Beavers become partners in restoration.

By Lisa Owens Viani

As public support for trapping has waned, beavers are making a comeback in urban waterways around the country. In Seattle, they are now said to be found in every suitable stream and water body, and some project designers now see them as partners in wetland restoration rather than nuisances. They say the benefits beavers bring to an ecosystem outweigh the challenges, and point out that working with them is far less expensive—and more humane—than trapping.

“Beavers construct wetlands that hold back and store water, allowing for groundwater recharge and pollution sequestration, and increasing biodiversity,” says Ben Dittbrenner, the aquatic ecologist and executive director of Beavers Northwest. “We do the same thing for hundreds of thousands of dollars, but they do it for free.” This past October, Dittbrenner, the biologist David Bailey, and Ken Yocom, ASLA, an associate professor and chair of the department of landscape architecture at the University of Washington, published a study that examines the influence of beavers on three wetland projects in Seattle and makes recommendations for managing them adaptively.

In Golden Gardens Park, beavers surprised project designers by moving into a stormwater treatment lagoon and felling some large alders to build a dam. Their actions added water storage capacity and complexity to what had been a relatively simple habitat, but they also threatened to flood a boardwalk. Park staff adapted by installing devices specifically designed to manage water levels behind beaver dams. The devices—systems of pipes with filters, with product names like Castor Master—keep the pool of water behind the dams at a selected elevation while disguising the flow of water. (When a beaver hears the sound of running water, it wants to repair its dams.)

At Magnuson Park, when building a stormwater treatment wetland and vernal pool habitat, the landscape architects anticipated that beavers might move in but did not take specific proactive design steps, says Guy Michaelsen, ASLA, a principal at Berger Partnership. When the beavers came, they built two large dams, increasing the size of some of the ponds by 20 to 25 percent. “They helped change and backwater all of our ponds, so we have a significant increase in our wetlands than what we had designed,” Michaelsen says.

Dittbrenner says that as urban beaver populations increase, designers should assume that beavers will colonize their project, especially if the animals are already in the vicinity. “It makes sense to stop and think about how these animals might affect these urban designs before we spend all this money to build them.”

Designers at Thornton Creek knew beavers would move into the restoration project because a colony lived nearby, so they purposefully created a wide floodplain, avoided putting pathways and bridges in low-lying areas, and replaced a culvert with a floodplain-spanning bridge.

They also installed in-stream wood structures, knowing that beavers would put them to use. “What beavers do to create landscapes is phenomenal,” Yocom says. “Your design is just the beginning. We have to let go and be willing to work with ecological processes instead of being invested in a strict aesthetic.”