Dam Removal Restores River's Natural Flow and Improves Local Ecosystem

The purpose of a dam is to block the flow of a body of water but Ohio State University researchers suggest the removal of some river dams could help restore local ecosystems. For instance, when a dam was taken down in Washington State, researchers noted that songbirds and other wildlife rebounded faster than expected.

Until it was torn down, the dam had created a divided ecosystem that was harmful to several species, including a songbird known as the America dipper that thrives on a salmon-rich diet. Dippers that claimed the free-flowing part of the river thrived, but those that nested on the side that was starved of fish (and the nutrients they leave behind) struggled mightly, according to a news release. 

"It's exciting to be able to show a real positive outcome in conservation. We don't always get that," Christopher Tonra, study leader and assistant professor of avian wildlife ecology at Ohio State University, said in the release. "That these rivers can come back within our own generation is a really exciting thing."

Dippers (Cinclus mexicanus) are aquatic songbirds that have a transparent second eyelid. this oddity allows them to dive below the river's surface while being able to see insects in their larval stage as well as juvenile salmon near the river bottom.

Researchers spend four years studying dippers living in and around Washington's Olympic National Park. The Elwha River winds through the park and is the site of the largest dam removal in history, researchers say. Removing this dam started in 2011 and concluded in 2014, finally freeing the path for migratory fish.

While salmon do most of their growing in the ocean, they carry marine-derived nitrogen and carbon back into freshwater systems when they return to spawn or die. In turn, salmon benefit local plants and animals by transporting vital nutrients and acting as a direct food source.

"They're truly fertilizing the river and so that makes its way all the way up through the food chain," Tonra added in the University's release.

Compared to dippers living along rivers blocked to salmon, researchers found those that had access to salmon were in better physical condition and had improved reproductive success: the latter bred more in a season and produced larger offspring. Furthermore, birds with access to marine-derived nutrients had an annual adult survival rate that was 11 percent higher than their salmon-deprived counterparts. However, within a year of the Elwha Dam removal, researchers observed an increase in salmon-derived nutrients found in American dippers.

Their study was recently published in the journal Ecography.

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