Making the pond it needs
Why Does a Beaver Build a Dam?
A beaver cannot defend itself on land. So it floods the land, and puts its front door underwater.
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Step 1 of 5
On land a beaver is slow and easy to catch.
It is heavy, short-legged and cannot run. A wolf or a bear would have no trouble with one caught out in the open.
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Step 2 of 5
So it makes the water deeper instead of moving house.
It drags branches and stones across the narrowest part of the stream and packs the gaps with mud. The water backs up behind and spreads out into a pond.
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Step 3 of 5
Then it builds a lodge with the door under the surface.
The pile of sticks in the pond is the home. Every entrance is below the waterline, so anything that wants to get in has to swim in the dark.
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Step 4 of 5
The teeth that cut the trees never stop growing.
A beaver's front teeth grow all its life. Gnawing wood is what keeps them worn down to a working length — a beaver that stopped chewing would be in trouble.
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Step 5 of 5
The pond ends up feeding a great many other animals.
Still water lets frogs spawn, fish shelter, and water plants take hold. Ponds made by beavers are some of the busiest places in a whole valley.
Beavers keep building while they can hear running water. Play a recording of a stream beside a dry bank and they will pile sticks against the speaker.
The short version
A beaver builds a dam to make a deep pond, because deep water is the one place it is safe and it can put its front door under the surface.
Try it yourself
Dam a trickle of water in a gutter with leaves and grit. Notice you do not build where the water is widest — you build where it is narrowest, and so does a beaver.