the reservoir. Until recently, smallmouth bass were extremely rare in Grand Canyon, downriver of Lake Powell. During much of the 21st century, natural runoff in the Colorado River basin has declined, while water use has remained relatively constant, leading to historically low reservoir storage. These low reservoir elevations:
- Increase rates of fish passage through dams due to decreasing depth to dam intakes, and
- Increase the temperature of dam releases, improving suitability for warm water nonnatives.
Smallmouth bass live in the surface waters of Lake Powell. Lower water levels mean that the surface is closer to Glen Canyon Dam’s turbines. In 2022, due to lower water levels, water from Lake Powell’s upper layer was released downstream into the Grand Canyon segment of the Colorado River, dramatically warming river temperatures and carrying smallmouth bass through the dam into the Colorado River. This led to the first records of reproduction of smallmouth bass below the dam.
The Grand Canyon segment of the Colorado River, located downstream of Lake Powell, is the stronghold for threatened humpback chub, which are recently experiencing some recovery success. However, if reservoir levels remain low for an extended period of time, smallmouth bass are likely to pose a continued threat to the recovery of humpback chub. To assist managers concerned about this invasion, researchers have developed models that
- Predict immigration rates at different reservoir elevations, and
- Link reservoir storage/operations, water temperatures, and smallmouth bass population dynamics to forecast population growth rates under different climate and management scenarios.
Researchers also assist managers to assess the effectiveness of various mitigation efforts to help humpback chub continue their path to recovery.
The USGS Grand Canyon Monitoring and Research Center models used by the Bureau of Reclamation for NEPA analyses can be accessed at:
In the News:
How a ‘cold shock’ of water from Lake Powell could thwart invasive Grand Canyon bass - Arizona Republic, March 2023
Plunging water levels push invasive bass toward Grand Canyon - Arizona Republic, March 2023
What’s the best strategy to keep this invasive fish upstream of the Glen Canyon Dam? - St. George News, March 2023
Officials fear ‘complete doomsday scenario’ for drought-stricken Colorado River - The Washington Post, December 2022
A shock of cold water could help control exotic fish in Grand Canyon - NPR KNAU Arizona Public Radio, October 2022
Electrofishing begins below Glen Canyon Dam to kill smallmouth bass - NPR KNAU Arizona Public Radio, October 2022
As Lake Powell shrinks, voracious smallmouth bass are staging for a Grand Canyon invasion - Arizona Republic, July 2022
Invasive smallmouth bass make it through Glen Canyon Dam into mainstem Colorado River, threatening native fish - NPR KNAU Arizona Public Radio, July 2022
‘Worst fears confirmed' in biologists' fight to save ancient Colorado River fish - USA Today, July 2022
Declining levels at Lake Powell increase risk to humpback chub downstream - The Aspen Times, June 2022
Biologists try to save ancient fish as Colorado River fades - USA Today, June 2022