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Interior testing “Bug Flow” at Glen Canyon Dam

The U.S. Department of the Interior is conducting a Macroinvertebrate Production Flow (also known as a Bug Flow) at Glen Canyon Dam from May 1 through August 31 to improve egg-laying conditions for aquatic insects, which are the primary food source for endangered and native fish in the Colorado River.

Aquatic insects lay and cement their eggs to rocks, vegetation and other materials near the river’s edge. If flows are too variable, water levels may drop below where eggs are laid, causing them to dry out and die.

During the Bug Flow experiment, the Bureau of Reclamation will make targeted adjustments to water releases from Glen Canyon Dam and Lake Powell. That adjusted release schedule will include low and steady flows during weekends, while weekday operations will maintain normal flows to meet hydropower demands. Weekday release rate hourly changes will remain unchanged.

Glen Canyon Dam impounds water for a 1,312-MW hydro powerhouse.

This is the third consecutive year for the Bug Flow under the Glen Canyon Dam Long-Term Experimental and Management Plan.

“Findings indicate that some aquatic insects are already benefiting from the bug flows, which also benefits fish and other animals that eat them,” said Scott VanderKooi, chief of the U.S. Geological Survey’s Grand Canyon Monitoring and Research Center. “For example, our research suggests that caddisflies, an extremely rare aquatic insect in the Grand Canyon over the past several decades, increased nearly four-fold during the first year of the experiment in 2018, before returning to pre-Bug Flows numbers in 2019. In contrast, non-biting midges, another type of aquatic insect that is a key food source for fish and other wildlife, may have increased, and a third year of Bug Flows should help verify this finding.”

Recreational fishing at Lees Ferry also improved during Bug Flows, with anglers catching an average of one to two more rainbow trout per day during Bug Flow weekends, when flows were low and steady, compared to weekdays.

The decision to conduct this experiment was based on technical input and recommendation from a collaborative team of scientists and technical experts from federal agencies and states involved in the Glen Canyon Dam Adaptive Management Program. This team includes representatives from Reclamation, the National Park Service, USGS, U.S. Fish and Wildlife Service, Bureau of Indian Affairs, Department of Energy’s Western Area Power Administration, Arizona Game and Fish Department, Upper Colorado River Commission and all seven Colorado River Basin states.

Experiments are designed to maximize benefits to the Colorado River ecosystem through the Grand Canyon, while meeting water delivery requirements and minimizing negative impacts to hydropower production. This experiment is expected to benefit aquatic insects and the fish, birds and bats that feed on them, while providing scientific information for future decision making.