The Colorado River is not a naturally flowing river, not anymore. With Glen Canyon Dam upstream and Hoover Dam downstream, the Colorado River in Grand Canyon is one of the most highly regulated water systems in the world. Its flow generates hydroelectricity, irrigates crops and provides water to nearly 40 million people across seven U.S. states, 30 Tribal Nations and two Mexican states.  

Managing the Colorado River Basin is complicated. Federal, state and Tribal agencies balance the needs of many user groups, from anglers to farmers to city municipalities. They also care for the river as an ecosystem, home to rare fish and the foundation of Grand Canyon, one of the Nation’s natural treasures. In an era of heat waves and drought, when there is less water than ever to go around, managers increasingly need high-quality science to respond to emerging challenges. 

The USGS provides critical science to resource managers in the Colorado River and Grand Canyon. Our stream gages monitor water quality and flows, our researchers track fish populations and our modelers forecast how resources may respond to future conditions. We help managers anticipate new threats and consider potential outcomes of management decisions. 

And on a scorching day in June 2022, the summer Lake Powell reached its lowest water level in five decades, we sprang into action when one of our predictions became suddenly real. 

Did you hear what they caught in Lees Ferry? 

For the first time, National Park Service staff caught baby smallmouth bass in the lower Colorado River, south of the Glen Canyon Dam holding back Lake Powell. While this voracious, predatory fish had previously been caught in very low numbers in the relatively pristine Grand Canyon ecosystem, such captures had been rare, and they had never been observed reproducing. 

The finding raised fresh concerns about the future of native fish of the Grand Canyon. 

Smallmouth bass were originally stocked in Lake Powell as a valued catch for anglers and have since established healthy populations throughout the lake. But with low lake levels in recent years, smallmouth bass can be sucked through the dam and spat into the Colorado River. Worse, extended drought means river temperatures are warmer than usual, creating especially hospitable conditions for the warm-water fish to proliferate. 

To slow the spread, Eppehimer and USGS research statistician Charles Yackulic worked with academic, state and federal cooperators to develop models predicting when and where the fish might invade, based on projected temperatures and Lake Powell water levels. These models help the National Park Service prioritize locations for smallmouth bass monitoring and eradication.  

Adding extra urgency: Smallmouth bass threaten to erase years of conservation gains for the threatened and endangered species of Grand Canyon. Most of the fish in the park today are native species, a hard-fought accomplishment in an era of constant non-native species invasions. And the humpback chub was recently downlisted from “endangered” to “threatened” after successful conservation efforts from park staff. 

But smallmouth bass are a particularly lethal threat. Laboratory predation trials by the USGS and U.S. Fish and Wildlife Service (USFWS) show that smallmouth bass eat native fish at all life stages, from small babies to grown adults. 

“Most of the sport fish species have big mouths and big teeth and they like to eat native fish,” says David Ward, fish biologist and assistant project leader for USFWS Conservation Office in Flagstaff, AZ. “When you get all those species preying on the chubs at all different life stages, they just don’t get a break.” 

If managers want to prevent smallmouth bass from becoming a permanent addition to Grand Canyon, they need to act fast. Once a species becomes established, it becomes virtually impossible to eradicate completely. 

Smallmouth bass management is a high priority for the Glen Canyon Dam Adaptive Management Program (GCDAMP) and the Adaptive Management Work Group (AMWG), a Federal Advisory Committee in the Colorado River Basin. Led by the Bureau of Reclamation (BOR), this group brings together twenty-five stakeholder and rightsholder groups representing different interests, including states, Tribal Nations, economic sectors, non-profit environmental organizations and hobby groups. Together, they provide recommendations to the U.S. Secretary of the Interior for how to manage flows from Glen Canyon Dam.  

The USGS’s Grand Canyon Monitoring and Research Group (GCMRC) is a fixture of these quarterly meetings, tasked with providing science to help members understand environmental change happening on the landscape and how different management alternatives may perform under future conditions.

A major discussion point for the advisory committee is how water should flow out of the dam – how often water should be released, how much water at a time, which part of the dam it should be released from, etc. These questions are important, impacting everything from hydroelectricity production to downstream rafting conditions. 

Eppehimer, Yackulic and other USGS researchers created models to predict how changes to Glen Canyon Dam flows may affect different systems, including energy production, river hydrology and sandbar formation. Of particular interest: they explored how pumping cold water from the dam’s deep bypass jet tubes could impact smallmouth bass viability below the dam. They identified ideal water temperatures for bass to grow and reproduce and modeled how cooling river temperatures using dam flows could impact overall population growth.  

This work served as the foundation for dam flow alternatives presented in the Glen Canyon Dam Long-Term Experimental and Management Plan and the supplemental Environmental Impact Statement.  

Using one of the USGS-modeled alternatives, the Bureau of Reclamation has begun modifying Glen Canyon Dam flows to try to prevent smallmouth bass spawning. When river temperatures reach 60°F (15.5°C) in the Colorado River at the confluence with the Little Colorado River tributary (76 miles downstream from the dam), the BOR releases deeper, cooler flows from Glen Canyon Dam to create less favorable conditions for smallmouth bass growth and reproduction. They began these releases on July 9, 2024, and are now working with the USGS and other DOI agencies to actively monitor the effects on river conditions and smallmouth bass populations.  

This work was funded by USGS’s Southwest Climate Adaptation Science Center (Southwest CASC), Ecosystems Mission Area, Water Mission Area, and the U.S. Bureau of Reclamation. The project embodies the USGS’s actionable science model, which prioritizes applied research designed to meet on-the-ground needs. 

“It is an excellent example of partnership-based science,” says Sarah LeRoy, Research Coordinator with the Southwest CASC. “From the very beginning, managers asked a question about what's going to happen to fish, native and invasive, in the Colorado River Basin, and the scientists answered their questions in a way that helps them better care for the river in the future.”