Bottom currents and fluviomarine sedimentation on the Mississippi Prodelta Shelf: February‐May 1984

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Abstract
Surface winds and currents, temperature, and water transparency at heights of 1 and 5 m above bottom in 60 m of water were measured during the period from February 23 to May 2, 1984, at a site on the Mississippi delta. The objective was to better understand the influence of near‐bottom flow on the character of sedimentation on the prodelta shelf. Data analysis showed currents to be parabathic, with periodicities of 5 and 20 days. Flow showed a strong east‐west polarization, with net flow 0.4 cm/s onshore at the lower level and 0.5 cm/s offshore at the upper. With easterly flow, water temperature declined (indicating upwelling), and current veering between 1 and 5 m was 12°–15° in the sense of a bottom Ekman layer. With westerly flow, water temperature increased (indicating downwelling), and veering was of the same magnitude but in the sense of a surface Ekman layer. Flow and suspended sediment data suggest that during the period of study, deposition of sediment from the turbid plume of the Mississippi River was continuous. Calculations of sediment‐settling velocities indicate that the surface plume of turbid river water transported, in suspension, particles with hydraulic equivalent diameters ranging from clay to fine‐sand size. Computations of friction velocity indicate that here on the outer prodelta shelf, the observed bottom currents were not competent to erode bottom sediment. Short‐period (1–3 days) fluctuations in mean bottom turbidity may be due to variations in the supply of suspended sediment as the surface plume moves laterally in response to the wind and, in at least some cases, to a vertical redistribution of sediment in suspension by currents too weak to initiate resuspension. These fluctuations are probably responsible for the silty and sand laminae (generally a millimeter thick or less) observed in bottom cores. The thickness of the annual flood deposit may approach 1 m in the vicinity of the plume.