U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information


Document NameEffects of river discharge, wind stress, and slope eddies on circulation and the satellite-observed structure of the Mississippi River plume.
Document AuthorWalker, N.D.
Wiseman, W.J.
Rouse, L.J.
Babin, A.
Short DescriptionJOURNAL OF COASTAL RESEARCH 21 (6): 1228-1244 NOV 2005
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2005
Text:

Abstract: Satellite measurements of suspended sediment, temperature, and chlorophyll a are used in combination with surface current measurements to investigate surface circulation and structure of the Mississippi River plume. River discharge changes affect frontal locations, areal extent, and suspended sediment loads of the plume. During high river discharge ( > 20,000 m(3) s(-1)) in spring, the sediment plume extends 23 km southwestward, covers 2700 km(2), with maximum concentrations of 360 mg L-1. Plume temperatures vary seasonally from 10 degrees to 28 degrees C, with maximum surface fronts of 3.3 degrees C km(-1) in winter. East winds, prevalent in autumn, winter, and spring, drive a westward flow of river waters around the delta, linking two isolated shelf regions and increasing river discharge onto the Louisiana/Texas shelf. During peak river flow, this westward current exhibits velocities of 40-90 cm s(-1), is 20 km wide, and transports 140,000-165,000 m(3) s(-1) of river and shelf water. It usually turns toward the coast between 89.5 degrees W and 90 degrees W, feeding a clockwise gyre in the Louisiana Bight and a westward coastal current. The prevalent east winds trap river water and associated nutrients on the shelf where hypoxia later develops in late spring/summer. During autumn and winter, short-term wind reversals from frontal passages rapidly reverse plume direction, initiate off-shelf transport, and reduce residence times for river waters and associated sediments, nutrients, phytoplankton, and carbon. During summer, persistent southwest and south winds force river water eastward, where cross-margin transport is likely due to the relatively narrow shelf. Slope eddies and the Loop Current control river water after leaving the shelf.