Why Do Beavers Make Dams? How Do They Work?

(Last Updated On: December 4, 2022)

They’re certainly not as weird as like… the platypus, but beavers are kind of weird when you think about them. They’re mammals with scaly tails, webbed feet, and they also have teeth made for going through wood. Of all the weird beaver things you’re aware of, you probably grew up learning that they make dams out of twigs and branches, which sounds like… a lot of engineering. So why do beavers make dams? How do they work anyway?

How Are Beaver Dams Built?

For starters, beavers don’t always build dams. Beaver dams need a water depth of at least 2-3 feet, or else the dam entrance will be blocked by ice during the wintertime. If the water is particularly deep, beavers might just build a separate lodge—dams are built when the water isn’t particularly deep. 

The purpose of the dam, then, is to create a water depth suitable for a beaver lodge. Beavers use branches and logs jammed into the mud to form a foundation; the rest is built from sticks, bark, rocks, mud, and really whatever the beavers can pick up off the ground to block the water. Eventually the dam will be large enough to flood the immediate area, creating a water depth allowing for a beaver lodge. 

Normally, beaver dams sit between around 10 feet and 330 feet, which is a pretty big range but that comes with the territory of damming entire canals. The largest beaver dam is in Alberta, Canada (specifically the Wood Buffalo National Park), and it’s 2,790 feet long. Also it can be seen from space. This dam is made of at least two different dams that ended up merging, and holds at least two beaver lodges inside of it. Apparently, satellite imaging suggests other beaver dams near this behemoth could merge with it too, adding anywhere between 160 and 330 feet to it in the next decade

What Do They Do?

Building a dam sounds like it alters the environment quite a bit, which is kind of true. Beaver dams often create or restore wetland areas, which is actually really important, since wetlands are among the most productive ecosystems. Over a third of the threatened/endangered species in the US live exclusively in wetlands, so preserving the wetlands is kind of a big deal. 

Broad strokes, beaver dams serve as important tools for flood control, since the dams control water flow. Rivers with beaver dams have been shown to have lower high-water regions and higher low-water regions. Beaver dams also accumulate a lot of bacteria, especially at their bottoms. This helps denitrify the surrounding soil as well as purify the space of some herbicides or pesticides that may end up in the stream water.

Though where beavers have been introduced without natural predators, beavers have ended up flooding a lot of land with excessive dam building. This messes with the people who live there, of course, but in the case of Tierra del Fuego it has also resulted in the destruction of endangered habitats. 

Beaver Lodges

So dams are used to set up beaver lodges. The beaver lodge itself is where the beaver engineering really happens. 

Beaver lodges serve a pretty straightforward purpose: they protect beavers during the winter and from predators. Building is sped through during the night, and beavers normally spend the fall doing dam and lodge maintenance to prepare for the winter. The dams functionally create a moat around the beaver lodge, which serves as a deterrent to some predators. Points of entry and exit to a beaver lodge are located underwater, so the less aquatically inclined will have trouble getting in. 

Beavers are native to the UK and North America, and while in North America lodges serve as a nice protective place, beaver lodges are rarely used for protection in the UK. Why? Well beavers don’t really have predators in Europe anymore thanks to humans. They still make lodges though. 

Anyway, beavers can’t breathe underwater. So beaver lodges have a big air pocket in them where the beavers themselves actually live. It works similarly to how you create an air bubble if you put an upside-down bucket under water. Neat. 


Beavers are typically associated with Canada, see if you know your other Canadian animals here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.

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