From Pacific Standard (Nathan Collins):
…United States Geological Service biologist Theodore Kennedy and his colleagues point out, the danger to ecosystems could go beyond an immediate threat to fish populations. In particular, no one’s really looked at the consequences of hydropeaking, in which dam operators release more water through the dam during the day to produce more electricity when it’s most in demand. Like the underlying electricity demand, the hour-to-hour changes in river flow are enormous. In some places, river flows change by as much as a factor of 10 over the course of the day, leading to a cycle of drying and re-wetting along the shore of a river. That creates intertidal zones more akin to what you’d see at an ocean beach than a typical river.
The dry phases, Kennedy and his team point out, could be very bad for mayflies, caddisflies, and other insect species that birds and fish rely on for food—in particular, it could be very bad for their eggs. To test that idea, the researchers first collected mayfly and caddisfly eggs from Utah’s Green River, downstream of the Flaming Gorge Dam, and tested them under hydropeaking-like cycles of wet and dry. Very few survived.
To see how much impact hydropeaking had in the real world, however, the researchers turned to citizen scientists—specifically, river rafters on the Colorado River downstream from the Glen Canyon Dam. Each night, rafters collected insects at various points along the river. Combining that data with a model of hydropeaking’s effect on river flow at those points, the team was able to estimate the effects hydropeaking had on insects.
The results: While different species responded to hydropeaking differently, insects that lay their eggs right at the river’s edge, such as mayflies, had all but vanished from the Colorado River. Midges, which lay their eggs in somewhat broader areas, were most abundant at places where hydropeaking had the smallest effect on water levels. A follow-up study of 16 rivers in the Western U.S. confirmed that hydropeaking had a strong negative impact on insect biodiversity.
Whether or not you care about insects themselves—and many people don’t—the authors point out that healthy insect populations are essential for species that we do care about. “For instance, recent food-web and bioenergetics studies demonstrate that in the popular Lees Ferry sport fishery downstream of the Glen Canyon Dam, the maximum size and growth of rainbow trout are limited by the abundance and overall small size of their invertebrate prey,” Kennedy and his team write.
The lack of insect biodiversity also makes for unstable fish populations—an issue that fisheries managers need to take into account.