FLOODPLAIN SEDIMENT TRAPPING AND STORAGE ALONG TRIBUTARIES TO THE CHESAPEAKE BAY

Floodplains along tributaries to the Chesapeake are a critical element in the maintenance of water quality by trapping and storing large amounts of sediment and associated contaminants. These floodplains are among the last places for sediment storage before entering critical estuarine nursery areas for fish and wildlife. We have monitored sediment deposition along 10 streams using dendrogeomorphic and artificial horizon (clay pad) analyses. Extensive riparian wetlands within the Coastal Plain regions of the Bay may trap as much as 70,000 kilograms of sediment per year along a 2-kilometer reach. However, discrete net deposition rates vary from near 0 to 8 mm/yr; some locations near levee crevasses are erosional while others near hydraulically connected sloughs have the highest deposition rates. Sedimentation rates are highest where alluvial streams receive runoff from either agricultural or urbanizing areas with high-suspended sediment loads. Channelized reaches trap 10 times less sediment than unchannelized reaches with typical overbank flooding regimes. Substantial amounts of nutrients (N and P) may also be stored with sediment permitting biogeochemical remediation. Quantification of fluxes in floodplain sediment is necessary for estimation of sediment budgets; while net trapping information may now be available, retention time remains poorly understood.