Montshire Minute: Beaver Dams

Originally aired during the week of June 19, 2000

Monday

Imagine taking a walk in the woods and stumbling across a quiet pond created by a beaver dam. It's easy to assume that the pond has been there for a long time, but in reality you are really witnessing a snapshot in time, one moment in the ever-changing history of the surrounding landscape. If you are observant, and you know a little bit about the natural history of beavers, you can tell when the pond was made. If the beaver lodge is vacant, you can also make some guesses as to how long the inhabitants have been absent, and what the landscape might look like in a few years time. Nature leaves many clues behind. Tom Wessels, author of Reading the Forested Landscape, has learned how to read many of them, and he shares his knowledge with us in a two-day field trip he's leading at Montshire next week. Listen in as we learn more about how to read some riddles posed by beaver dams.

Tuesday

We know that beavers are spunky engineers that can whip up a lodge in just a few days. They gnaw down trees, drag them to the lodge site, dam up moving water, and create a landscape populated with hardwood stumps. In short, they cause a lot of disruption. Then, when the beavers have used up the trees around their dam, they move on to another site. Wait a minute - the beaver is a scourge of the environment? A woodland equivalent to a human housing developer? Well yes and no. Beavers may stay in their lodge for perhaps 5-20 years, and the ponds they leave behind may last decades longer. But abandoned beaver dams also accumulate rich silt. When the pond dries out, a fertile meadow emerges on which new forest growth can take place. Geologists believe that centuries of ceaseless beaver activity helped produce the fertile valleys settlers found when they arrived in America.

Wednesday

Beavers are great swimmers. But when they're on dry land, searching for food or new material for their lodge, they are rather slow-footed. And these are the times they are most vulnerable to predators. In his book Reading the Forested Landscape, naturalist Tom Wessels says this is one reason why beavers have such a strong instinct to dam up running water. By creating a pond, the beaver can swim near the borders of its territory, limiting the amount of time it has to linger on land. Researchers have noted that beavers will get busy building dams on dry land in response to recorded sounds of running water. Beavers may do quite a job on the hardwood forests surrounding the pond, but they are actually very skilled recycles. They collect sticks and store them, feeding off the bark in the winter. Then, in the warmer months, they use the sticks to reinforce or repair the lodge.

Thursday

After beavers have run out of trees, they may move on to another dam site. How can you tell if a pond has been abandoned by the resident engineers? Naturalist Tom Wessels tells us there are signs of abandonment even a few weeks after the beavers depart. Without daily repair by the beavers, the dams will spring leaks and the water level may drop by as much as a foot. This exposes some of the mud around the pond's edge, where herbaceous plants soon appear. Looking at stumps left behind also provides insights into how long the pond has been abandoned. A tree cut within one year's time has blondish colored wood on the stump. Darker-colored stumps indicate that the tree was gnawed down by the beaver more than a year previously. Nature leaves many clues like these behind. Tom Wessels has learned how to read many of them, and he shares his knowledge with us in a two-day field trip he's leading at Montshire next week.

Friday

When we see an abandoned beaver pond, we are looking at the beginnings of a whole new ecosystem. The water level of the pond will begin to sink noticeably just a few weeks after beavers have departed. But if the pond is relatively strong, the pond will slowly fill with sediment and develop into a wetland with rushes and cattails - a "beaver meadow." As dead plant material decays, the sedges may give way to wetland trees and shrubs. Finally, enough organic matter accumulates to support red maple, and a wet-sited forest emerges. If the abandoned pond drains quickly, trees may take root more quickly. It takes many years for this to happen, of course. But the succession from wetland to forest environment will eventually present dam-building opportunities for a new family of beaver. Sometimes, a new dam will be built years later on the site of an old dam.