Beaver dams: Beneficial for watersheds?

Two bearded men, one of them standing next to and one  in a beaver dam pool,
Billy Beck (right), Iowa State Extension and Outreach forestry specialist, and Andrew Rupiper, graduate student in natural resource ecology and management, at a beaver dam study site in central Iowa. Iowa Learning Farms photo. 

AMES, Iowa – A novel research project investigating beavers and the dams they build is exploring the influence of this industrious, little-known animal on water quality and hydrology (water movement) within Iowa watersheds.

“Once plentiful, beavers were almost completely eradicated from Iowa and the whole United States in the late 1800s and early 1900s, due to market forces,” said Billy Beck, assistant professor of natural resource ecology and management and Iowa State University Extension and Outreach forestry specialist.

“As demand for pelts declined, the species has slowly recovered, and they have seen significant increases in Iowa. More is known about beavers and their impacts, especially on fish populations, in regions like the Pacific Northwest, but there isn’t a lot known about them in the heavily altered agricultural landscape of Iowa and the Midwest.”

Beck is leading a project to learn more, with assistance from Andrew Rupiper, a graduate student in natural resource ecology and management. Their three-year study, supported by the Iowa Nutrient Research Center, is looking at beaver dams across north-central Iowa’s Des Moines Lobe region, with a focus on dams at two locations. One is along Prairie Creek, at the Smeltzer Farm near Ft. Dodge -- a larger watershed almost entirely in row crops, where the stream is more steeply incised, and the water runs faster. The other is along Caton Branch, near Woodward, Iowa -- a smaller, wider stream with more tree cover, in a watershed of about 70% cropland.    

“We’re really starting from scratch to try and understand if these fascinating rodents have an appreciable impact on our watersheds, and if so, what it might be,” Rupiper said.

Beck and Rupiper will discuss their study, ”Beavers: Superheroes for Water Quality?” at a free, virtual field day on Thursday, Feb. 9, from 1-2 p.m. The event is hosted by the Iowa Learning Farms in partnership with the Iowa Nutrient Research Center and the Conservation Learning Group.

“It’s been difficult to develop a sampling regimen for this kind of project, but we’ve been enjoying the challenge,” Rupiper said. He and Beck are documenting the beavers’ locations and characteristics of their dams, and measuring water quality above, within and below dams, as well as sediment and water levels associated with the dams.

While the team’s findings are preliminary, they can share some interesting initial observations. For one thing, it has been commonly thought that beaver dams are somewhat porous and act to filter the water that flows through them, according to Beck.

“However, by taking a closer look at and into the dams, we see that they are relatively solid and interact with streams much like human-constructed dams,” he said. “Cross-sections show they have an intricate matrix of sediment, grass, logs and other material, sometimes large rocks or other surprising debris that beavers find nearby. They are always busy shoring up their habitat. These solid structures help to maintain impressive quantities of water in pools, even during drought conditions.”

In terms of water quality, beaver dams act as natural settling basins, similar to wetlands. They back up water for varying periods, capturing sediments and allowing the natural microbes and plants to process excess nutrients, such as nitrate. The researchers have found that, in general, nitrogen processing in the pools behind the beaver dams is reducing nitrate concentrations leaving the pools by several milligrams per liter during period of higher rainfall like late spring and early summer compared to water from stream reaches above the dam. 

“That may not sound like a lot,” Beck said, “but it can significantly reduce total nutrient loads leaving the watershed.”

These retention ponds also impact watershed hydrology by slowing runoff from the landscape.

“Their hydrological impacts depend on a number of factors,” Rupiper said. “That includes the type of stream system, the number of beavers and their pools – and how much the water can spread out behind the pools. Especially in smaller watersheds, it looks like the beaver dams can make a difference by reducing the energy of flow during high water and associated erosion and sediment transport.”

The research team is working with others on geographic information system-based modeling to explore the potential impacts of beaver dams in different rain events.  

“Beavers aren’t always appreciated,” Rupiper said. “But in the right context -- in locations where they are farther away from crop fields or other places where they could back up water in a detrimental way and where they have access to the woody vegetation they prefer -- our preliminary work suggests that beavers may function as a virtually free, natural conservation practice with multiple benefits.”

The Iowa Nutrient Research Center pursues science-based approaches to evaluating the performance of current and emerging nutrient management practices, providing recommendations on implementing the practices and developing new practices. Since 2013, INRC has invested approximately $15 million in 127 nutrient-related water quality projects.

American beaver chewing on woody stem with leaves
American beaver (Castor canadensis).