Every school-age child knows that beavers dam up streams, and it is a compliment to be described as being “busy as a beaver.”
These two thoughts are related, since beavers do spend long hours cutting down trees and then weaving the branches into a functional dam. Their impoundments require constant upkeep and additions of new branches. Along with the woody components, beavers scoop up mud from the bottom of the impoundment and pack it in between the stick framework.
Beaver dams vary in size and height but are frequently 2 to 3 feet high. The beaver dam I studied for my master thesis was more than 200 feet wide and created a pond of almost half an acre.
There are a couple important benefits for the beaver that justify the extensive energy output during dam construction. Perhaps the most obvious benefit is to raise the water level upstream of the dam. The deepened, backed-up water provides a quick escape when confronted with potential predators like cougars, wolves or bears. Besides the safety aspect, the water inundates channels that then provide safe access to feeding areas upstream. These upstream areas are where beavers cut down trees that are used for food or construction materials. While beavers will often take down larger diameter trees, they seem to prefer the young trees that are only a couple inches in diameter. The smaller trees are completely covered with tender and edible bark. It’s like a beaver’s version of a corn dog on a stick.
The beaver pond can provide a series of shipping lanes for moving tree limbs to the lodge or to the dam. Another major role the impoundment plays is as a food locker. Beavers will store hundreds of freshly trimmed limbs and branches underwater. During winter, they can swim from their lodge (or from tunnels into the bank) to the underwater pantry to retrieve a submerged limb. After feeding on the bark, the now-bare sticks can be used for springtime dam repair.
Ecologists often describe beavers as a keystone species in streams. This connotation refers to the major impact beavers have on a stream and the surrounding habitats. The presence of beavers can change many important aspects of the stream, as well as conditions in the surrounding riparian areas.
Beaver impoundments inundate large areas of terrestrial vegetation and will literally drown out most of the original vegetation. Virtually all the inundated trees and shrubs will die. This usually leads to an increase in solar radiation, which provides opportunities for new species of plants and algae to grow. Sometimes sedges and other wetland plants will appear along the edges or right on the breast of the dam.
Construction of a beaver dam immediately slows the stream water and that causes deposition of many materials being washed downstream. Organic materials like leaves, twigs, grasses and fish carcasses, as well as mineral sediments, will collect on the bottom of the dam. This accumulated material will eventually become very rich soil.
Because of the deepened water in the beaver pond, many fish will use these areas as an overwintering refuge. Many resident salmonids in Alaska move out of streams in the winter months and seek out areas like beaver dams where they can be assured of unfrozen waters. Given the opportunity, young silver salmon fry will use a beaver pond for a wintering area or may use it for a year-round place to feed and hide.
Because of the excavations by beavers and scooped-up mud for stabilization of the stick-built dam, there tends to be a fair amount of fine silt downstream of beaver dams. Those silty stream conditions cause changes in the stream’s invertebrate community downstream of the impoundment. These differences in turn can cause alterations in the downstream fish populations, too.
Beaver dams are ephemeral structures. After beavers utilize most of the available trees, they begin to prospect for greener pastures, or areas with more young trees. With no beavers to perform repairs and maintenance on the dam, it will eventually give way and drain out. When a dam washout occurs, those previously underwater areas become terrestrial habitats again. With all the deposited materials creating rich soil, the newly exposed pond bottom quickly becomes revegetated with herbs and woody vegetation.
The released stream cuts down through the deposited silt material and returns to its original physical flow patterns. Soon, the original stream and riparian biota return too. Several years later, when the trees have regrown, the stream section will once again be an attractive site for another beaver dam.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the biology of the Kenai River watershed.