ESTUARINE AND BATHYMETRIC CONTROLS ON SEASONAL SEDIMENT STORAGE IN THE LOWER MISSISSIPPI AND ATCHAFALAYA RIVERS

Multibeam bathymetry, sidescan, CHIRP subbottom, CTD casts, gravity cores (<3m), and surface grab samples were collected in 2001 during three cruises to the lower Mississippi and Atchafalaya Rivers and were timed to coincide with peak (March), falling (June) and low (November) water discharge. Three, 10-15 km long river sections were surveyed: the lower Mississippi below Venice (0 to 20 km above Head of Passes), upstream at English Turn (120-140 km above Head of Passes), and the lower Atchafalaya (5 to 20 km upstream of the subaerial delta). During peak flow (>55,000 m3/sec), active bedload transport was indicated by the presence of large (>3m high) sand waves in the river thalweg at the Venice (30-45 m deep) and Atchafalaya (<20m deep) sites, while the CHIRP data showed a compacted relict pavement between sand fields. During low discharge (9,000 m3/sec), a layer of fine-grained (previously suspended) sediment was observed draping the sand waves and relict surfaces. This layer was up to 3 meters thick, composed of >90% silts and clays and was found primarily in the thalweg. CTD casts during the October cruise and USACOE data indicate the salt-water wedge is located in this zone or further upriver when discharge is below 10,500 m3/sec. This suggests that the mud layer is formed by estuarine stratification brought on by the convergence of the intruding salt-water wedge and riverine fresh water. During rising flow (December – February) the salt-water wedge is pushed out to the Gulf of Mexico and the surface mud layer is resuspended and transported to the continental shelf.

Data collected above the salt-water wedge at English Turn differ in that the temporary mud layer is <0.5 m thick and found in shallow water (<20 m) on point bars. Since the English Turn site is located upriver of the salt wedge even at low discharge, shallow water storage of muds during low flow is probably controlled by reduced benthic shear stress over point bars. Point bar storage of muds during low flow likely takes place far upstream and involves an even larger fraction of the Mississippi River’s overall annual sediment budget than estuarine storage.