Mississippi River Mouth Processes: Effluent Dynamics and Morphologic Development

Abstract

Repeated observations from the mouths of the Mississippi River from 1968 to 1973 indicate that the relative contributions of outflow inertia, turbulence, bottom friction, buoyancy, and marine forces to river mouth outflow dynamics and consequent sediment deposition vary spatially and temporally. Effluent behavior varies significantly with river stage and between four discrete dynamic regions. During low and normal river stages, when a distinct salt wedge intrudes into the distributary channel, Region I, which extends from the mouth to about four channel widths seaward, is characterized by buoyancy-dominated lateral effluent expansion and vertical thinning and by vertical saltwater entrainment. In Region II, which is situated over the distributary-mouth bar crest, the density interface approaches closest to the surface, densimetric Froude numbers attain maxima, internal waves break, and intense mixing occurs. Region III lies approximately between six and ten channel widths seaward of the mouth and is characterized by an internal hydraulic "jump" in which densimetric Froude numbers decrease to subcritical values and the depth of the density interface increases. In Region IV, which extends from about ten channel widths seaward of the mouth to the seaward limits of the effluent, densimetric Froude numbers are subcritical ; the surface effluent expands under the influence of buoyancy and is subject to mixing by marine forces. When the river is flooding, the salt wedge is flushed seaward beyond the bar crest. Turbulent mixing, strong bottom shear, and seaward bed-load transport prevail throughout Region I. Under these conditions, the effects of buoyancy are weak relative to the effects of inertia and friction. Throughout most of the year, the combined effects of effluent buoyancy and saltwater entrainment create lateral convergence of flow near the bottom and beneath the effluent. These three-dimensional flow tendencies may be responsible for inhibiting the divergence of incipient subaqueous levees in Region I, thereby causing high depth/width ratios at the outlet and allowing stratified distributaries to prograde with straight, parallel banks and a minimum number of bifurcations. During the extreme flood of spring 1973, prolonged discharge of large volumes of bed load, together with high outflow velocities, strong bed shear, and turbulent diffusion resulted in rapid shoaling of the South Pass distributary-mouth bar.