Two large dams on the Elwha River, Olympic Peninsula, Washington, will be removed over a 2–3 year period starting in 2011. Sediments that have accumulated in reservoirs behind these dams will be exposed to new physical and chemical conditions that could affect P distribution and availability in the oligotrophic river system. Coarse sediments from a reservoir delta and fine sediments from a reservoir bottom were collected. The sediments were analyzed for physical and chemical characteristics relevant to P availability. The fine sediments had 20 to 200% greater concentrations of C, N, amorphous Fe, Fe-bound P, Ca-bound P and organic P than the coarse sediments. Both sediment types had relatively low P concentrations compared with published values for eutrophic systems. Both fine and coarse sediments immobilized large quantities of added P, but fine sediments maintained dissolved P concentrations at half the level of coarse sediments. A 300 h incubation of sediments under diverse environmental conditions indicated released P was not affected by short-term exposure to oxygen. For coarse sediments, P release was greater in freshwater than saltwater throughout the incubation, for fine sediments this occurred only initially. Results of sediment characterizations are important in understanding potential post-dam conditions. Release of P from eroded and resuspended sediments will likely be of sufficient magnitude to increase downriver P concentrations. The Ca-bound P in non-eroded dewatered sediments will likely be sufficient to meet the P demand of vascular vegetation that establishes in the new riparian zone.
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Vol. 86 • No. 2