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Sedimentation on a Humber saltmarsh

Abstract Sediment deposition was measured on two transects down the profile of Skeffling marsh in four short term studies, ranging from two tidal covers to a spring tide cycle, and in longer term studies of four intervals over 18 months. Sedimentation in Spartina and bare areas at the same elevation at the marsh front was measured to determine the role of Spartina in accretion, and the effect of two levels of stem density on short term deposition was compared. In terms of sediment deposition per number of tidal inundations at the different elevations, accretion was greatest on the upper parts of the marsh. At lower elevations, subjected to more frequent inundation, deposition generally increased with the number of tidal covers in three of the four short-term studies, although there was evidence of resuspension and/or bedload movement. During an autumn high spring tidal cycle, large amounts of sediment were deposited on the upper marsh (up to 4–5 mm), but very little sediment was retained on the lower marsh and erosion was observed on the bare mud at the marsh edge. The longer term studies revealed accretion at all sites on the marsh compared with the initial baseline measurements, ranging between 3.2 and 6.3 cm after 18 months, depending upon the transect and elevation. At both transects, accretion was generally continuous at the upper levels, but deposition was more variable in the lower zones, particularly the bare mud at the marsh edge. This area accreted the most during the summer months, but lost material during the winter of 1996–1997 while the upper zones continued to build up. There was no evidence to suggest that Spartina enhances deposition in the pioneer zone at Skeffling. The tide advances rapidly across the expanse of mudflats and scours channels around the vegetation at the marsh edge. Spartina is probably more effective as a sediment stabilizer during periods of erosion and any resulting net accretion is likely to be apparent only when measured over longer time scales.