Sedimentation of beaver ponds in an Oregon Coast Range stream Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/12579v125

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  • Through the trapping of sediment behind dams, the presence of beaver in a watershed may cause substantial changes to fluvial and geomorphic processes. In an effort to investigate sedimentation patterns in beaver ponds in the Oregon Coast Range, a case study was made of the annual accumulation of sediment in a series of ponds on Flynn Creek, a small watershed in the Alsea River drainage. Depth, texture, and organic content of sediment deposited during the 1992-93 highflow season were measured. However, three out of four of the ponds studied drained between January and March 1993 because of dam failures. Sediment retained in these ponds varied between -0.9 and 3.5 m3. Cumulatively, these breached ponds retained 4.3 m3 or 3% of their potential storage. The volume of sediment retained behind the intact dam was 10.3 m3, or 29% of potential. The net quantity of sediment retained in the four ponds was estimated to be 22 t (i.e., 11 tonnes per square kilometer of watershed area) and represents about 11% of the average annual sediment yield from the watershed. Depth, sediment particle size and organic matter content patterns were highly variable within the drained ponds, but were relatively orderly within the intact pond. Particle size of the deposited sediment tended to decrease in the downstream direction. Organic content was significantly inversely related to the logarithm of the mean particle size (p<O.O1). Sediment trapping efficiency of the intact pond was estimated for a range of streamflows and particle sizes. For the highflow event of the 1992-93 water year, the pond was estimated to be 100% efficient in trapping particles 0.4 mm and larger in diameter. The variations in the depth, texture and organic content of the sediment captured in the beaver ponds indicate a complex depositional environment occurs within the ponds. The intact pond captured substantial quantities of suspended sediment-sized material that might have otherwise been transported to low-gradient reaches further downstream. These results indicate that beaver ponds not only alter local rates and quality (i.e., particle size distribution) of sediment accumulations in the valleys of small coastal watersheds, but that they also affect the downstream routing of these materials. Over long periods of time they may thus have a significant influence on the morphology and occurrence of floodplain features in the Oregon Coast Range.
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