Why do beavers build dams?
Quick answer
Beavers build dams across streams to create deep, still ponds. The water protects them from predators like wolves, since their lodge entrances are underwater, and lets them store branches for winter food under the ice. Beaver dams change river ecosystems so much that beavers are called “ecosystem engineers” and a keystone species.
How a dam is built
A family of beavers can build a working dam in a few nights, and they keep improving it for years.
- Beavers fell trees by gnawing around the trunk.
- They drag or float branches to a narrow spot in a stream.
- They wedge sticks into the streambed and pile up branches, stones and mud.
- They patch leaks constantly; the sound of running water is a strong trigger to start repairs.
Beavers then build a lodge, a dome of sticks and mud with an underwater entrance and a dry chamber above the waterline.
Teeth that never stop growing
A beaver’s front teeth grow throughout its life and are worn down by gnawing. Their outer layer contains iron, which makes them orange and extra hard.
How dams change landscapes
| Effect | Result |
|---|---|
| Creates wetlands | Habitat for fish, frogs, insects, birds and moose |
| Slows floods | Water is held back and released gradually |
| Raises groundwater | Keeps land green during droughts |
| Filters water | Sediment and pollutants settle out |
| Firebreaks | Wet areas can slow wildfires |
The longest beaver dam
The longest known beaver dam, in Wood Buffalo National Park, Canada, is about 850 m (2,790 ft) long, roughly twice the length of the Hoover Dam. Researcher Jean Thie spotted it on satellite images in 2007. It’s so remote that the first person reached it on foot only in 2014.
Two species
| Species | Range |
|---|---|
| North American beaver | Canada, US, northern Mexico |
| Eurasian beaver | Europe and Asia |
Both were hunted almost to extinction for their fur, especially for felt hats. Today, beavers are being reintroduced in parts of Europe, including the UK, to restore wetlands and reduce flooding.
Beaver facts
- The beaver is Canada’s national animal and appears on its 5-cent coin.
- Beavers can stay underwater for about 15 minutes.
- Their flat, scaly tail is used for balance, steering and slapping the water as an alarm.
For another animal that reshapes its habitat, see which marine mammal uses rocks to open shellfish, about sea otters protecting kelp forests.
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