Aquatic insects can be used as living indicators of how a river’s ecosystem is doing. When scientists look at the Colorado River running through the Grand Canyon, evidence shows that these insects are being severely affected by the Glen Canyon Dam. Science Moab talked with ecologist and bug lover Anya Metcalfe about her research with aquatic invertebrates in the Grand Canyon and how this work might help sustain the weakening food web within this river corridor.
Science Moab: Can you explain an insect’s life cycle?
Metcalfe: Life cycles describe the different forms and phases that an animal goes through during its life. With insects, most have three life stages, an egg, a larva and an adult. Others, like butterflies, have a fourth life stage where they pupate before they become an adult.
My favorite aquatic insects are caddisflies. Those are like butterflies of the river as they’ve got four life stages. This is something that’s really cool about aquatic insect life cycles: most of the time, they’ll start those first three life stages in the river and as adults they’ll sprout wings and fly off into a whole new terrestrial world. This is interesting from a food web perspective because while they’re in the river, they’re really important food for fish. Then once they emerge as adults, they also become important prey for lizards, spiders, birds and bats. They’re fueling food webs both in and out of the water.
Science Moab: Specifically, you’re studying the aquatic insects in the Colorado River through the Grand Canyon. How are these insects being affected by the fact that they’re below a very large dam?
Metcalfe: So the pre-dam Colorado River was very warm and turbid with a lot of suspended sediment, fluctuated in temperature with the seasons of the year, and it used to flood from 1000 cubic feet per second up to 80,000 CFS. Now, we have a cold, clear, fairly “stable” river. It’s fairly stable throughout the whole year because there are no natural floods, but there are hydropeaking flows when water is released from the dam in response to peak power demand. So you can have these artificial tides in the river where the water is going up and down throughout the day in response to power needs. Since you don’t get a tide in a natural river ecosystem, we’ve been finding evidence that these hydropeaking operations are having an effect on aquatic insect populations.
Many aquatic insects lay their eggs pretty close to the shore on rocks and submerged vegetation, not expecting that the river’s water level is going to drop out in an hour. So what we found is that aquatic insects are laying their eggs and then the tide is dropping, and those eggs are being exposed to dry air and drying up. So that’s a really interesting pattern that we found and attributed to the hydropeaking operations.
Science Moab: How is that affecting fish and other critters that eventually would eat these insects? How are those populations doing?
Metcalfe: A lot of research has been done on fish in the Grand Canyon section of the Colorado River. What people are finding through complex population models or directly measuring fish fat or just going out and fishing and paying attention to what they’re catching is that fish are hungry. Down in the Grand Canyon, we’re just seeing skinny fish and drastic swings in fish populations. Much of it has been attributed to low food availability. These fish are relying on aquatic insects and sometimes there’s just not enough around to eat. This applies both to natives, many of them endangered fish like the humpback chub, and also to the rainbow trout found up at Lee’s Ferry.
Science Moab: What research is being done to help address this problem with food shortages?
Metcalfe: The relationship between insect populations and hydropeaking is really exciting because hydropeaking is a problem that’s manageable since it’s human created in the first place. With those findings, the Glen Canyon Dam Adaptive Management Program decided that it was worth running an experiment to see if less hydropeaking might lead to more aquatic insects and therefore help those food webs in the Grand Canyon. We’ve been calling them “bug flows.”
From 2018 to 2020, there was an experiment where from May to August each year, there was no hydropeaking on the weekends to give the bugs a weekend off from the water going up and down and drying out their eggs. Across those three years, we found that natural processes improved in the Grand Canyon and we saw a huge increase in caddisflies more than anything else. So that was a really exciting outcome.
Science Moab is a nonprofit dedicated to engaging community members and visitors with the science happening in Southeast Utah and the Colorado Plateau. To learn more and listen to the rest of Anya Metcalfe’s interview, visit www.sciencemoab.org/radio. This interview has been edited for clarity.