The Grand Canyon is a haven for native fish. But that’s now threatened by smallmouth bass, an exotic fish that eats native species and has recently escaped through Glen Canyon Dam. Scientists say a shock of cold water released through the dam might help keep their numbers down. KNAU’s Melissa Sevigny spoke with fish biologist Drew Eppehimer about the tradeoffs involved.
Tell me why smallmouth bass are becoming a problem.
So if we look to the Upper Basin, so parts of the river in Colorado and Utah, smallmouth bass have been a problem for years. They’ve been stocked throughout West as a recreational fishery, and they’re a ton of fun to catch, but they often have detrimental consequences for native fish and native ecosystems…. but historically the section of the Colorado River between Lake Powell and Lake Mead has been insulated from that. Because Glen Canyon Dam served as a barrier.
And what’s happening now is the dam isn’t acting as much of a barrier anymore, because the reservoir shrinks, the zone where the fish live, they can now pass through the hydropower turbines?
Yes. It certainly still is a barrier but it’s becoming increasingly permeable as reservoir levels get lower…. and also as the reservoir levels go lower, the water temps in the river are getting warmer and warmer. Which now means that fish like smallmouth bass can successfully spawn.
What are some of the options for addressing this problem?
One that’s currently being considered by the Bureau of Reclamation is some sort of physical barrier in Lake Powell, so in the forebay in front of the dam, some sort of a net or screen to limit the passage of fish through the dam. There are also different ways to operate Glen Canyon Dam to try to disadvantage or prevent smallmouth bass from getting established, so that’s what the research team that I’m a part of has been working on…. The two main ways to pass water through the dam: you have penstocks, which are where the turbines are, that’s where most of the water is usually pulled through. But also a hundred feet deeper in Lake Powell at the dam, you have bypass jet tubes. The deeper water is also colder. One potential alternative is you are able to mix both the penstock and bypass to cool the river temperature year-round so it never gets to the threshold where smallmouth bass can start to reproduce, that’s estimated to be 16 C… Another option you can use for selective withdrawal is kind of a cold water shock. You push the vast majority of the water through those deeper bypass jet tubes on weekends through the spawning season, that’s to get the water really cold to disrupt spawning behavior that smallmouth bass might be having.
Why on weekends?
Typically weekends are when hydropower would be least impacted, by using the deeper water bypass tubes which don’t have turbines and can’t generate hydropower.
Because the demand for energy isn’t as high on weekends.
Exactly.
So there’s a tradeoff here where we can maybe handle the fish problem but lose a little bit of hydropower production.
Correct. Yeah, complicated system, a lot of different stakeholders involved. Any option you look at doing on the Colorado River will have tradeoffs, so its up to managers and stakeholders to weigh those pros and cons, to see what potential management options will work best under the circumstances.
Drew, thank you so much for speaking with me.
Thank you.
