Legacy sediments have, over the last two centuries, accumulated in valley floodplains inthe eastern United States from the ubiquitous clearing of forests during 19~(th)centuryagricultural and industrial development. Concomitant with the clearing of forests was theconstruction of small and medium sized mill dams along streams in the eastern UnitedStates. Sediments accumulated behind these mill dams, which in some cases werelocated every few miles along streams.As these mill dams have collapsed or been purposefully removed, the legacy sedimentsthat accumulated in the inundated floodplain behind these dams have been eroding andthe sediments and nutrients carried downstream. A recent study showed five legacysediment streams with dams long removed were eroding laterally with an average 323tons of sediment, 747 lbs of nitrogen, and 348 lbs of phosphorus eroded per 1,000 feet ofstream over a 4.4 month monitoring period.The headcut that develops up the stream channel once a dam has been removed is thecomparative short-term sediment impact. The lateral stream bank erosion of legacysediments accumulated in the impounded floodplain is the long-term impact that willcontinue to transport sediments and nutrients to downstream waters and estuaries. Damremoval planning and design needs recognize these long-term loading impacts fromlegacy sediments.
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