Beavers are famous as engineers, but their siting skills are often overrated. They are like people – some are smarter than others. Many beaver dams are washed away or broken because the beavers picked a bad location. Often, they could avoid much labor merely by moving a short distance up or down stream. Yet, once the beavers start to build, no other spot will do. And whatever the location, it’s their persistence that gets the job done.
Every October, beavers build a dam across the creek below our cabin, and every October I destroy it. Last year, I even tried to shoot them. They finally moved on but not before cutting down dozens of the mountain laurel, witch hazel, swamp dogwood, and sweet bay magnolia trees that grow on the creek bank. They are back this year and have built two dams – a big dam in the usual place and a small one a short distance upstream. And last weekend, I destroyed both dams and tried again to shoot the beavers. My sister-in-law from New York City thinks I am a horrible person.
When a dam site is selected by newly paired beavers, they set to work felling shrubs and saplings. Larger pieces are added later, but the bulk of the dam is constructed with relatively small material. Alder, which grows abundantly in streamside thickets, is frequently used. Each piece is dragged to the construction site as soon as it is cut and placed with the butt end facing upstream. But beavers will put anything they can find into the dam – live wood, dead wood, broken chairs, aluminum cans, old tires, river cane, grass and rocks. And they work fast because they are vulnerable until there is a pool of water into which they can retreat for safety. Two beavers can build a substantial dam in three or four nights.
A beaver dam is built on good engineering principles. The base is always much wider than the top to counteract the greater pressure on the bottom of the dam. The upstream face of the dam isn’t steep, but is tapered like a wedge against the water flow.
Beavers raise the crest of their dam uniformly across the stream. If they built the dam in sections, they would have no way of building it on the same level throughout. They don’t have carpenter’s levels to guide their work – they use the waterline, which makes a perfect natural level. At first, the dam leaks badly but the beavers dredge mud from the stream bottom and plaster it on the dam’s upstream side. The flow of water downstream washes the mud among the sticks, gradually clogging the leaks. They use rocks to compress and weigh down the brush in the dam. Sediment and leaves collect against the structure and seal it.
I appreciate the effort beavers put into building a dam because I know how much work it takes to dismantle one. I don’t appreciate the damage their work does to the trees and shrubs along the creek. But I have hope. Constant persecution will cause beavers to abandon a site and even give up dam building altogether. The beavers of Europe once built dams and lodges like those built by American beavers. But because of constant harassment by people, they gradually stopped and began living in dens excavated in the banks of ponds and streams. Now they are protected, their numbers have increased, and they are building dams and lodges again. This tells me I can’t let up.
Beavers aren’t protected in Alabama, and I am persistent, too. By the end of the month, the beavers on my creek will decide to find another creek to dam and different trees to cut ... or else. Maybe my sister-in-law is right.
Tate is a Carrollton resident and local nature and bird enthusiast.