Acoustic observations of a fluid mud layer transporting sediment on the northern California shelf

During the past few years, STRATAFORM field efforts off Eureka, CA have involved numerous investigators studying the source, deposition and movement of sediment brought to the sea by the Eel River. During the winter of 1997-98, WHOI investigators deployed tripods and moorings across the shelf to observe the sediment movement. The 60-m tripod had an acoustic backscattering sensor (ABS) that profiled the bottom meter of the water column at 1-cm intervals. In late January 1998 a series of high river discharge events coupled with high waves produced a series of ~30 g/l suspended sediment layers, ~20 cm thick that propagated past the tripod. The optical backscattering sensor at 0.5 m saw only a maximum of 3-g/l peak and so missed these thin “fluid mud” layers. Such fluid mud layers can be a major source of sediment transport, and are often not well resolved by traditional optical techniques, but are observed with remote acoustic profiling techniques