How Do Beaver Dams Affect Water Quality?

Published June 11, 2024

Considered nature’s engineers, beavers build dams across streams to create ponds. The lodges within these dams can only be accessed through underwater entrances, keeping beavers safe from predators like bears and wolves. After historical overhunting, beaver populations are growing—in part because of recent reintroduction programs—and are settling down in places they’ve never been found before, including Tierra del Fuego and the Arctic. Beavers are a keystone species because of their significant impact on streams, the movement of water, water quality, and the other animals that live there. Beavers can alter their environments in many ways, especially through dam construction. The effects of these dams can be different in geographical regions (also known as biomes), but scientists do not have a clear understanding of how they impact water quality, habitat, and sedimentation in floodplains. 

“Due to limited study in many biomes, some research scientists and land use managers must make decisions on how the conservation, expansion, and reintroduction of beavers can alter their local streams based on findings from ecosystems that are more frequently studied and better understood,” said EPA researcher Ken Fritz.

The Scientific Question

Because stream ecosystems are complex, it can be difficult to understand how disturbances and changing environmental conditions will impact the ecosystem. Additionally, the impacts of beaver dams may vary widely across biomes because the underlying watershed characteristics are different.

EPA scientists Ken Fritz, Tammy Newcomer-Johnson, Heather Golden, and Brent Johnson, in collaboration with researchers from Miami University, Ohio, conducted a scientific literature review to better understand how beaver dams impact stream systems across different biogeographical regions. Their paper, “A global review of beaver dam impacts: Stream conservation implications across biomes,” used 267 peer-reviewed studies to quantify the effects of beaver dams.  Literature reviews summarize the main points of scientific research already published on a specific topic, which helps determine future efforts. The paper provides a current understanding for environmental managers on how the conservation, expansion, and reintroduction of beavers can alter streams in different geographical locations.

What the Scientists Discovered

The literature review found that beaver dams had significant environmental effects across all studied biomes. The impacts on stream morphology (the shape of river channels and how they change in shape and direction over time) and stream hydrology (water movement) were similar across geographical regions. Stream integrity, or health, also appeared to improve with beaver conservation in all biomes. The geographical region influenced how water quality and plant and animal life changed in response to beaver dams.

Specifically, results show that while nitrate and suspended sediments (which block the sunlight that bottom-dwelling plants need to survive) decreased downstream from beaver dams, pollutants like methyl mercury, dissolved organic carbon, and ammonium concentrations increased. Total nitrogen and phosphorus concentrations tended to not be affected by beaver dams. The effects beaver dams have on pollutants vary depending on environmental conditions —like temperature, sunlight, water velocity and depth – that aid in changing and transporting certain pollutants. EPA scientist Heather Golden noted that “the effects of beaver dams on water quality can often vary with time of year, or season.” 

On a larger scale, beaver dams slow water flow and increase sedimentation, and most pollutants likely settle out of the water into sediments upstream of the beaver dam. These areas could become zones of high concentration of some pollutants and harmful hotspots for exposed wildlife. For certain pollutants like nitrogen, this temporary storage can provide time for microbes to convert nitrate pollution into harmless nitrogen gas, a process known as microbial denitrification.

“When you clean your drinking water in your home, you throw away the dirty filter and put in a new one. This doesn’t happen with beaver dams,” said EPA researcher Tammy Newcomer-Johnson. “Dams slow the flow of water so that heavier particles settle out. Over time, storms and floodwaters can damage the dams and wash the sediments stored behind them downstream.”

The paper found that beaver dams can significantly influence the areas around them. These findings can be useful for stream conservation and restoration efforts that introduce or protect beavers. The review also found that the impacts of beaver dams were most often studied in temperate forests. Additional studies are needed in dry or cold biomes historically occupied by beavers and in new environments where beaver populations are currently expanding.

Learn More About the Science

Read the journal article: A global review of beaver dam impacts: Stream conservation implications across biomes

This article was written by EPA's Melissa Payne.