SPATIAL VARIATIONS IN SEDIMENT TYPE, ACCRETION RATES, AND RECORDS OF N ISOTOPIC CHANGE IN FRESHWATER TIDAL MARSH CHANNELS

Marsh equilibrium requires accretion of mineral and organic sediment on marsh surfaces and in tidal channels at a rate equal to sea level rise. Sediment loads in many coastal streams has varied significantly in the past 300 years. Reductions in sediment loads are associated with recovery of aquatic vegetation that produce and trap organic matter. Growth of aquatic vegetation, however, may decrease velocity and up-marsh suspended sediment transport. This study examines the transport and deposition of sediment in freshwater tidal channels of the upper Patuxent River. Channel morphology, vegetation characteristics, flow velocity, and tidal characteristics were measured at locations within the channel network. These data were used to model suspended sediment within the tidal network. Sediment cores taken within aquatic vegetation zones of the tidal channels were analyzed to determine their sedimentary and geochemical characteristics. Surface sediment decreased in bulk density and increased in organic matter with up-marsh distance from tidal inlets. The cores were analyzed for ¹⁵N, ¹³C, and ³⁴S abundances. Pronounced shifts in ¹⁵N in inlet cores may reflect increases in nitrogen fertilizer use in the upstream watershed. Cores from up-marsh locations did not record this N history, but field measurements indicate significant winter erosion and summer deposition at these sites. Sharp declines in bulk density in the cores may record mineral sediment reduction associated with dam construction upstream on the Patuxent River. The coupled stratigraphic and geochemical shifts were used as time constraints in order to approximate depositional rates in the tidal marsh channel. Estimations of sediment accumulation rates at tidal inlets match the rate of sea level rise, at upstream locations, however, the evidence of reworking suggests periods in which erosion may be greater than sediment accumulation, particularly when aquatic vegetation was not abundant.