Sedimentation in a salt marsh-tidal channel system, southern New Jersey

Abstract

The tidal channels in a low-mesotidal back-barrier salt marsh of southern New Jersey are comprised of large through-flowing (TF) channels which separate marsh islands and smaller dead-end (DE) channel networks on the marsh islands. TF channels have at-a-station hydraulic geometries similar to other tidal marsh channels and DE channels have hydraulic geometries similar to rivers. The silt-clay (mud) content of DE channel perimeters is high (averaging 78%) and width:depth ratios are low (5–21) in contrast to TF channels with higher width:depth ratios (34–129) and lower contents of silt-clay at the perimeters (averaging 9%). Three distinct environments are associated with the channels: (1) a subtidal thalweg region, (2) an intertidal channel-margin flat (vegetated and unvegetated), and (3) a channel-margin marsh (levee and back-levee). Interchannel sediment trends for each environment proceeding from large TF to small DE channels are: (1) a decrease in grain size of the sediments from sands to silts and clays, (2) increase in total organic matter with increasing mud content, and (3) a change from predominantly physical to biological sedimentary structures. Intrachannel trends at individual channel cross sections comprise a decrease in grain size of the sediments with distance from the thalweg region and increasing total organic matter content from the thalweg to the marsh. TF and DE channels have different origins. TF channels evolved from flood tidal delta channels that became fixed in position with the stabilization of delta shoals by vegetation and cohesive bank-margin sediments. DE channel networks, on the other hand, originated as ebb drainage patterns on the unvegetated substrates of marsh islands. They also became fixed in position by vegetation and cohesive bank margins. The two mechanisms of channel formation have resulted in little to no migration through time after this stabilization.