Why Beaver Dams Shouldn’t Get a Bad Rap

February 24, 2017
beaver dams
S.H. LeeDam valuable! Before you curse beaver dams, learn what work they do for the environment.

Beaver dams get a bad rap. Sure, they can be a nuisance, wreaking havoc on roads, cellars, and culverts across the Northeast and inspiring officials to extend beaver-trapping season and install beaver-proof pipes to drain flooded areas. But the busy rodents and the ponds they create are also cleaning our waterways and protecting fish that live downstream.

For that reason, the animals deserve more respect, according to Arthur Gold, professor and chair of the Department of Natural Resource Science at the University of Rhode Island. “There’s value in these beaver ponds,” he says.

Nasty Nitrogen

To understand the impact of eager beavers, first you need to know about nitrogen, a chemical that’s increasing in the region’s waterways due to agricultural fertilizers and urbanization. Nitrogen seeps into streams and ponds and travels to estuaries, where rivers meet the sea. As nitrogen levels have risen, coastal waterways in the Northeast have been experiencing a cascade of negative effects: loss of sea grasses, more algae blooms, less oxygen, and eventually the creation of dead zones, where fish and other marine life die off because conditions are so noxious.

Into this 21st-century disaster steps the heroic beaver, armed only with strong teeth and natural engineering talent, yet up for the task of removing at least some of the nitrogen from the water before harm is done.

Mighty Microbes

In a 2015 study, Gold and other researchers revealed how beavers help prevent nitrogen from reaching vulnerable estuaries. Beavers start by building dams, which in turn create ponds. Water that would have flowed freely stays within these ponds for a while, flooding areas from a quarter acre to several acres while still allowing water to continue slowly downstream.

Ponds allow more direct sunlight into the water than streams do, and many plants thrive in these sunnier, slower-moving waters. “We saw a riot of rooted aquatic plants and algae,” Gold says.

As those plants die and decay, rich organic matter builds up on the pond floor, providing food for microbes. These microorganisms need oxygen to break down the organic matter, and as Gold’s team showed in tests using sediment samples, the microorganisms use the elements in the water, altering it in the process. “They start peeling off the oxygen from the nitrate- nitrogen,” he says. As a result, the pond gives off nitrogen gas (Gold calls it “good nitrogen”), which is not a greenhouse gas. Depending on the size of a beaver pond, 5 to 45 percent of nitrogen in the water can be removed through this process, called denitrification.

Beaver ponds aren’t always a benefit, though. Research conducted in the Rocky Mountains suggests that beaver dams can cause greater accumulation of toxic mercury in the food chain downstream. Beaver ponds are also a source of methane gas, a potent greenhouse gas. Even so, they are important habitats for amphibians, as well as powerful nitrogen filters, Gold says.

Worth the Portage

Whether the nitrogen-cleanup rate of beavers in the Northeast can keep pace with nitrogen pollution is unclear. Still, Gold is pleased by the proliferation of beaver ponds and hopes they will be allowed to remain wherever they aren’t causing hazards.

“When beavers make a dam, your trail gets flooded, maybe your backyard gets flooded,” he says. “When you’re paddling, I recognize it means you have to get out of your canoe or kayak and pull it around, but that’s not the end of the world. Beavers have a place in the ecosystem.”


Read more about the natural world in outdoors.org/wildwisdom.


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Heather Stephenson

AMC Outdoors, the magazine of the Appalachian Mountain Club, inspires readers to get outside and get engaged. Learn more.