Riparian Zone
"Riparian" means by the side of a river, stream or lake or pond of fresh water. Beavers need a reliable year-round source of water. They also need trees whose bark they can eat in winter. Their favorites are willow, birch aspen, cottonwood, poplar and alder, and these are common in riparian zones throughout North America.
Climate
North American beavers live all through the temperate zone from rainforest to desert, but not in the arctic or tropical zones. The true arctic regions lack trees, and in the tropics the beaver's niche is filled by another large aquatic rodent, the capybara.
Terrain
Beavers live in high mountain valleys, where the snow lies heavy and ice covers the water all winter. Here they build lodges in their ponds with the trees they cut down in the summer, and eat the cambium underbark all winter. Beavers also live in flat country by large rivers with fast-moving water and here they dig a riverside den rather than build a lodge.
Habitat Adaptation
If beavers find a place that has water and trees, but the water is not permanent or isn't big enough for their needs, they will change the environment to suit themselves. They are second only to humans in this ability. They create freshwater wetlands, a rare and valuable land-based ecosystem, by damming shallow dry valleys so that water from rain and melted snow collects and forms marshes and ponds. These wetlands created by beaver dams nurture the beaver, but they also support biodiversity, control flooding and drought, prevent erosion and purify water.