Aquatic insect populations have declined in many rivers downstream from hydroelectric dams, threatening fish and other wildlife that depend on them for survival. Now researchers from the US Geological Survey (USGS) together with collaborators from Oregon-, Utah- and Idaho State Universities, have found that hydropeaking — a practice widely used in the management of hydroelectric dams that involves increasing river flow rates during the daytime when demand for electricity is high — is likely the cause. Their findings suggest that these negative impacts could be mitigated if alternative hydropower methods were used.

Aquatic insects form a vital part of food webs in freshwater ecosystems and provide the primary source of food for many different species, including fish, reptiles, bats and birds, as well as other wildlife. The study, which was recently published online in the scientific journal BioScience, assessed the impact that sudden changes in water levels have on aquatic insects at all stages of their lifecycle, and provides the foundation for determining how to improve the health of rivers downstream from hydropeaking dams all over the world.

The study was partly assisted by data collected in a citizen science project where educational groups, river guides and other citizen scientists collected over 2,500 insect samples from a stretch of Colorado River downstream from the Glen Canyon Dam in the Grand Canyon. The researchers compared insect diversity of 16 large dammed river systems across the western regions of the US that have varying degrees of hydropeaking to assess how the impact of abrupt changes in water levels affected river health.

"For the first time, this study determines the ecological impacts of hydropeaking separated from other dam-imposed stressors, and identifies the specific cause-and-effect relationships responsible for biodiversity loss below hydroelectric dams," explains lead author Ted Kennedy, a USGS scientist. "These results may help resource managers improve river health while still meeting societal needs for renewable hydroelectricity."

Hydroelectric power provides approximately 19% of the electricity used globally, and far exceeds that produced by wind, solar or other renewable energy sources combined. Many hydroelectric dams practice hydropeaking, where the volume of water released can vary dramatically within a day, resulting in large hourly shifts in river levels that create artificially induced tides along the shoreline of rivers, which freshwater organisms simply are not adapted to cope with.

Many insects of ecological importance, such as mayflies, stoneflies and caddisflies, attach their eggs onto a substrate such as aquatic vegetation or rocks that are submerged just beneath the surface of the water, where the eggs soon hatch. But should water levels drop rapidly, exposing the eggs to the sun and air, they can become desiccated causing the larvae to die within the egg before hatching.

"These large daily rises and peaks in river flows due to hydropower dams are not normal. Prior to the construction of dams, there were almost no major daily changes in river levels in places like the Grand Canyon," said David Lytle, a professor at Oregon State University and co-author of the paper. "This can interrupt the egg-laying practices of some species, and the impact of this was poorly appreciated. Until now, no one really looked at this aspect, and our results show that it causes serious problems for river health."

For decades, aquatic scientists have been puzzled by the conspicuous absence of stoneflies, mayflies and caddisflies on the Colorado River in the Grand Canyon. This study sheds light on the reason for their absence — the hydropeaking activities of Glen Canyon Dam is partly responsible. The researchers suggest some mitigation measures that dam managers may want to consider, including maintaining low, stable water levels in rivers during times when power demands are low, such as over weekends, to give insects a couple of days to successfully lay (and hatch) their eggs.

"If mitigation flows are successful, a more diverse community of aquatic insects should improve the health of the Colorado River ecosystem in Grand Canyon, including the largest remaining population of endangered humpback chub," said Kennedy.

With over 2,500 samples collected by citizen scientists over a 3-year period, this project highlights the important contribution that citizen science makes to research of this nature. The research team partnered with Grand Canyon Youth who provided over 150 samples, to the project — watch them in action here. Having this extensive dataset enabled scientists to reach scientific conclusions which would otherwise have not have been possible.

"Many urgent questions in ecology remain unanswered, because scientists are bumping up against data limitations where it is impossible for them to collect sufficient data to answer complex questions across large landscapes," notes Kennedy, pointing out that this study highlights the powerful contribution citizen scientists can make to fundamentally advancing knowledge and gaining valuable new scientific insights.

Journal Reference

Kennedy TA et al. Flow Management for Hydropower Extirpates Aquatic Insects, Undermining River Food Webs. BioScience (2016) 66(6) doi: 10.1093/biosci/biw059