Beaver dam

A minimum water level of 0.6 to 0.9 metres (2.0 to 3.0 ft) is required to keep the underwater entrance to beaver lodges from being blocked by ice during the winter.

[citation needed] In lakes, rivers and large streams with deep enough water, beavers may not build dams, and live in bank burrows and lodges.

[1] Then sticks, bark (from deciduous trees), rocks, mud, grass, leaves, masses of plants, and anything else available are used to build the superstructure.

Beavers can transport their own weight in material;[citation needed] they drag logs along mudslides and float them through canals to get them in place.

It takes a beaver about 20 minutes to cut down a 15-centimetre (5.9 in) wide aspen, by gnawing a groove around the trunk in an hourglass shape.

[7] The largest known beaver dam is in Wood Buffalo National Park in Alberta, Canada, and is 775 metres (2,543 ft) long.

Wetland benefits include flood control downstream, biodiversity (by providing habitat for different species), and water cleansing, both by the breakdown of toxins such as pesticides and the retention of silt by beaver dams.

Impeded fish movement because of dams, siltation of spawning habitat and low oxygen levels in ponds were the most often cited negative impacts.

Afterward the dam gradually releases the extra stored water, thus somewhat reducing the height of the flood wave moving down the river.

Farming along the banks of rivers often increases the loads of phosphates, nitrates and other nutrients, which can cause eutrophication and may contaminate drinking water.

Additionally, bacterial populations absorb nitrogen and phosphorus compounds as they pass by in the water stream and keep these and other nutrients in the beaver pond and the surrounding ecology.

The dams provide calm water which means that the young salmon can use energy for growth rather than for navigating currents; larger smolts with a food reserve have a better rate of survival when they reach the sea.

Beaver dams have been shown to be beneficial to frog and toad populations, likely because they provide protected areas for larvae to mature in warmer, well-oxygenated water.

By stimulating the growth of species of plants that are critical to populations of songbirds in decline, beaver dams help create food and habitat.

While this is sometimes necessary, it is typically a short-lived solution, as beaver populations have made a remarkable comeback in the United States (after near extirpation in the nineteenth century) and are likely to continually recolonize suitable habitat.

Introduced to an area without its natural predators, as in Tierra del Fuego, beavers have flooded thousands of acres of land and are considered a plague.

Warming temperatures in the Arctic allow beavers to extend their habitat further north, where their dams impair boat travel, impact access to food, affect water quality, and endanger downstream fish populations.

The rich thick layer of silt, branches, and dead leaves behind the old dam is an ideal habitat for some wetland species.

Finally the meadow will be colonized by riverine trees, typically aspens, willows and such species which are favoured by the beaver.

Research is sparse, but it seems likely that parts of the bottomland in North America was created, or at least added to, by the efforts of the generations of beavers that lived there.

BDA builders may use construction techniques beyond the beaver's capabilities, such as driving wooden posts into the stream bed to brace horizontal branches that would otherwise be washed away.