The Mississippi, one of the world’s great rivers, has transported a vast quantity of sediment to the Gulf of Mexico during its history. Understanding the oceanography around the Mississippi’s mouth increases our understanding of the sediments which have formed there and also the oceanography of other similar areas where fresh water meets and flows into the sea.
Despite the size of the Mississippi, it is small relative to the entire Gulf of Mexico, and ocean currents off its mouth can be compared with currents in a large tank off a very small rubber hose. The large body of water flows first one way and then another and carries with it the minor volume of hose discharge. Currents due to hose discharge are relatively small swirls localized about the orifice.
The regional gulf current pattern provides the background on which are superimposed local conditions off the delta. Regional or semi-permanent currents are produced both by winds blowing over the entire gulf and by regional density differences acted upon by gravity. Density differences are due to fresh water input at many places and to gulf-wide evaporation, heating, and cooling. The semi-permanent current flows northward in the central gulf, and on approaching land south of the delta, apparently divides into two tongues. These tongues diverge and flow eastward and westward away from the delta.
In addition to the regional currents, there are local currents produced by: (1) tides, (2) winds, and (3) river discharge. Any single current measurement is a vector containing components produced by all these causes. Tidal currents affect the entire water column; in inshore waters they alternate in direction and speed with changes in normal tide-producing forces, but in the open gulf they probably are rotary. Wind-driven currents are caused directly by stress on the sea surface and also by piling of water against a boundary such as the shore. Locally they change direction with the winds and are of about the speed usually attributed to wind currents.
Currents in the gulf produced by river discharge are the swirls off the small hose. They are the result of residual momentum of issuing river water, hydraulic head, and density differences between river and sea water. The direction in which these currents flow usually is determined by the direction of tidal, wind, or semi-permanent currents; their speed is related first to the volume of river discharge. Residual momentum currents persist seaward from the largest distributaries to a maximum distance of about 25 miles. Hydraulic head and density differences produce the usual two-layer estuarine circulation found in other areas where fresh water flows into the sea, but there are important differences from other areas because of the relatively large volume of discharge and the geography of the system.
In general, repetition of local circulation features is brought about by repetition of similar conditions of tide, wind, and river discharge. However, in the area south of the delta between the two diverging current tongues, the east- or west-directed component of the current is related mainly to local winds. Because of this, data on average monthly winds and river discharges can be used to estimate river water and sediment distribution seaward in the gulf. About 55-65 per cent of the river water is distributed westward, about 25-35 per cent goes east, and the remainder (about 10 per cent) goes south. The river delivers a huge but still unknown amount of sediment to the gulf each year. Most of this load is carried during the high discharge months of late winter and spring when the winds are persistent easterlies, and because of this as much as 70-75 per cent of the Mississippi’s suspended load may start westward in the gulf. A large part of the total clastic load is deposited on the sea floor immediately around the delta, and it is only partly fractionated by size.
Interesting hydraulic processes also occur within the distributaries. Channels of the lower distributaries are defined seaward to the bars at all times. The bar crests and seaward ends of the natural levees constitute the discharge orifices of the distributaries. During low river stages of late summer and fall there is intrusion of salt water into the bottoms of the channels under the river water. This bottom water is called the “salt wedge,” and it oscillates with the tide but apparently has a small upstream current on the average. When the salt wedge is present, bed load can not be transported downstream and the amount of suspended load diminishes. Only during high river is the full channel volume required to transport fresh water; only then can bed load be moved out to the sea. Principal topographic features of the lower distributaries are: (i) the channel-mouth bars, (2) the channel-bottom ramps sloping upward toward the sea, and (3) tlie typical bell-shaped mouths formed by outward flaring of the natural levees. These features are all related in origin, and they are shaped by both deposition and erosion during, and in response to, seasonal high river stages where light river water flows over dense salt water.
Waves in the gulf usually are small, and the largest occur seasonally. Frequency of wave-stirring of the sea floor decreases with increasing water depth and occurs at greater depths mostly during fall and winter or during the rare hurricanes. Greatest turbidity in the shallow water around the delta occurs during fall and winter storms when some of the earlier deposited load is resuspended. Salt content and temperature of the inshore water are highly variable and are related to characteristics of both river and gulf waters. Temperature in the protected sounds also is related closely to local air temperature and insolation.
Turbidity currents on the delta’s face apparently take place, but probably only under the special hydrographic conditions of fall and winter. During these seasons temperature and chlorinity are uniform vertically below a thin surface layer, there is little or no density stratification in the water column and the concentration of suspended matter frequently is high.
The delta’s oceanography is the sum of effects of the gulf, the river, and the atmosphere. These elements combine to create a complex natural sedimentation system which at present is not amenable to theoretical analysis.