 |
A beaver dam or beaver impoundment is a dam built by beavers; it creates a pond which protects against predators such as coyotes, wolves and bears, and holds their food during winter. These structures modify the natural environment in such a way that the overall ecosystem builds upon the change, making beavers a keystone species and ecosystem engineers. They build prolifically at night, carrying mud with their forepaws and timber between their teeth.
A minimum water level of 0.6 to 0.9 metres (2.0 to 3.0 ft) is required to keep the underwater entrance to beaver lodges from being blocked by ice during the winter.[ citation needed ] In lakes, rivers and large streams with deep enough water, beavers may not build dams, and live in bank burrows and lodges. [1]
Beavers start construction by diverting the stream to lessen the water's flow pressure. Branches and logs are then driven into the mud of the stream bed to form a base. [1] Then sticks, bark (from deciduous trees), rocks, mud, grass, leaves, masses of plants, and anything else available are used to build the superstructure. Beavers can transport their own weight in material;[ citation needed ] they drag logs along mudslides and float them through canals to get them in place. [2] Once the dam has flooded enough area to the proper depth to form a protective moat for the lodge (often covering many acres), beavers begin construction of the lodge. [3]
Trees approaching a diameter of 90 centimetres (3.0 ft) may be used to construct a dam, although the average is 10 to 30 centimetres (3.9 to 11.8 in). Log length depends on the diameter of the tree and the size of the beaver. There are recorded cases of beavers felling trees of 45 metres (148 ft) tall and 115 centimetres (45 in) in diameter. Logs of this size are not intended to be used as structural members of the dam; rather, the bark is used for food, and sometimes to get to upper branches. It takes a beaver about 20 minutes to cut down a 15-centimetre (5.9 in) wide aspen, by gnawing a groove around the trunk in an hourglass shape. A beaver's jaws are powerful enough to cut a 1.5-centimetre (0.59 in) sapling in one bite. [3]
Maintenance work on the dam and lodges is often done in autumn.[ citation needed ]
If beavers are considered central place foragers, their canals may be considered an extension of their "central place" far beyond the lodge, according to a 2004–2012 study that mapped beaver ponds and cut stumps. [4]
Some people consider[ like whom? ] that by building dams beavers are expressing tool use behaviour. [5]
Beaver dams typically range in length from a few meters to about 100 metres (330 ft). [6] Canals can be over 0.5 kilometres (1,600 ft) long. [7] The largest known beaver dam is in Wood Buffalo National Park in Alberta, Canada, and is 775 metres (2,543 ft) long. [8] Satellite photos provided by NASA WorldWind show the dam did not exist in 1975, but it appeared in subsequent images. It has two or more lodges and is a combination of two original dams. Google Earth images show new dams being built which could ultimately join the main dam and increase the overall length by another 50 to 100 metres (160 to 330 ft) during the next decade. [9] Coordinates: 58°16′15″N112°15′6″W / 58.27083°N 112.25167°W .
Another large beaver dam measuring 2,139 feet (650 m) long, [10] 14 feet (4.3 m) high and 23 feet (7.0 m) thick at the base was found in Three Forks, Montana. [3]
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. [11]
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). [12]
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. [13] [14]
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. [15]
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. [16]
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.
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. [17] 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. [18]
Beaver dams and the associated ponds can provide nurseries for salmon and trout. [19] 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. [20]
There are several reasons why beaver dams increase salmon runs. [21] [22] [23] 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.
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. [24] 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." [25]
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. [26] They can also have positive effects on local waterfowl, such as ducks, that are in need of standing water habitats. [27]
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. [28] 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 [ citation needed ] 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. [29] Pools formed by the dams store heat, thus changing local hydrology and causing localized thawing of permafrost that in turn contributes to global warming. [29]
If a beaver pond becomes too shallow due to sediment accumulation, or the tree supply is depleted, beavers will abandon the site. Eventually the dam will be breached and the water will drain out. The rich thick layer of silt, branches, and dead leaves behind the old dam is an ideal habitat for some wetland species.
As the wetland fills up with plant debris and dries out, pasture species colonize it and the wetland may eventually become a meadow suitable for grazing in a previously forested area. This provides a valuable niche for many animals which otherwise would be excluded. Beaver dam creation also increases the plants the dams were made from (such as willows) to reproduce by cutting, encouraging the growth of adventitious roots.
Finally the meadow will be colonized by riverine trees, typically aspens, willows and such species which are favoured by the beaver. Beavers are then likely to recolonize the area, and the cycle begins again.
Each time the stream life cycle repeats itself another layer of organic soil is added to the bottom of the valley. The valley slowly fills and the flat area at the bottom widens. Research is sparse, but it seems likely that parts of the bottomland in North America was created, or at least added to, by the efforts of the generations of beavers that lived there. [30]
Humans sometimes build structures similar to beaver dams in streams, either to get the benefits of beaver dams in places without beavers, or to encourage beavers to settle in a particular area. [31] These are often called "beaver dam analogs" (BDA) although other names are also used. [32] When the goal is to attract beavers, sometimes the site is unsuitable in its present condition, such as being too eroded for beavers to build a dam in their usual way. BDA builders may use construction techniques beyond the beaver's capabilities, such as driving wooden posts into the stream bed to brace horizontal branches that would otherwise be washed away. The hope is that beavers who wander by or are brought in will choose to live there and take over construction and maintenance of the dam.
Beavers are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver and the Eurasian beaver. Beavers are the second-largest living rodents, after capybaras, weighing up to 50 kg (110 lb). They have stout bodies with large heads, long chisel-like incisors, brown or gray fur, hand-like front feet, webbed back feet, and tails that are flat and scaly. The two species differ in skull and tail shape and fur color. Beavers can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges.
Salmon is the common name for several commercially important species of euryhaline ray-finned fish from the genera Salmo and Oncorhynchus of the family Salmonidae, native to tributaries of the North Atlantic (Salmo) and North Pacific (Oncorhynchus) basins. Other closely related fish in the same family include trout, char, grayling, whitefish, lenok and taimen, all coldwater fish of the subarctic and cooler temperate regions with some sporadic endorheic populations in Central Asia.
A weir or low-head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of lakes, ponds, and reservoirs. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level. There is no single definition as to what constitutes a weir.
The Napa River is a river approximately 55 miles (89 km) long in the U.S. state of California. It drains a famous wine-growing region called the Napa Valley, in the mountains north of the San Francisco Bay. Milliken Creek and Mt. Veeder watersheds are a few of its many tributaries. The river mouth is at Vallejo, where the intertidal zone of fresh and salt waters flow into the Carquinez Strait and the San Pablo Bay.
Freshwater ecosystems are a subset of Earth's aquatic ecosystems. They include lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ecosystems, which have a larger salt content. Freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation. There are three basic types of freshwater ecosystems: Lentic, lotic and wetlands. Freshwater ecosystems contain 41% of the world's known fish species.
The North American beaver is one of two extant beaver species, along with the Eurasian beaver. It is native to North America and has been introduced in South America (Patagonia) and Europe. The North American beaver is one of the official national wildlife of Canada symbols and is the official state mammal of Oregon and New York. North American beavers are widespread across the continental United States, Canada, southern Alaska, and some parts of northern Mexico.
The Kickapoo River is a 126-mile-long (203 km) tributary of the Wisconsin River in the state of Wisconsin, United States. It is named for the Kickapoo Indians who occupied Wisconsin before the influx of white settlers in the early 19th century.
An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environment. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Freshwater ecosystems may be lentic ; lotic ; and wetlands.
Coyote Creek is a river that flows through the Santa Clara Valley in Northern California. Its source is on Mount Sizer, in the mountains east of Morgan Hill. It eventually flows into Anderson Lake in Morgan Hill and then northwards through Coyote Valley to San Jose, where it empties into San Francisco Bay.
The watercress darter is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is endemic to the eastern United States where it is only known from the Black Warrior River drainage basin near Birmingham, Alabama.
Cross-boundary subsidies are caused by organisms or materials that cross or traverse habitat patch boundaries, subsidizing the resident populations. The transferred organisms and materials may provide additional predators, prey, or nutrients to resident species, which can affect community and food web structure. Cross-boundary subsidies of materials and organisms occur in landscapes composed of different habitat patch types, and so depend on characteristics of those patches and on the boundaries in between them. Human alteration of the landscape, primarily through fragmentation, has the potential to alter important cross-boundary subsidies to increasingly isolated habitat patches. Understanding how processes that occur outside of habitat patches can affect populations within them may be important to habitat management.
Pescadero Creek is a major stream in Santa Cruz and San Mateo counties in California, United States. At 26.6 miles (42.8 km), it is the longest stream in San Mateo County and flows all year from springs in the Santa Cruz Mountains. Its source is at 1,880 feet (570 m) above sea level on the western edge of Castle Rock State Park, with additional headwaters in Portola Redwoods State Park, and its course traverses Pescadero Creek County Park and San Mateo County Memorial Park before entering Pescadero Marsh Natural Preserve at Pescadero State Beach and thence to the Pacific Ocean 14.4 miles (23 km) south of Half Moon Bay.
A riparian buffer or stream buffer is a vegetated area near a stream, usually forested, which helps shade and partially protect the stream from the impact of adjacent land uses. It plays a key role in increasing water quality in associated streams, rivers, and lakes, thus providing environmental benefits. With the decline of many aquatic ecosystems due to agriculture, riparian buffers have become a very common conservation practice aimed at increasing water quality and reducing pollution.
Alhambra Creek is a stream in Contra Costa County, in the East Bay region of the San Francisco Bay Area in northern California.
Freshwater biology is the scientific biological study of freshwater ecosystems and is a branch of limnology. This field seeks to understand the relationships between living organisms in their physical environment. These physical environments may include rivers, lakes, streams, ponds, lakes, reservoirs, or wetlands. Knowledge from this discipline is also widely used in industrial processes to make use of biological processes involved with sewage treatment and water purification. Water presence and flow is an essential aspect to species distribution and influences when and where species interact in freshwater environments.
Flow devices are man-made solutions to beaver-related flooding problems. Flow devices are relatively cost-effective, low-maintenance solutions that regulate the water level of beaver dams and keep culverts open. Traditional solutions have involved the trapping and removal of all the beavers in an area. While this is sometimes necessary, it is typically a short-lived solution as beavers have rapidly recolonize suitable habitat.
The North American beaver had a historic range that overlapped the Sierra Nevada in California. Before the European colonization of the Americas, beaver were distributed from the arctic tundra to the deserts of northern Mexico. The California Golden beaver subspecies was prevalent in the Sacramento and San Joaquin River watersheds, including their tributaries in the Sierra Nevada. Recent evidence indicates that beaver were native to the High Sierra until their extirpation in the nineteenth century.
Weister Creek is a stream, some 25 miles (40 km) long, in Vernon County in southwestern Wisconsin in the United States and is a tributary of the Kickapoo River. It lies in the Driftless Area which is characterized by hills and valleys apparently missed by the last glacial advance during the Pleistocene. Much of the lower half of Weister Creek is surrounded by wetlands and lies in the Kickapoo Valley Reserve.
Salt Creek is a 24.0-mile-long (38.6 km) tributary of the East Arm Little Calumet River that begins south of Valparaiso in Porter County, Indiana and flows north until it joins the East Arm Little Calumet River just before it exits to Lake Michigan via the Port of Indiana-Burns Waterway.
The beaver is a keystone species, increasing biodiversity in its territory through creation of ponds and wetlands. As wetlands are formed and riparian habitats enlarged, aquatic plants colonize newly available watery habitat. Insect, invertebrate, fish, mammal, and bird diversities are also expanded. Effects of beaver recolonization on native and non-native species in streams where they have been historically absent, particularly dryland streams, is not well-researched.
