Beavers occupy small and medium-sized permanent water bodies in forested areas (Whitaker and Hamilton 1998). All manner of aquatic habitats that can be dammed to form calm pools and ponds are used. In larger still or slow flowing water bodies (lakes and rivers), beavers only build lodges or use burrows, dug by themselves or other animals into earthen banks at water's edge (Webster et al. 1985). In mountainous areas, beavers typically have most success at building ponds in valleys and flattened areas. Therefore, populations are more apt to be concentrated at lower elevations (Linzey and Linzey 1971, Whitaker and Hamilton 1998). However, they can be found far up the tributaries of mountain streams (Webster et al. 1985).

Beavers rely on the inner bark, twigs and buds of nearby woody plants for food, harvesting these plant parts from shrubs near the ground or cutting down trees to gain access to them (Brown 1997). In doing this, they perpetuate early (seedling/sapling) successional stages of forest around their ponds.

NATURE SERVE GLOBAL HABITAT COMMENTS:

Beavers inhabit permanent sources of water of almost any type in their range, which extends from arctic North America to the Gulf of Mexico and arid Southwest, and from sea level to over 3400 m in the mountains. They prefer low gradient streams (which they modify; see below), ponds, and small mud-bottomed lakes with dammable outlets (Slough and Sadleir 1977, Beier and Barrett 1987, Novak 1987a, McComb et al. 1990). Beavers readily occupy artificial ponds, reservoirs, and canals if food is available. They generally avoid lakes with strong wave action and fast- moving streams. Waters with greatly fluctuating flow or water levels generally are poor habitat. In larger rivers (9th order or larger streams), beavers use floodplains and backwaters. In north-central Minnesota, one-third of the beaver families occupied bogs, two-thirds occurred in streams or lakes; most bog impoundments were at brushy minerotrophic sites (Rebertus 1986). Rebertus felt that beaver colonization of the bogs may have occurred when food supplies at lake sites were exhausted. In South Dakota, beavers depend on streamside riparian habitat for both food and shelter (Dieter and McCabe 1989). In the north, they require water that is deep enough such that it does not freeze to the bottom and allows the accumulation of a substantial food pile beneath the ice.

Beavers are associated with deciduous tree and shrub communities. These may be either edaphic equilibrium communities (typically on alluvial soils and on hydric shorelines) or transient communities (usually following fires but also associated with windthrow, insect outbreak, and logging) (Slough and Sadleir 1977). Conditions that may make an area unsuitable or marginal for beaver include high stream gradient or inadequate food supply (e.g. see Beier and Barrett 1987), especially in winter (see section below on diet). In eastern Oregon, beavers selected areas with gentle bank slopes, low stream gradient, and abundant deciduous canopy cover; they did not build dams at sites with a rock substrate (McComb et al. 1990). In northern Minnesota, beavers appear to select optimal pond sites first, then flood more marginal areas as their population increases and resources are depleted (Naiman et al. 1988).

In many situations beavers modify existing stream courses or other bodies of water and make them more suitable for their own use (see section below on dam building). They may occupy deep sections of streams and existing lakes or ponds without constructing a dam.

In South Dakota, beaver preferentially occurred in areas where the riparian zone was ungrazed, providing plenty of small trees for food and building materials (Dieter and McCabe 1989).

See Allen (1983) for a beaver habitat suitability index based on food and water characteristics. See Novak (1987a) for references to other models that have been developed in attempts to predict beaver family density.

Dam Construction

In many parts of the range, the first thing a beaver does when arriving in an area is begin building a dam (Remillard et al. 1987). Most dam building occurs April-June and especially August-October (Hill 1982), though repairs may be made at any time. All family members, except young in their first year, may assist in dam construction. More than one family group may cooperate in repairing a common dam (Schwartz and Schwartz 1981). The length and height of a dam may vary greatly, depending on the topography. In Quebec, the density of dams on small streams (4th order or less) averaged 10.6 per kilometer (range 8.6-16.0) (Naiman et al. 1986). In the northeastern U.S., a beaver family maintains an average of 2.5 dams.

Generally trees, together with mud and rocks, supply the material for a dam. In some areas where trees are not available, shrubs (e.g. sagebrush) are used, though the resulting dam may be relatively weak. Some dams may be made mostly of rocks.

Streams dammed by beavers typically are 2nd to 4th order streams; rarely do they build on 1st-order streams except at uncommonly high population densities. Dams in the main channel of streams of 5th order or larger often are destroyed by spring flooding (Naiman et al. 1986). In some areas (e.g. Missouri), dam building does not occur as commonly as in the north or west, perhaps due to the occurrence of deep swift rivers subject to severe flooding (Schwartz and Schwartz 1981).

Dams typically are built across streams or at the outlet of a lake, but beavers also may dam seepage areas associated with bogs (Rebertus 1986). During periods of flooding due to heavy rains or snow melt, beavers may construct a spillway in the dam. Dam repairs apparently are stimulated by the sound of running water.

In northern Minnesota, Johnston and Naiman (1990) found that during a period (1940-1986) of beaver population expansion and stabilization, 75% of the pond sites that ever were used were established in the first two decades of beaver colonization. Thereafter, creation of new ponds was constrained by geomorphology; ponds created after the first two decades averaged much smaller than those duirng the initial beaver population increase. (In 1986, at the end of the study period, beaver density was approximately one colony per square kilometer.)

Lodges

A characteristic beaver sign is the lodge, a hollow mound of tree branches (or, in the far north, mainly nonwoody material) and mud within which the beaver family sleeps and raises young. A lodge may be built against a bank, on an island, or in shallow water, but it must be near permanently deep water such that the entrance of the lodge is under water and so that the food cache can be reached even when the body of water is ice-covered. In bogs in northern Minnesota, beavers sometimes built lodges on the bog mat and many lodges were floating (Rebertus 1986). The walls of the lodge may be up to about two feet (60 cm) thick, with a thinner roof allowing for some ventilation. A family may maintain and use more than one lodge in summer in some areas (e.g., Colorado, Massachusetts). In some localities beavers use a different lodge each winter; it is more likely that the same lodge will be used if food in the area is abundant (Buech 1985). Along the Big Sioux River in South Dakota, all but 2 of 33 active lodges were built into river banks (Dieter and McCabe 1989).

Beaver dens are not always in lodges. In some areas, beavers den in burrows in the banks of streams or lakes. This is common in areas subject to floods and fluctuating water levels. A den in a bank burrow is reached through a long tunnel 30-45 cm in diameter, with the entrance usually below water (Schwartz and Schwartz 1981). A den in a bank has a vent hole and a covering of sticks and mud (Novak 1987a). Bank burrows are more likely to be used at a natural pond or lake and along a large river (Hamilton and Whitaker 1979). In many areas both lodges and bank dens are used (Hill 1982). Missouri beavers may build lodges in marshes, lakes, or backwaters, but most burrow in stream banks (Schwartz and Schwartz 1981). In Texas, beavers burrow into cut banks of streams and lakes. Dens have one or more entrances. Beavers normally stay within 0.8 km of the den throughout the year (Schwartz and Schwartz 1981).

Swimming and Diving