How dams can cause fish declines

The Glen Canyon Dam was the last big dam built in the United States. Spanning the Colorado River above the Grand Canyon, it provides hydropower for the region and regulates the flow of water. Until the dam was built, the river would experience spring floods during snowmelt followed by low flow in the summer, especially during drought years. Now water below the dam flows at about the same rate year-round.

This more orderly flow has not been good for some of the native fish in the river.  Four native fish species are no longer found in the Grand Canyon, and a fifth, the humpback chub, is endangered.

Many people think that the problem with dams is that they block fish from swimming upriver to spawn. But it’s more complex than that.

“In addition to blocking migrations upstream, the dams actually change the temperature of the river. So instead of the water coming down from the river, it’s coming from a lake.  And from the bottom of a lake, the water is really cold. And this changes the physiology of fish.”

Emma Rosi-Marshall is an aquatic ecologist at the Cary Institute.

“In the case of the humpback chub, the humpback chub’s ideal temperature for spawning is 18 degrees Celsius.  And the Colorado River now is between 9-12 degrees Celsius.  And so the  dam changes the physical conditions within the river.”

Although some big dams in the U.S. are being decommissioned, there is little chance of that happening at Glen Canyon. But it’s possible that management practices like changing how water is released could benefit the humpback chub. Good science can help inform the best management for these fish in the Colorado River.

Produced in collaboration with WAMC Northeast Public Radio, this podcast originally aired on January 1, 2013. To access a full archive of Earth Wise podcasts, visit: www.earthwiseradio.org.

Photo: RP Franklin

Cary Institute of Ecosystem Studies | Millbrook, New York 12545 | Tel (845) 677-5343

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