Engineers use drains to deter busy beavers
Beavers are much busier. They build dams, cut trees, dig channels, build lodges — and in so doing create rich wildlife habitat and help control floodwater and stream flows.
But they also can create headaches for humans. This becomes increasingly apparent as suburban development encroaches on beaver habitat and even affects areas where beavers are protected.
The key is gurgling water. When beavers hear it, they go for it, bringing with them sticks, branches, twigs and mud. Then they plaster over the source of the gurgle until the gurgle goes away.
This is great for beaver dams, but it's bad for culverts.
Engineers want culverts to gurgle, because a gurgling culvert is doing the job it was made to do, that is, to keep water moving.
Humans don't always win the contest: Often, as soon as one beaver dam is cleared from a culvert, another is under construction.
It's enough to make engineers chew their pencils in half.
In a couple of places in East King County, engineers have resorted to something called a beaver deceiver, a device that lets water flow but doesn't let beavers get to the gurgle. Usually it consists of a plastic pipe inserted through a beaver dam. Its upper end is anchored below the surface of the water in the pool. The lower end of the pipe also is submerged.
This makes it difficult for beavers to hear where the water is escaping. But to make sure they can't plug the pipe, engineers construct a wire-mesh enclosure around the openings to keep beavers away.
Once installed, the drain prevents water levels from rising higher.
"You go to a place which is a controlling dam, and you place one of these," said Jon Hansen, senior ecologist with the King County Department of Natural Resources. "The water level will be reset, but not completely gone. The beavers won't get what they want, but they won't go someplace else.
"We're still trying to find out how you do those things in different settings."
One setting is Peterson Pond, sometimes called Swan Lake, east of Redmond near Union Hill Road and 238th Avenue Northeast. The pond drains through a fish ladder equipped with a grate over the upstream end of the ladder.
That grate, designed to catch trash and debris and keep it out of the ladder, worked just as well as a foundation for beaver engineering.
A King County Web site tells the tale:
"While the Peterson Pond beavers had demonstrated their ongoing commitment to damming the pond outlet, the local residents strenuously objected to any removal or relocation of the beavers."
That was in 2000. At the time, the King County Wildlife Program also was working with the Humane Society of the United States and Skip Lisle, the inventor of the beaver deceiver, to present a "Solving Conflicts with Beaver" workshop in King County.
Lisle invented the deceiver while working as a wildlife biologist with the Penobscot Indian Nation in Old Town, Maine. He came up with the idea after seeing how beavers had plugged culverts under the Penobscots' logging roads, causing them to wash out; subsequently, more than 30 deceivers were installed successfully.
Lisle and another speaker at the workshop, John Hadidian of the Humane Society, readily agreed to lend their expertise to the Peterson Pond beaver problem.
They concluded that a beaver deceiver would solve it, which it did. However, at another site, the Carnation marsh near the Snoqualmie River west of Carnation, things are not nearly so clear-cut. A beaver deceiver was installed there a year ago, with the idea that it would help regulate summertime water levels in the complex 120-acre wetland by providing an outlet that beavers couldn't plug.
The Seattle Audubon Society owns and manages the marsh with the goal of preserving it and its wildlife and keeping it from being drained.
But local beavers have been just a bit too busy, said Chuck Adams, chairman of the society's nature-preserves committee. The beavers' extensive network of ponds and dams had increased water levels to the point at which the adjacent Snoqualmie River Road had actually flooded during dry summer months, and neighbors said the higher water levels have drowned a number of big, mature Sitka spruce trees near the road.
These concerns reached the King County roads department, where Don Althauser is the supervising engineer for Surface Water Engineering Services.
The first work was a small beaver deceiver put in by a Washington Conservation Corps crew, he said. Later, a roads crew installed a culvert underneath the roadway, built catch basins and repaired the road. The work required a slew of environmental permits for work in the wetland.
Althauser's goal was to keep the water level below the road. But he readily admits that he has no expertise in wetlands, beavers or their ecology, and that building beaver deceivers in such areas is new.
"On large capital works, this is the first one we've done," Althauser said. "This is a dicey one. It's not a normal thing at all."
Nor is it easy to gauge the success of the project. Althauser noted that the marsh, which is recharged from groundwater and periodic flooding of the Snoqualmie, discharges those waters at different places at different times. It is nearly impossible to say what effect, if any, the deceiver is having.
Hansen, the King County ecologist, said water levels in the marsh probably also change from year to year, "based on weather conditions, the beaver, a whole host of things."
But he, too, has little experience with beaver deceivers — or beavers, for that matter.
Kate Stenberg, however, is an expert. With a doctorate in wildlife management, she owns an environmental consulting firm and volunteers as chairwoman of the urban-wildlife working group of the Wildlife Society.
She said beaver deceivers, if properly designed, should have no adverse effects on the beavers.
"However, one of the things that concerns me is, if they are doing the type that maintains water levels, they may not provide enough of a pool for the beavers," Stenberg said.
That's a critical issue for beavers, who must have pools to survive and reproduce. Stenberg explained that beavers rely on the water for protection from predators, because they are vulnerable on dry land. At the same time, they must venture onto land for food. Rising water levels benefit them by expanding their safe habitat into new areas where food is easily reached.
"They dig these huge channels around the edges, deep enough to swim in," Stenberg said. "They are really doing a lot of earth moving, dam building, hydro works — which is then what supports all the other animals that depend on those habitats."
Stenberg had not seen the beaver deceiver installed in the Carnation marsh, but she said it might have no effect on flooding caused by beavers and otherwise be benign, or it might affect water levels in the entire 120 acres.
"And that may be a severe impact," Stenberg said.
Conversely, the beavers' own reworking of the environment should not be cause for alarm. The dying spruce trees, for example, very likely were killed by rising water levels, but Stenberg said that is part of a natural cycle. And the snags are valuable habitat for a lot of birds who nest nowhere else.
"The other thing that's happening is, there is a lot of development on the plateau above the Carnation marsh," Stenberg said. "Folks say it isn't dense enough to cause runoff, but there is an increase in impervious surface. It's going to be difficult to measure."
One thing that is clear is that the beavers seem to have moved away from the deceiver. The site shows no signs of dam repairs.
Stenberg said this, too, is normal.
"Beavers move around the landscape," she said. "Eventually, they come up against the laws of topography and they move on."
Jon Savelle: 206-464-3192 or jsavelle@seattletimes.com