When Climate-Change Flooding Strikes—Call in the Beavers

by Michael J. Caduto

American beaver (Castor canadensis). Photo by Tim Umphreys on Unsplash

It is not surprising that an Ojibwe creation story tells of how the remnants of a giant dam built by Beaver (Amik) created the 30,000 Islands in Lake Huron. With sharp incisors, flat tails and an industrious character that is legend, few animals match the beaver’s ability to transform its surroundings. Beavers are strong and live up to their industrious reputation. Large adults weigh nearly 70 pounds and can heft a submerged, ten-pound rock, then carry it by pressing it against the chest with the forearms. Like humans, beavers can alter wide swaths of habitat that transcend all boundaries.

When it comes to territorial disputes with beavers, we humans rarely suffer competition with grace, or sometimes even compassion. Imagine the number of beaver conflicts that began with a biological impulse triggered by the mere sound of running water. A tiny trickle, a muted gurgle—the beaver’s clarion calls to build a dam. The resulting floods have inundated roads, forests and farmland. And a beaver’s got to eat, so the surrounding poplar, aspen and birch come down while water lily roots, grasses, sedges and cattails disappear.

Fortunately, the local nuisances that beavers create are just a subplot in a long-running story that plays out on a grander scale. In the Connecticut River Valley, following the 1955 hurricane, flood control dams were built by the Army Corps of Engineers along the tributaries. Behind each dam is a basin that catches water during heavy rains. These waters are slowly released during the following days and weeks in order to prevent flooding downstream. Whoever devised this scheme must have been a student of beavers. In this age of intense, episodic rain events and extreme floods driven by global warming, beavers are nature’s climate-change mitigation team.  

“Beavers create flood storage impoundments that change the water regime,” observes Francis C. Golet, Professor Emeritus in the Department of Natural Resources Science at the University of Rhode Island and renowned international expert on wetlands ecology. “They build a dam that holds water back in a basin. When flooding occurs, wetlands form.”

Golet sites one study in Wisconsin—within the larger homeland of the Ojibwe—which found that the greater the percentage of a watershed covered in wetlands, the greater the flood reduction. “Landscapes with very few wetlands, such as those where many have been filled or drained, create major downstream flooding problems.”

Beavers prefer to build their dams where food is abundant and in wide valleys that have a stream gradient of less than 6 percent. In this kind of environment there is a smaller chance of severe floods.      “The composition of a landscape can have a major impact on the safety of people living downstream,” says Golet. “By building a dam, beavers are decreasing the gradient of a stream by flattening it out with a basin. This both delays runoff getting into the stream and reduces the peak flood levels downstream. Beaver pond basins drain for a period of several weeks after a storm and slowly release storm runoff, so you don’t get the flash-flood effect. This is called de-synchronization, where runoff going into a stream doesn’t drain all at once.” (An excellent local example of this phenomenon occurs in the beaver-dam wetland bounded by Whitmore Circle in Reading, VT, which can be clearly seen from Route 106, some 7.5 miles south of the center of the Village of South Woodstock.)

Many of the traditional villages constructed by New England’s Native peoples were located on the rich, flat, well-drained soils of river terraces and floodplains, often within 60 feet of the water level and within 300 feet from the water’s edge. Some of these villages existed for thousands of years when beaver were more prevalent; the multi-tiered steps of dams and wetlands created by our largest native rodent protected indigenous people from sudden, disastrous floods. They were the Beaver Corps of Engineers during an earlier era.

A complex of beaver dams and wetlands in stream and river valleys creates a series of terraces which slows the rate of water flow, increases sedimentation behind the dams, helps to control erosion and improves water clarity in the free flowing waters. Beaver complexes increase the height of the water table associated with the wetlands, create recreational areas and aid in restoring riparian (riverbank) environments whose quality has been reduced by human activity. The canals used by beavers in their impoundments act like liquid arms that spread the water out over a larger area.

Some researchers have even explored the role of periodic flooding by beavers in helping to control invasive species. “Beavers create habitat that increases biodiversity, and they have an impact on an area that lasts for a while,” says Golet. “They create marshes, wet meadows and dead trees for insectivores and cavity nesters.”

But that isn’t the end of the story. Beaver activity is cyclical: They come, they build, they eat and reproduce, then move on. Beavers really do eat themselves out of house and home. During the growing season they prefer grasses, sedges, cattails and lots of water lily roots, which they also store for winter food. In late autumn, winter and early spring, much of a beaver’s diet is woody plants—the inner bark, growing tips and leaves of aspen, willow, alder and white birch are among their favorites. Red maple will do in a pinch. Some winter food comes from an underwater cache of green branches and tree trunks which they create in deep water close to the lodge.

After beavers exhaust the food supply around their impoundment, they leave the area. The dam gradually breaks down, the water level falls and wetlands grow up to more advanced stages of succession. Each stage provides habitat for some different species. Eventually, plants that offer food for beavers become prevalent again; beavers arrive, build a dam and the cycle begins anew.

Michael J. Caduto is the author of Pond and Brook: A Guide to Nature in Freshwater Environments, and the children’s picture book, Riparia’s River.


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