Beaver dam

A beaver dam is a structure beavers build to create a pond. This pond serves as a safe haven from predators and a place to store food for winter. By altering their surroundings so profoundly that the entire ecosystem adapts to the change, beavers act as a keystone species and ecosystem engineers. They are most active at night, hauling mud with their front paws and carrying wood clamped in their teeth.

To keep the underwater entrance to their lodge from freezing shut, beavers need a water depth of at least 0.6 to 0.9 meters. In lakes, rivers, or large streams where the water is already deep enough, they may skip the dam and live in bank burrows or lodges instead.

Construction begins with diverting the stream to reduce water pressure. Beavers then drive branches and logs into the streambed mud to form a foundation. They build the rest of the dam using sticks, bark from deciduous trees, rocks, mud, grass, leaves, plant masses, and any other available material. A beaver can carry its own weight in material, dragging logs along mudslides or floating them through canals. Once the dam has flooded enough area to create a protective moat around the lodge—often covering many acres—they start building the lodge itself.

Trees up to about 90 centimeters in diameter may be used, though the average is 10 to 30 centimeters. Log length depends on the tree’s diameter and the beaver’s size. There are records of beavers felling trees 45 meters tall and 115 centimeters in diameter, but such large logs are not used as structural parts of the dam; instead, the bark is eaten, and sometimes the beaver reaches upper branches. A beaver can cut down a 15-centimeter-wide aspen in about 20 minutes by gnawing an hourglass-shaped groove around the trunk. Its jaws are strong enough to sever a 1.5-centimeter sapling in a single bite.

In colder climates, beavers are most active maintaining their dams in fall and spring. Where ice is not a concern, they may work year-round, though activity drops in summer. A study in Poland found that beavers could rebuild a destroyed dam and restore water levels in roughly eight hours. Some researchers view beaver canals as an extension of the lodge, their "central place," based on a 2004–2012 study mapping ponds and cut stumps. Some people consider dam-building a form of tool use.

Beaver dams typically range from a few meters to about 100 meters long. Canals can exceed 500 meters. The largest known beaver dam is in Wood Buffalo National Park in Alberta, Canada, measuring 775 meters long. Satellite images show it did not exist in 1975 but appeared later. It contains two or more lodges and is a combination of two original dams. Google Earth images suggest new dams may eventually join the main one, adding another 50 to 100 meters over the next decade. Another large dam, 652 meters long, 4.3 meters high, and 7.0 meters thick at the base, was found in Three Forks, Montana.

Dam-building can help restore damaged wetlands. Benefits include downstream flood control, increased biodiversity by providing habitat for various species, and water cleansing through the breakdown of toxins like pesticides and the retention of silt. Beaver dams reduce erosion and lower turbidity, which can limit some aquatic life. These benefits may be long-term and go unnoticed without close monitoring. Nearly half of endangered and threatened species in North America depend on wetlands.

A 2012 systematic review of beaver dam impacts on fish and fish habitat, mostly in North America (88%), found the most frequently cited benefits were increased habitat heterogeneity, rearing and overwintering habitat, flow refuge, and invertebrate production. The most common negative impacts were impeded fish movement, siltation of spawning habitat, and low oxygen levels in ponds. Benefits (184) were cited more often than costs (119).

A beaver dam may have a freeboard above the water level. During heavy rains, the river or lake fills up, and the dam gradually releases the extra stored water, reducing the height of the flood wave downstream. By raising the stream level, the dam reduces the gradient of the water table above it, causing water near the dam to flow more slowly into the stream. This can also help reduce flood waves and increase water flow during dry periods. In short, beaver dams smooth out water flow by increasing the wetted area, allowing more water to seep into the ground and eventually return to the stream. Rivers with beaver dams in their headwaters experience lower high water and higher low water levels. By raising the water table in wetlands like peatlands, beaver dams can stabilize fluctuating water levels, influencing both carbon and water cycles. A 2017 study of beaver dam hydrology in a Rocky Mountain peatland found that the dams increased groundwater storage and regional water balance, which could help prevent drought. The study also suggested potential for improving water availability.