By Eric Barendsen
Beginning May 1, WAPA and the Bureau of Reclamation, along with other partners, kicked off a third year of “bug flow” experiments on the Colorado River below Glen Canyon Dam. The investigation, part of the Department of the Interior’s Glen Canyon Dam Long-Term Experimental and Management Plan, aims to improve egg-laying conditions for aquatic insects in the Grand Canyon by tweaking the weekly schedule of hydropower releases.
The midges, caddisflies and other bugs that could benefit from the lowerflow weekends are an important food source for the endangered humpback chub and for prized sport fish such as rainbow trout. The fly-fishing community and conservation groups are interested in improving the habitat for these fish in a way that doesn’t also impact other important resources such as hydropower production at Glen Canyon Dam.
The bug flows are an ecological experiment conducted at the dam with the dual purposes of trying to improve downstream habitat while also increasing the value of hydropower. Operators from the Bureau of Reclamation, in coordination with WAPA, will maintain routine hydropower production flows on weekdays—with slightly higher flows than normal—but dial releases back to relatively low, steady flows on weekends until the end of August. Hourly weekday release patterns and total release volumes will remain unchanged. The reduction in flows on the weekends allows for more water to be released on the weekdays when electrical demand is higher and power is more valuable.
Many aquatic insects lay their eggs on rocks, vegetation and other materials near the edges of rivers. Scientists from the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center are looking to see if eggs laid during low weekend flows will survive better than eggs laid during fluctuating flows that occur throughout the rest of the week.
“Results from the first two years have been a little ambiguous,” said Fish Biologist Craig Ellsworth. “The data collected so far have yet to show an increase in bugs that we believe are related to the bug flow experiment.”
A large increase in the caddisfly population measured on the river in 2018 was initially attributed to the 2018 bug flow experiment, Ellsworth explained. But because these caddisflies have a one-year lifecycle, they had actually emerged from eggs laid in 2017, the year before bug flows started.
At the same time, water conditions in Grand Canyon were unusually clear during the fall and winter of 2017-18. This complicates the story that poor egg-laying conditions, and not some other factor, are to blame for the shrinking caddisfly populations in Grand Canyon.
The number of bugs collected during last year’s experiment returned to pre-bug flow levels. This could be an indication that other factors, such as the muddiness of the river following winter and summer storms, may have a stronger effect on bug populations than hydropower impacts do.
“The scientists want to conduct this experiment for at least three consecutive years to allow the numbers of aquatic insects to grow over time, kind of like compounding interest at the bank, which we hope will give us a strong signal to interpret whether the experiment was a success or not,” said Fish Biologist Shane Capron. “If successful, this offers us another tool to improve the productivity of the river below Glen Canyon Dam.”
The sampling cycle
The evolving COVID-19 pandemic and response has affected researchers’ ability to monitor the experiment. Specifically, light trapping—a way of catching insects for scientific study—is conducted by river guides and other people rafting down the Grand Canyon as citizen scientists. The closure of Grand Canyon National Park delayed this year’s citizen-scientist sampling by a few weeks.
However, the delay shouldn’t put a damper on the overall study. “Even if these light trap and other sampling activities begin later than usual, they will still provide valuable data to assess the experiment,” said Ellsworth.
Because of the one-year lifecycle of most of the bugs in the canyon, assessing any improvements in egg survival due to this year’s experiment would be done through monitoring the bug populations that are produced next year.
The U.S. Geological Survey has been in discussions with Grand Canyon National Park looking for other innovative ways of collecting light-trap samples. If approved, it would yield some monitoring coverage when access to the park is limited.
Solving for ecological success
The decision to conduct a third year of bug flow experiments was based on input from a collaboration including WAPA, Reclamation, the USGS, the National Park Service, the Fish and Wildlife Service and the Bureau of Indian Affairs. The team also receives input from the Arizona Game and Fish Department, the Upper Colorado River Commission and all seven of the Colorado River Basin States.
The experiments are meant to benefit the Colorado River ecosystem through the Grand Canyon while meeting water delivery requirements and minimizing negative impacts to hydropower production.
“By directly experimenting with flows, researchers gain valuable insights about the aquatic ecosystem in the Grand Canyon,” said Capron. “Beyond the potential benefits to bugs and the wildlife that relies on them, this third year of bug flows will increase our knowledge base, enabling better future decision making on Glen Canyon Dam operations.”
Note: Barendsen is a public affairs specialist.
Follow the Bug Flow experiment on the Glen Canyon Adaptive Management Program’s wiki. Visit gcdamp.com and search for “bugflow”.
Citizen scientists use a light trap to assess aquatic insects in the Grand Canyon. (Photo courtesy of the U.S. Geological Survey and Freshwaters Illustrated.)
Moving water from the weekend to the weekday—when electrical demand is higher and power is more valuable—helped offset the cost of the experiment.
Last modified on September 12th, 2023