|left Dam building can help to restore damaged wetlands. Wetland benefits include
flood control downstream, biodiversity (by providing habitat for different species), and water cleansing, both by the breakdown of toxins such as pesticides and the retention of silt by beaver dams. Beaver dams reduce erosion and decrease the turbidity that can be a limiting factor for some aquatic life. The benefits may be long-term and largely unnoticed unless a catchment is monitored closely. Almost half of endangered and threatened species in North America rely upon wetlands. In 2012, a systematic review was conducted on the impacts of beaver dams on fishes and fish habitat (biased to North America (88%)). The most frequently cited benefits of beaver dams were increased
habitat heterogeneity, rearing and overwintering habitat, flow refuge, and invertebrate production. Impeded fish movement because of dams, siltation of spawning habitat and low oxygen levels in ponds were the most often cited negative impacts. Benefits (184) were cited more frequently than costs (119).
Flood control 's 1887 book
Eleven years in the Rocky Mountains and a life on the frontier.A beaver dam may have a
freeboard above the water level. When heavy rains occur, the river or lake fills up. Afterward the dam gradually releases the extra stored water, thus somewhat reducing the height of the flood wave moving down the river. The surface of any stream intersects the surrounding water table. By raising the stream level, the gradient of the surface of the water table above the beaver dam is reduced, and water near the beaver dam flows more slowly into the stream. This may also help in reducing flood waves, and increasing water flow when there is no rain. In other words, beaver dams smooth out water flow by increasing the area wetted by the stream. This allows more water to seep into the ground where its flow is slowed. This water eventually finds its way back to the stream. Rivers with beaver dams in their head waters have lower high water and higher low water levels. By raising the
water table in wetlands such as
peatlands, they can stabilize a fluctuating water table, which influences the levels of both carbon and water. In a 2017 study of beaver dam hydrology, monitored beaver dams in a Rocky Mountain peatland were found to increase
groundwater storage and regional
water balance, which can be beneficial for preventing drought. The study also suggested potential to improve
carbon sequestration.
Excess nutrient removal Beaver ponds can cause the removal of nutrients from the stream flow. Farming along the banks of rivers often increases the loads of phosphates, nitrates and other nutrients, which can cause
eutrophication and may contaminate drinking water. Besides silt, the beaver dam collects twigs and branches from the beavers' activity as well as leaves, notably in the
autumn. The main component of this material is
cellulose, a
polymer of
β-glucose monomers. (This creates a more crystalline structure than is found in
starch, which is composed of
α-glucose monomers. Cellulose is a type of
polysaccharide.) Many bacteria produce
cellulase which can split off the
glucose and use it for energy. Just as algae receive energy from sunlight, these bacteria derive energy from cellulose, and form the base of a very similar food chain. Additionally, bacterial populations absorb nitrogen and phosphorus compounds as they pass by in the water stream and keep these and other nutrients in the beaver pond and the surrounding ecology.
Pesticide and herbicide removal Agriculture introduces
herbicides and
pesticides into streams. Some of these
toxicants are metabolized and decomposed by the bacteria in the cellulose-rich bottom of a beaver dam.
Denitrification Some scientists believe that the
nitrogen cascade – the production of more fixed nitrogen than the natural cycles can turn back into nitrogen gas – may be as much of a problem to Earth's ecology as carbon dioxide production. Studies have shown that beaver dams along a stream contribute to
denitrification (the conversion of nitrogen compounds back into nitrogen). Bacteria in the dirt and the plant debris, which collects at the dams, turns nitrates into nitrogen gas. The gas bubbles to the surface and mixes with the atmosphere once more.
Salmon and trout ,
Sweden Beaver dams and the associated ponds can provide nurseries for
salmon and
trout. An early indication of this was seen following the 1818 agreement between the British government of Canada and the government of America allowing Americans access to the Columbia watershed. The
Hudson's Bay Company, in a fit of pique, instructed its trappers to extirpate the fur-bearing animals in the area. The beaver was the first to be made locally extinct. Salmon runs fell precipitously in the following years, even though none of the factors associated with the decline of salmon runs were extant at that time. There are several reasons why beaver dams increase salmon runs. They produce ponds that are deep enough for juvenile salmon to hide from
predatory wading birds. They trap
nutrients in their ecology and notably the nutrient pulse represented by the migration of the adult salmon upstream. These nutrients help feed the juveniles after the
yolk sac has been digested. The dams provide calm water which means that the young salmon can use energy for growth rather than for navigating currents; larger
smolts with a food reserve have a better rate of survival when they reach the sea. Finally, beaver dams keep the water clear which favours all
salmonoids.
Frogs Beaver dams have been shown to be beneficial to
frog and
toad populations, likely because they provide protected areas for larvae to mature in warmer, well-oxygenated water. A study in Alberta, Canada, showed that "Pitfall traps on beaver ponds captured 5.7 times more newly metamorphosed wood frogs, 29 times more western toads and 24 times more boreal chorus frogs than on nearby free-flowing streams."
Birds Beaver dams help migrating
songbirds. By stimulating the growth of species of plants that are critical to populations of songbirds in decline, beaver dams help create food and habitat. The presence of beaver dams has been shown to be associated with an increased diversity of songbirds. They can also have positive effects on local waterfowl, such as ducks, that are in need of standing water habitats.
Disruption . The dam is about high. Beaver dams can be disruptive; the flooding can cause extensive property damage, and, when the flooding occurs next to a railroad roadbed, it can cause
derailments by washing out the tracks. When a beaver dam
bursts, the resulting
flash flood may overwhelm a
culvert. Traditional solutions to beaver problems have been focused on the trapping and removal of all the beavers in the area. While this is sometimes necessary, it is typically a short-lived solution, as beaver populations have made a remarkable comeback in the United States (after near extirpation in the nineteenth century) and are likely to continually recolonize suitable habitat. Modern solutions include relatively cost-effective and low maintenance
flow devices. Introduced to an area without its natural predators, as in
Tierra del Fuego, beavers have flooded thousands of acres of land and are considered a plague. One notable difference in Tierra del Fuego from most of North America is that the trees in Tierra del Fuego cannot be
coppiced as can willows, poplars, aspens, and other North American trees. Thus the damage by the beavers seems more severe. The beaver's disruption is not limited to human geography; beavers can destroy nesting habitat for endangered species. Warming temperatures in the Arctic allow
beavers to extend their habitat further north, where their
dams impair boat travel, impact access to food, affect water quality, and endanger downstream fish populations. ==Stream life cycle==