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Beaver dams are dams built by beavers to provide ponds as protection against predators such as coyotes, wolves, and bears, and to provide easy access to food during winter. These structures modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species. Beavers work at night and are prolific builders, carrying mud and stones with their fore-paws and timber between their teeth. Beavers can rebuild primary dams overnight, though they may not defend secondary dams as vigorously. ==Construction== A minimum water level of 0.6 to 0.9 m is required to keep the underwater entrance to beaver lodges from being blocked by ice during the winter. In lakes, rivers and large streams with deep enough water, beavers may not build dams and instead live in bank burrows and lodges. If the water is not deep enough to keep beavers safe from predators and their lodge entrances ice-free, beavers build dams. Beavers start construction by diverting the stream to lessen the water's flow pressure. Branches and logs are then driven into the mud of the stream bed to form a base. Then sticks, bark (from deciduous trees), rocks, mud, grass, leaves, masses of plants, and anything else available, are used to build the superstructure. The average height of a dam is about 1.8 m with an average depth of water behind the dam of 1.2 to 1.8 m. The thickness of the dam is often around 1 m or more. The length depends on the stream width, but averages about 4.5 m long. Beavers vary the type of dam built and how they build it, according to the speed of water on the stream. In slow-moving water, they build a straight dam, whereas in fast-moving water they tend to be curved. Spillways and passageways are built into the dam to allow excess water to drain off without damaging it. Dams are generally built wider at the base and the top is usually tilted upstream to resist the force of the current. Beavers can transport their own weight in material; they drag logs along mudslides and float them through canals to get them in place. Once the dam has flooded enough area to the proper depth to form a protective moat for the lodge (often covering many acres), beavers begin construction on the lodge. Trees approaching a diameter of 90 cm may be used to construct a dam, although the average is 10 to 30 cm. The length depends on the diameter of the tree and the size of the beaver. There are recorded cases of beavers felling logs of as much 45 m tall and 115 cm in diameter. Logs of this size are not intended to be used as structural members of the dam, but rather the bark is used for food, and sometimes to get at upper branches. It takes a beaver about 20 minutes to cut down a 15 cm wide aspen, by gnawing a groove around the trunk in an hourglass shape. A beaver's jaws are so powerful they can cut a 1.5 cm sapling in one bite.〔 Maintenance work on the dam and lodges is particularly heavy in autumn. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beaver dam」の詳細全文を読む スポンサード リンク
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