Hydropeaking Dams = Fewer Insects

Hoover Dam, photo © US Bureau of Reclamation

Although we’ve heard of dams causing environmental and community problems before, we’ve also seen how they can be beneficial to society, and it’s clear that they’re a double-edged sword. Most recently on the topic, we learned that with proper planning and design, hydropower can be less of an enemy to conservation. Now, research highlighted in Conservation Magazine displays the possibility of helping native river-reliant insect populations by adjusting how dams “hydropeak,” or change river flow to compensate for electricity demand. Sarah DeWeerdt reports:

Scientists know that hydropower dams often decrease the abundance and diversity of aquatic insects downstream. But until now it wasn’t clear why—after all, dams cause a range of environmental stressors such as alterations in water flow, temperature, and sedimentation.

A massive new study led by the U.S. Geological Survey lays much of the blame on hydropeaking, the practice of varying river flows below a dam depending on electricity demand. Because of hydropeaking, the amount of water released from a dam can vary by as much as ten-fold throughout the day, creating an artificial intertidal zone that propagates for hundreds of kilometers downstream.

The hour-by-hour variation in water levels is a major problem for aquatic insects, which are a key element of river food webs and important prey for fish, birds, bats, and other wildlife, researchers reported last week in the journal BioScience. But lowering river flows during times of peak egg-laying and low electricity demand could give those insects a boost.

Consulting a database of insect life-history traits, the researchers determined that more than three-quarters of aquatic insects lay their eggs in shallow water at the river’s edge or by cementing eggs to the undersides of partially submerged rocks. These species could be vulnerable to hydropeaking, they reasoned, because eggs laid during high river flows are likely to be exposed to the air once the hydropeaking tide passes.

To follow up on this hunch, the researchers collected mayfly and caddisfly eggs from the Green River in Utah and exposed them to varying periods out of the water. Indeed, even an hour of desiccation was enough to kill most of the eggs, they found.

And to see how this all operates on a real river, the scientists selected a 400-kilometer stretch of the Colorado River in the Grand Canyon, downstream of the Glen Canyon Dam. Since the Grand Canyon is remote and difficult to access, they turned the project into a huge citizen-science effort, recruiting participants on rafting tours to collect more than 2,500 aquatic insect samples over the course of three years.

Microcaddisflies, which lay their eggs along the river’s edge, were rare in the Grand Canyon, the researchers found. Most of the microcaddisflies collected in the study were probably from nearby tributaries, not the main stem of the river.

In fact, scientists have been puzzled for decades by the fact that mayflies, stoneflies, and caddisflies are virtually absent from the Colorado River downstream of the Hoover and Glen Canyon Dams, but rich and abundant upriver. The new study explains why: all those insects are river’s-edge egg layers, and have likely been extirpated from the river due to hydropeaking.

Read the rest of the article here.

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