By Eric Barendsen
Photos courtesy of Kevin Bestgen

Invasive fishes and woody plants. Declining Colorado pikeminnow numbers. Struggling razorback sucker larvae. These challenges, not to mention longstanding drought conditions in the region, have added stress to the Colorado River and Green River ecosystems in recent decades.

In particular, non-native smallmouth bass pose an existential threat to native fish throughout the Upper Colorado River Basin.

“They’re highly predaceous, so they eat lots of endangered native fish,” said Fish Biologist Derek Fryer. “They reproduce and become established, and they’re very hard to get rid of.”

He compared smallmouth bass to weeds in a garden in the way they compete for resources.

“We’re continually weeding the smallmouth bass to try to keep their numbers down,” he explained.

“The big predator in the system, pikeminnow, evolved to eat some of these other native fish and doesn’t have a large gape,” Fish Biologist Shane Capron said. “Smallmouth bass have quite a large gape to be able to swallow large fish compared to their body size. They’re a unique predator in our system, to be able to chomp on our fish and not have any big fish out there to take care of them.”

Upper Colorado River Endangered Fish Recovery Program ecologists on the Green River in northeastern Utah are taking a holistic approach to the problem. They believe that disrupting the reproductive cycles of smallmouth bass through well-timed, short, abrupt increases in river flow could hold one key in their quest.

“The whole goal here is generally to wipe out or disrupt as much of that smallmouth bass spawning as possible,” Fryer said. The larger they get, he explained, the more native fish they eat. “The idea is to lessen the number of fish that are available to grow into adulthood and become big predators.”

A 2018 report authored by Colorado State University scientist Kevin Bestgen proposed taking that approach to disadvantaging bass in the Green River. The study recommended using an early summer, cold-water high-flow spike from Flaming Gorge Dam.

While preparing for the flow spike, ecologists have continued to reduce smallmouth bass through electrofishing—electrically charging the water to temporarily stun the fish and removing as many adults as possible. States also encourage or require recreationists to remove them when fishing.

When new introduction locations are found, the program targets them to discourage new populations from becoming established.

The program also funds the construction of screens to limit movement of non-native fish between ponds, reservoirs and rivers with endangered fish habitat.

“It’s kind of an all-hands-on-deck scenario,” Capron said. “This flow spike is another thing we can do to further that impact. We’re trying to break the back of the reproduction cycle, and it’s a really hard thing to do.”

Powerful partners

Before the study, WAPA recruited experts from Argonne National Laboratory to help gauge whether or not the maximum “power plant” flow of 4,600 cubic feet per second could provide the conditions for a successful experiment.

The goal was to use all of the high flow to generate electricity, rather than running some of the water through a bypass to create faster flows. That kept the costs down for WAPA’s customers and ensured that all of the water produced power.

“We worked with Argonne and developed this feasibility project, and the evaluation came back that, ‘yeah, we can get some pretty good flows and water levels in these side channels. We think this will be a good experiment and worth doing just with power plant,'” Capron said.

WAPA worked closely with the Bureau of Reclamation to pattern the ramp up and down to maximize power value.

Collaborating with Reclamation, WAPA seeks to offset the cost of the experiment through additional water releases and energy sales from Flaming Gorge Dam during summer and fall.

First spike, ask questions later

In June, WAPA, CSU, Reclamation and other partners in the Endangered Fish Recovery Program conducted the first flow spike targeting smallmouth bass eggs and larvae.

The experiment sought to hobble the invasive bass’ advantage over native fish such as the Colorado pikeminnow and razorback sucker. Scientists timed the study to occur before pikeminnow larvae arrive in the Green River from upstream spawning grounds on the Yampa River.

“The spawns are very closely timed, but there is enough separation there that the program thought it was safe to conduct the study,” Fryer explained.

From 12 p.m. on June 21 to 4 p.m. on June 24, Reclamation rapidly ramped up flow at Flaming Gorge from about 860 cfs to 4,600 cfs. This surge increased the river’s depth by three feet in many locations and dramatically increased flow velocities.

Cold water from Flaming Gorge Reservoir plunged the temperature of the river from around 70 degrees to about 60 degrees.

“That cold shock, in addition to the high flows, could discourage them from spawning and cause them to abandon the effort,” Fryer said.

Slowing the spread

Smallmouth bass rapidly expanded in the Upper Basin in the early-to-mid 2000s, and their numbers continue to grow.

The adult male guards the nest. Because they are poor swimmers and susceptible to predation, larvae and newly hatched fish stay nearby. But the high flow may disrupt the spawning process, get the male to abandon the nest and sweep the eggs and larvae downriver to be preyed upon.

Flushing colder water downriver may also help by confusing the bass’ natural temperature cues to spawn.

Scientists believe that timing the flow spike to occur in the middle-to-later portions of the spawning period will have the maximum effect. Bass can still lay eggs after the flow spike, but late-season spawners produce late young, which have a harder time surviving winter.

Drought

While higher-flow years naturally limit bass reproduction, low-flow years provide conditions favorable to spawning, growth and survival. With continuing drought in the West, the past few years have provided favorable habitat conditions for reproduction.

“That’s why the program pushed really hard to get this flow spike this year, because we’re in such a drought right now with such low flows,” explained Capron. “The potential effect was much bigger and more important this year.”

The program sees the driest years as the best for impacting bass, but those are also the most challenging years for WAPA.

“Even though we support the Endangered Fish Recovery Program, these are very difficult years for us to release that kind of water,” Fryer said.

Data capture and early indicators

CSU scientists developed the study plan, determined the timing of the flow spike and headed into the field to collect data.

Prior to the water release, they sampled 19 sites in Lodore Canyon and Whirlpool Park for connecting flows and physical habitat. The researchers gathered data on habitat characteristics including temperature, depth and velocity.

During the flow spike, they resampled those 19 locations for physical habitat characteristics, plus some limited biological sampling, such as collecting bass larvae in drift nets. If young bass are found in drift nets, it indicates that the young are being swept from the nest and that the experiment is having the desired effect.

In July and September, the team is revisiting those locations and collecting biological data to measure larval and juvenile smallmouth bass abundance, which will be compared to prior years’ estimates.

Early indications show that the effects varied by site but, overall, they look positive in many locations. Time-lapse photos captured the high flows inundating spawning habitats. The data shows approximately three-foot depth increases at many locations with fairly high velocity water flowing through them.

“I’m not sure how big of a systemwide effect we are going to have,” Fryer said. “I think the potential is there.”

“How far downriver this flow is going to be effective is a real question,” Capron said. “You’re trying to do something with a relatively small flow, and we’ll see. It could be really effective. It’s hard to gauge, but worth trying.”

Future programming

Enhancing recovery of native fishes and mitigating the impact of invasive smallmouth bass remains the overarching goal in Green River ecosystem management.

Going forward, program stakeholders, including WAPA, Reclamation, Fish and Wildlife, the Park Service and the Upper Basin states, will continue to meet several times per year to develop workplans. Together, they set the agenda for future scientific studies, review the progress of removal efforts and prioritize screening locations.

WAPA’s Colorado River Storage Project Management Center will continue to protect native fish while preserving hydropower.

“It’s really critical for us to recover these species,” Capron said. “But also, in the future, to be able to remove restrictions that limit the capacity and energy we produce because of protections for endangered species.” 

Note: Barendsen is a public affairs specialist.

Ecologists use electrofishing to reduce the smallmouth bass population just downstream from Winnie’s Rapid in the Green River.

Riverbed during flow spike, with more water

Photos taken at the same location before and during the June flow spike illustrate the significant temperature, depth and velocity changes measured by researchers. 

Last modified on September 12th, 2023