One day some scientists looked around and saw that the rivers were very dirty, and that there were less fish to catch.
They also noticed that there were fewer birds and dragonflies, and that the trees were starting to die.
They couldn't find what was causing the problem.
But they noticed something else was missing—the beaver.
So scientists decided to study how beavers live so that they can bring them back to the rivers.
Wherever beavers live, they make big changes to nature.
In fact, the only creatures who affect nature more are humans!
However, unlike humans, the changes that beavers make are good for the other animals and plants around them.
Once a beaver finds an empty spot on a stream or river, it builds a dam.
As soon as the dam is built, the land upstream begins to fill with water.
The extra water changes the land behind the dam into a wetland.
A wetland is a wide area of mud and water, and wetlands are very important habitats.
In fact, over half of North America's endangered species need wetlands to survive.
Once a beaver makes its dam, it takes care to make sure the water level around the dam stays high.
The beaver does this to protect its home, but it also protects nature.
The dam slows down a river, which helps make the water cleaner.
And instead of the river carrying good soil away to the ocean, the dam keeps the soil in the forest.
Even though beavers cut down trees to make their dams, they actually help the trees to grow.
A beaver's favorite trees for eating and building are cottonwoods and aspens.
Both of these trees make hundreds of baby trees every year.
However, if all the baby trees lived, they would eventually get too crowded, and they all would die.
But when the beaver takes out some of the trees, it gives the rest more room to grow.
This helps keep the forests healthy.
Beavers eat only plants, and this makes its other animal friends happy.
The wetland created by a beaver dam gives otter, mink, muskrats, deer, bears, dragonflies, turtles, frogs, and ducks—
and lots of other birds—a place to grow and hunt.
The reason scientists couldn't find fish or dragonflies or birds is that the beavers weren't there building their dams.
Even though beavers keep the rivers clean, the trees growing, and the other animals alive, many people get angry with beavers.
When dams make wetlands, sometimes they ruin a farmer's field or flood a road.
Some dams make it hard for fish to travel in streams.
But when people try to trap beavers to take them away, a new beaver always moves in.
So scientists are studying ways to help beavers and humans live together so we can all make changes that will be good for nature.