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flow device
Flow devices are man-made solutions to beaver-related flooding problems. 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 beaver populations have made a remarkable comeback in the United States (after near extirpation in the nineteenth century) and rapidly recolonize suitable habitat. In fact, a 2006 survey found that trapping as a solution to beaver problems had a 79% failure rate within two years due to resettlement by new beavers. Flow devices are relatively cost-effective, low-maintenance solutions that regulate the water level of beaver dams and keep culverts open. A 2006 study by the Virginia Department of Transportation found that for every $1 spent on flow-device installation relative to historical preventive maintenance, road repairs, and beaver population control activities, $8 was saved, for a return on investment of nearly 8:1.
==Benefits of beavers==
Beavers faced near extinction from unregulated trapping across North America during the early to mid-nineteenth century. This caused widespread environmental damage including the drying up of many streams and rivers formerly sustained by the high water tables associated with beaver dams. In addition, beavers create wetlands which increase biodiversity and improve water quality by removing sediment and pollutants. In fire-prone areas, beaver ponds serve as fire breaks. Salmon and trout easily cross beaver dams and scientific evidence shows that fish size and fish populations are larger when beaver are present.〔 A keystone species, beavers create habitat for numerous other species, as exemplified by ponds created in Alhambra Creek in Martinez, California, by a new beaver colony in 2007 which colony, in turn, led to the return of numerous birds, steelhead trout (''Oncorhynchus mykiss''), river otter (''Lontra canadensis'') in 2008, and mink (''Neovison vison'') in 2009.
The most common beaver-related flooding issues that people encounter are caused by blocked road culverts or freestanding beaver dams. Beaver-related flooding issues can usually be resolved with properly designed and installed water control devices, also known as flow devices. Flow devices are either specially designed pipes installed through beaver dams, or pipe and/or fence systems that protect road culverts from being blocked. The beneficial effects of beavers on stream flow, riparian habitat, salmon and trout, and wetland creation can be sustained with application of these inexpensive technologies, which require little maintenance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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