<P> Conventional wisdom has held that beavers girdle and fell trees and that they diminish riparian trees and vegetation, but the opposite appears to be true when studies are conducted longer - term . In 1987, Beier reported that beavers had caused local extinction of Quaking aspen (Populus tremuloides) and Black cottonwood (Populus trichocarpa) on 4--5% of stream reaches on the lower Truckee River in the Sierra Nevada mountains; however willow (Salix spp .) responded by regrowing vigorously in most reaches . He further speculated that without control of beaver populations, aspen and cottonwood could go extinct on the Truckee River . Not only have aspen and cottonwood survived ongoing beaver colonization, but a recent study of ten Sierra Nevada streams in the Lake Tahoe basin using aerial multispectral videography has also shown that deciduous, thick herbaceous, and thin herbaceous vegetation are more highly concentrated near beaver dams, whereas coniferous trees are decreased . These findings are consistent with those of Pollock, who reported that in Bridge Creek, a stream in semiarid eastern Oregon, the width of riparian vegetation on stream banks was increased several-fold as beaver dams watered previously dry terraces adjacent to the stream . In a second study of riparian vegetation based on observations of Bridge Creek over a 17 - year period, although portions of the study reach were periodically abandoned by beaver following heavy utilization of streamside vegetation, within a few years, dense stands of woody plants of greater diversity occupied a larger portion of the floodplain . Although black cottonwood and thinleaf alder did not generally resprout after beaver cutting, they frequently grew from seeds landing on freshly exposed alluvial deposits subsequent to beaver activity . Therefore, beaver appear to increase riparian vegetation given enough years to aggrade sediments and pond heights sufficiently to create widened, well - watered riparian zones, especially in areas of low summer rainfall . </P> <P> The surface of beaver ponds is typically at or near bank - full, so even small increases in stream flows cause the pond to overflow its banks . Thus, high stream flows spread water and nutrients beyond the stream banks to wide riparian zones when beaver dams are present . </P> <P> Finally, beaver ponds may serve as critical firebreaks in fire - prone areas . </P> <P> In the 1930s, the U.S. government put 600 beavers to work alongside the Civilian Conservation Corps in projects to stop soil erosion by streams in Oregon, Washington, Wyoming, and Utah . At the time, each beaver, whose initial cost was about $5, completed work worth an estimated $300 . In 2014, a review of beaver dams as stream restoration tools proposed that an ecosystem approach using riparian plants and beaver dams could accelerate repair of incised, degraded streams versus physical manipulation of streams . </P>

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