Satellite Assessment of Mississippi River Discharge Plume Variability

The Mississippi River is the major contributor of sediments, pollutants, and nutrients to the northern Gulf of Mexico continental shelf and slope. This study utilized four years of NOAA Advanced Very High Resolution Radiometer (AVHRR) satellite data to quantify which areas of the continental shelf and slope of the Gulf of Mexico are subjected to Mississippi Rive discharges. The eighty-three images analyzed revealed that the sediment plume varied greatly in size, from 450 km2 to 7700 km2. River discharge was found to exert some control over plume size; however, wide speed and direction were also important controlling factors in determining plume morphology and surface sediment distribution over the continental shelf and slope. Results of a compositing analysis revealed that under medium discharge conditions, the "mean" composite plume covered 2200 km2 of the continental shelf. Under high discharge conditions, the area of the "mean" composite plume doubled. The spatial extent of the "maximum" composite plume under high discharge conditions covered an extensive area of the continental shelf and slope (13,207 km2), extending from 88Ý 20' W to 90Ý 50' W, and offshore to the 1000 m isobath. Although the freshwater and sediments of the Mississippi River plume were generally confined to the continental shelf, substantial cross-shelf and shelf-slope exchanges of plume water occurred during strong wind events. Off-shelf movement of river water was initiated by strong winds from the west through northeast. Subsequently, the movement of river water over the continental slope was controlled primarily by slope currents, often associated with Loop Current eddies and filaments. This study has revealed that the proportion of water flowing westward from the eastern side of the delta may increase from October through March, as a result of cold-front passages. Under the influence of strong northeasterly winds, river water from the eastern passes and ambient coastal waters flow southward hugging the eastern delta region. They subsequently turn westward to join the discharge emanating from South Pass and Southwest Pass. Sediment transport towards the southeast and east is often inhibited by strong convergence zones associated with the prevailing southeasterly wind regime as well as by intrusions of the Loop Current water onto the shelf. To the west of the delta, the shelf circulation is generally anticyclonic within the Louisiana Bight although a portion of the plume often flows further westward. The net result is an accumulation of river sediments, pollutants and nutrients within the bight and on the shelf west of Southwest Pass.