How beavers build dams, and how to build a small one at home

Finding a beaver dam was a sure sign that a beaver was nearby, which mattered to the fur trappers and traders around Fort Vancouver. From the 1700s into the 1800s, beaver fur was made into hats and other fancy clothing worn in Europe, and trappers in the Northwest sent furs from Fort Vancouver across the ocean. This National Park Service page explains how beavers build dams and shows how to make a small one at home.

NPS photo of two beavers.

What a dam is for

A beaver dam is made of logs and mud and blocks or slows the flow of a river or stream, creating a pond behind it. Beavers cut trees and branches with their strong front teeth and add grass, rocks and mud. They build the dam to make a safe pond for their lodge, whose entrance is underwater, and the pond protects them from predators such as wolves, coyotes and mountain lions. The beavers do not live in the dam: the lodge, built of twigs, sticks, rocks and mud, is where they sleep, raise their babies, stay warm in winter and hide.

The ponds matter to others, too. They are habitat for wetland animals including birds and fish, and they help control soil erosion and reduce flooding. The Park Service calls the beaver a keystone species: it changes its environment in ways that help other animals and plants.

Were the beavers all trapped?

Trappers in the 1800s took a lot of beavers across North America, and there are fewer today than before the 1800s. Beavers are not endangered, though: the page says there are about 10 to 15 million North American beavers in the wild.

Build a dam at home

You need a waterproof container deep enough to hold water and a dam, such as a baking dish or plastic container; dam materials such as mud, dirt, sand, rocks and sticks; shovels or trowels, or just your hands; and water. Ask an adult for permission before taking materials from your yard.

Image from the National Park Service's page.

1. Gather materials and imagine your yard is a forest. What would a beaver use? Sticks, rocks, sand or plants all work, and dirt mixed with water makes mud to hold everything together.
2. Build the dam inside the container in layers of mud, sand, twigs or other natural materials.
3. Slowly pour a small amount of water on one side of the dam.

Image from the National Park Service's page.

If the water breaks through the dam, empty the container and try again. Some water will always trickle past: a beaver dam on a stream does not hold the water back completely. If your dam holds most of the water, you have built one like a beaver's.

Sources

• National Park Service, Fort Vancouver National Historic Site, "Build a Beaver Dam" (https://www.nps.gov/articles/buildabeaverdam.htm); rewritten in hubnx's own words.