Beaver chewing stick in water © Amy Vaughn
© Amy Vaughn

Beaver on felled tree at PV © Ed Anzures
© Ed Anzures

Easily recognizable by its long, broad tail, and its noticeably big and sharp front incisor teeth, beavers are most active at night.

The most obvious signs of beavers are the distinctive "pencil points" of gnawed tree trunks and the lodges and dams they build.


The largest North American member of the rodent family, the beaver (Castor canadensis) measures 32 to 48 inches and weighs anywhere from 27 to 67 pounds.

Beavers are well adapted for their aquatic environment, with broad, flat tails (at a length of 12 to 20 inches) that they use to scull or steer, and large, webbed hind feet. They use their small and un-webbed front feet for carrying and digging. Beavers sport brown, thick fur, with a waterproof underfur. 

People often mistake the much-smaller muskrat for a beaver because of its aquatic habits and similar shape. But the muskrat only weighs two or three pounds and has a narrow, laterally flattened tail.

Life Cycle

Beavers in North Pond at Pleasant Valley Wildlife Sanctuary (Photo: Bill Bernbeck)

A family unit—called a colony—consists of two adults, the young of the year, and the young of the previous year. Beavers become sexually mature at two years old, at which time they are driven off by the adults to seek mates and find territories of their own.

Beavers are typically monogamous and, unlike many mammals, the male stays with the female year-round. They mate between January and March, and the kits are born four months later. The litter can contain between one and nine kits, but a typical litter is four.

The kits are born fully furred with their eyes open, and become skillful swimmers by the time they reach one-week old. They're able to eat vegetation at two to three weeks, and are weaned by the time they reach three months.

Beaver Dams

Beavers are unique among mammals in that they alter their habitat to meet their needs by damming streams to form ponds. This behavior actually benefits other species (including people), as well. By building dams and flooding woodland swamps, beavers play an important part in the restoration of lost wetlands (over 50 percent of our wetlands have disappeared since European settlement in North America), providing habitat and food for a wide variety of plants and animals.

Beavers build their dams in order to create deep ponds that won’t freeze at the bottom in winter. Within these ponds, they build lodges to provide protection for the young in summer and for the entire colony in winter. Lodges are large structures, from 15 to 40 feet across and three to six feet above the water. They line the living space (located above the waterline) with dry plant material. Entrances are located underwater.

Because of the flooding beavers create, trees often die off, providing nesting sites for Great Blue Herons, Wood Ducks, Tree Swallows, and other birds. These new ponds become homes to amphibians, turtles, fish, otters, muskrats, and other animals.

Beaver-created wetlands also enhance human habitat by storing and slowly releasing floodwater, which controls downstream flooding. They improve water quality by removing or transforming excess nutrients, trapping silt, binding and removing toxic chemicals, and removing sediment. And finally, flooded areas can also recharge and maintain groundwater levels, and provide flow to streams even during droughts.

Beaver chewing stick in water © Amy Vaughn 1000
© Amy Vaughn


Beavers are vegetarians, feeding on any number of plants and trees in the summer. In the winter they feed on the inner bark of trees, particularly aspen, willow, birch, and alder. In the fall they collect edible branches and anchor them in the mud at the bottom of the pond near the entrance to their lodge. In the winter, the colony stays inside the lodge, leaving only to gather the cached food from the bottom of the pond.

Beavers in Massachusetts

Before colonization by European settlers, the beaver thrived in Massachusetts, but unregulated hunting and trapping combined with extensive deforestation resulted in its extirpation by the end of the 18th century.

With the decline of agriculture at the beginning of the 20th century, the forests that beavers needed for habitat began to regrow. In 1928, beavers were discovered in West Stockbridge, the first recorded occurrence in the state since 1750. To help restore beaver populations, three additional beavers were brought from New York and released at Mass Audubon’s Pleasant Valley Wildlife Sanctuary in Lenox. Today they live throughout the state, with the exception of Cape Cod.

Situations & Solutions

Beaver at Pleasant Valley Wildlife Sanctuary © Rene Laubach, Mass Audubon

Both humans and beavers change their surrounding to suit their needs (see Beaver Dams), and from time to time, these needs may come into conflict. Property owners, in particular, may experience unwanted flooding. Since beavers are largely beneficial, it’s in our interest to find the least draconian solutions to beaver problems.


Relocating wildlife is illegal in Massachusetts. It's also illegal in Massachusetts to disturb beaver dams or other structures without permits (the same applies to muskrat structures).

To get a permit, you must take the following steps:

1. Contact your local Board of Health to apply for a 10-day emergency permit that will allow you to take specific actions to resolve the problem. You must be able to show that beavers are causing the problem, and that the situation constitutes a threat to public health and/or safety.

2. If the Board of Health approves your application for the 10-day emergency permit, you must then get an emergency certification from your local Conservation Commission for permission to breach the dam or install a Water Level Control Device (WLCD).

Removal or Breaching of a Dam

Breaching a dam can cause severe flooding downstream, and draining a wetland affects many species—not just beavers. If a dam must be breached, it’s best to create a small notch to control the slow release water. Large breaches allow water to rush through, creating too much water pressure. This can result in the entire dam giving way.

The sound and movement of water stimulate beavers to rebuild and repair their dams, sometimes as quickly as overnight, so consider breaching a dam only as a short-term solution to flooding. Because beavers are nocturnal, it’s best to breach a dam early in the morning to allow water to flow all day.

Water Flow Devices

Many property owners find success with water flow devices, which prevent issues from flooding caused by beaver dams. Pipe system devices control the water level in a beaver pond—offering a long-term solution for homeowners while allowing beavers to continue maintaining precious wetlands.

A wide variety of devices exist on the market, with various ways to install them. The best products muffle the sound of moving water and minimize the sensation of flow—preventing industrious beavers from rebuilding.

Lethal Means

When conflicts arise, it’s better to seek long-range solutions. Destroying beavers just creates vacant territory for new beavers to move in. If you must turn to lethal options, you will need to get a permit from your local Board of Health. You must be able to show that beavers are causing the problem and that the situation constitutes a threat to public health and/or safety. Beaver elimination should occur only at the end of summer, to avoid orphaning dependent kits. Only licensed trappers and Problem Animal Control agents (PACs) can trap beavers.

Tree Damage

To prevent beavers from chewing on individual trees, surround the trunk with 4-foot galvanized garden fencing (2" x 4" mesh), making sure the mesh is flush with the ground. For groups of trees, standard fencing works well. Beavers are poor climbers, rarely burrow under fences, and generally do not chew fencing unless it’s wrapped tightly around the tree.

Beavers and Disease

Caused by the flagellate protozoan Giardia lamblia, Giardiasis (sometimes called “ beaver fever”) is a common cause of gastroenteritis in humans. Humans and other mammals, including beavers, can carry the disease, which can be transmitted via contaminated water.

While beavers are occasionally blamed for waterborne outbreaks, studies show higher levels of the Giardia cysts in water near high-use human recreational areas. Most contamination cases in Massachusetts derive from human origin—such as washed-out septic systems, untreated human sewage discharged into waterways, cabin toilets, and contaminated feces from campers and backpackers in rivers and streams.