The volume of water released from the Glen Canyon dam fluctuates a lot in the course of a day. Throughout May the Bureau of Reclamation (BOR) is releasing 15,000 cubic feet per second (cfs) during the day dropping to 9,000 cfs during the night. The reason for the change is because consumers use less electricity at night because the majority of them are sleeping.
In summer months it’s common to see the water released from the dam fluctuate between 9,000 to 17,000 cfs on a daily basis.
This fluctuating water level puts a strain on the populations of water insects, particularly black flies and midges, which lay their eggs just under the surface of the water along the shore of the river, usually on rigid surfaces such as rocks, logs and reeds.
If the insects lay their eggs while the water level is high the eggs may dry out, even if out for only  an hour or two.
A small fly hatch means less food for the trout that live in the Colorado River below the dam and depend on the flies as one of their main food sources. Small fly hatches mean smaller populations of trout and skinnier individual trout.
Starting on May 1 the BOR started a program where they release a steady water flow of 9,500 cfs on the weekends in an effort to be less interruptive to the life cycles of the midges and other flies laying eggs along the Colorado River.
The experiment is being touted as “the bug flows.”

This is the first time the BOR has done a bug flow experiment. The bug flow water releases will continue until Aug. 31. The bug flow releases will occur only during the weekend. According to a BOR press release, water release will return to normal flows from Monday morning until Friday afternoon.
“Many bugs lay their eggs at dusk when the water is high, then, at night, when flows are typically lower, the eggs dry out and die,” said Fish Biologist Craig Ellsworth. “We are going to see if we provide two days of low, steady water, if those eggs have better survival rates.”
Insect populations downstream of Glen Canyon have sharply decreased since 2012, and insect diversity has also been low. As the primary food sourc
e for fish, insect abundance and diversity of species are major concerns.
“We are seeing fish on a diet. The fish are skinny – all head, and the bodies felt like a piece of cardboard,” said Ellsworth. “This experiment is all about the fish. Fish live off these insect populations. We want to see if we can improve conditions for the bugs, so the fish populations should stabilize.”
In 2014, the trout fishery at Lees Ferry collapsed due to starvation, with a 90-percent mortality rate over the course of a single winter.
The more resilient humpback chub, recently proposed for reclassification from endangered to threatened, was also affected.
The 15 mile stretch of river below the dam is very popular among fly-fishermen and the skinny, weak fish lack appeal to anglers.  
Trout aren’t the only species that depend on the insects. So do bats and insect-eating birds such as swallows, say scientists.
“We expect this experiment will positively benefit crucial insect populations, which will benefit the entire ecosystem while limiting the impact on other resources and Colorado River users,” said Dr. Timothy Petty, Assistant Secretary for Water and Science.
According to the BOR, the decision to conduct this experiment was based on input from a collaborative team, including Department of the Interior agencies—Bureau of Reclamation, National Park Service, U.S. Geological Survey, U.S. Fish and Wildlife Service, Bureau of Indian Affairs—the Department of Energy’s Western Area Power Administration, six consulting American Indian Tribes, and all seven of the Colorado River Basin States. Before proceeding with these experiments, experts determined there would be no unacceptable adverse impacts on other resource conditions.
Technical experts with the Department of the Interior have coordinated the experiment’s design to optimize benefits to the aquatic ecosystem throughout the Grand Canyon while meeting all water delivery requirements and minimizing negative impacts to hydropower production.