the lodge, according to a 2004–2012 study that mapped beaver ponds and cut stumps. It is claimed by some that by building dams, beavers are expressing tool use behaviour. Size Beaver dams typically range in length from a few meters to about . Additionally, canals can be over in length. The largest beaver dam known to exist is in Wood Buffalo National Park in Alberta, Canada and measures in length. 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 during the next decade. Coordinates: 58°16'15"N 112°15'6"W Another large beaver dam measuring long, high and thick at the base was found in Three Forks, Montana. Effects Dam building can be helpful in restoring 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 as well as 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 as well as 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 A beaver dam may have a freeboard above the water level. When heavy rains occur, the river or lake fills up. Afterwards 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