Beavers typically start building dams in low lying areas with shallow, moving water. They will utilize natural or manmade objects such as a rock outcropping or a manmade stone wall, a constriction in the streambed, a tree stump, etc. to anchor their dams. So while somewhat predictable, they can select from a wide variety of spots in which to build a dam.
Once a pond is formed beavers do not have to travel far on land to gain access to new trees. The more area their dam floods, the more food they can safely access. Sometimes beavers will even excavate canals over a hundred feet long in order to bring water closer to stands of their favorite trees. This allows them to swim up close to the trees and retreat to the water quickly if they sense danger. In addition, they use the canal to float edible branches back to the pond.
Beavers predictably select sites to build their dams based primarily on topography and food supply. Preferred sites for damming will be in areas where the dam will flood a large flat area and there are plenty of desirable woody plants for food in the vicinity. Streams that are more than two feet deep or have strong currents are not generally dammed. Beavers often situate their dams where there are constrictions in the stream flow (natural or manmade). This is why beavers have a strong propensity to dam culverts. For relatively little work they can create a large dam and pond.
Each beaver colony will usually establish one large pond which where they will build their lodge. In addition to this primary pond other smaller dams up and downstream are usually built to create smaller ponds. These smaller ponds permit safe travel for the beaver as it seeks out new food supplies. The average beaver colony will dam a half-mile length of a small stream.
Beavers are monogamous and mate for life. They do not breed until they are two to three years old. In Massachusetts the female becomes pregnant during the winter and gives birth in May or June..
Beaver will have one litter of 1 - 6 kits per year. The availability of food appears to affect the size of the litter. Each established beaver "colony" consists of adult parents, and two years of offspring. Only the adult female breeds. The average number of beavers in an established family is typically six or seven beavers. We have seen as few as one and as many as thirteen.
Once a beaver reaches the age of two they will usually leave the colony to find a mate and establish a colony of their own. This is the most very dangerous time in the life of a beaver. Not only can they be killed by predators or cars, other beavers will attack them if they enter their ponds. As beaver populations expand uninhabited watersheds can be difficult to locate since suitable beaver habitat only comprises 1 - 2% of the landscape. Beavers have been noted to travel ten or more miles searching for a place to live.
Beavers have a highly organized social structure. Young beaver appear to play and wrestle with their siblings. This helps to develop their motor skills. They will groom each other using their hands and teeth. Young beaver have innate abilities to build dams and lodges, but improve these skills watching their parents or older siblings.
Beavers are typically social and peaceful animals, with a strong family structure. However, to protect their limited food supply, a beaver will not allow unrelated beavers to inhabit its pond. To mark their territories they surround their ponds with scent mounds. Scent mounds are piles of mud with the adult's castor oil mixed in. They act as warnings to any beaver that may be passing through the area. Adult beavers will defend their territory by attacking any beaver outside its family who enters it. However, other than territorial disputes or self-defense, they appear to have a strong inhibition towards biting and are not known to attack humans.