(Photo at left courtesy of Chris Bernstein)

If you drive slowly enough past the Veterans Memorial Park on Route 139 near Marshfield Center, you may notice a tree that has fallen down across the South River. For at time this past spring, before it fell, it appeared to be balancing on a fine point, as beavers had gnawed away at its trunk from all sides.

Beavers are native to Massachusetts. They have been around for thousands of years. Native American tribes hunted them for meat, skins and medicine. Early European settlers used beaver pelts as a form of currency. However from the late 1700s to the early 1900s, as lands were cleared to trees for shipbuilding and agricultural purposes, beaver habitat disappeared. That combined with widespread hunting drove beaver populations away from New England. Now that dense forests are coming back, the beaver has returned as well — in abundance.

Beavers can be found throughout much of North America, from northern Canada to northern Mexico, but they are rarely found in Southeastern Massachusetts, Cape Cod & the islands. They prefer flat terrain, and need water deep enough to provide habitat under winter ice. Generally they can be found in rivers, lakes and ponds, or in areas that can be converted to beaver ponds.

The beaver (Castor canadensis) is the largest rodent native to North America. It is dark or reddish brown in color, with long, shiny hair, small eyes and ears, and pointy incisor teeth. In adulthood, it can weight between 35 and 80 pounds. There is no size difference between male and female. A beaver’s body ranges from 2-2.5 feet long, with another 10-18 inches of wide, flat, leathery tail. The tail helps it to swim, to stand upright, to regulate body temperature, and to communicate with other beavers (by slapping their tails on the water). It also has large webbed hind feet, which helps it to rise up and look around, to gnaw on trees, and to gather food.

Beavers move slowly on land, and thus spend about 80% of their time in water. Among other places, you can see them on the South River, swimming under the Keville Bridge behind Marshfield’s CVS Pharmacy.

Beavers are strict vegetarians, and although they live primarily in the water, they do not eat fish. They feed on aquatic vegetation, especially water lilies, as well as the shoots, twigs, leaves and roots of other woody plants. They also enjoy the outer and inner bark of trees and shrubs. Their skulls are quite large, to accommodate the chewing muscles required for such a hearty diet.

The reason beavers fell trees is that it helps them to access more food than they otherwise could reach from the ground. Once such a tree is stripped of food sources, it is most often used to construct a dam or a lodge.

Beavers build dams to increase the overall size of their habitat. Damming a brook will create a pond for the beavers to inhabit. There they have access to food, shelter in the winter, and protection from land-based predators. They spend much of their days caring for and repairing the dams.

Their dams are constructed with mud and stones at the base, with logs and branches piled on top. More mud, stones, and aquatic vegetation provide a kind of glue to hold the structure together. Most beaver dams are less than 100 feet long. Several dams may be constructed along the same stream, creating terraces with standing water, an ideal environment where the kinds of vegetation beavers favor – water lilies, cottonwood, alder and willow — can grow.

Within a pond, beavers will construct a lodge from trees, sticks and mud. These tend to be 15-40 feet across the top and the base, and 3-6 feet above water. They include a vent for air circulation, and one or more underwater entrances – but none above ground. Especially in winter and spring, mating and birthing season, this provides them with a cozy home. Once food sources become scarce, or the build-up of silt renders the pond too shallow, beavers will abandon it, and seek a new location to build another.

According to the MassWildlife website, since 1996 when a Massachusetts ballot referendum banned the use of certain wildlife traps, the annual harvest of beaver dropped from 1700 to less than 100. As a result, the beaver population has grown exponentially, from 24,000 in 1996 to about 70,00 today.

Beavers have few natural predators. Adult beavers may live up to 20 years. Each year an adult pair will usually bear 3-5 young – or as many as nine. The kits stay with their parents for two years before setting off on their own to find or construct new habitat — and to mate. They mate for life.

A typical beaver family unit is comprised of the parents, the current year’s kits, and the previous year’s young. This is known as a colony, and it is usually comprised of 6-8 beavers of varying age. As beaver population increases, so does their need for habitat. That’s why we’re seeing beaver in so many new places now.

While beaver dams can cause all sorts of trouble for humans, resulting in the flooding of yards, roads and septic systems, ecologists generally view them as a good thing. The pond that results from a beaver dam is an expansion of the area’s wetlands, which in turn provides filtration for groundwater as well as habitat for plants and other wildlife. You may not feel that way if it’s on your property, though.

If beaver dams are flooding or otherwise damaging your land, first you must ask the Board of Health to assess the situation. If it is not a threat to human health or safety, they will refer you to the MassWildlife District office. Some solutions they may suggest include fencing off trees to prevent damage, obtaining a permit to breach a dam, or installing water control devices. They also might also encourage you to become or find a licensed trapper (trapping season is November 1 – April 15). If the work of the beavers is not harmful to your property or your safety, consider sitting back and just observing them. They’re fascinating to watch – both at work and at play.


Beavers in Massachusetts: Natural History, Benefits and Ways to Resolve Conflicts Between People and Beavers by Scott Jackson and Thomas Decker

By Kezia Bacon-Bernstein, Correspondent
September 2005

Kezia Bacon-Bernstein’s articles appear courtesy of the North and South Rivers Watershed Association, a local non-profit organization devoted to the preservation, restoration, maintenance and conservation of the North and South Rivers and their watershed. For membership information and a copy of their latest newsletter, contact NSRWA at (781) 659-8168.