ABSTRACT

The areal extent, the recent geologic history, and the landforms that have developed in the Mekong and Mississippi Deltas are generally similar. Discharge, velocities, and stage variation are reasonably alike. Sinuosity, the rate of bank migration, delta growth and subsidence, and shoreline configuration and growth are markedly dissimilar. A partial parallel in the two river systems is the upstream diversion of a sizeable portion of the Mekong's flow by the Bassac and of the Mississippi's flow by the Atchafalaya. Flood relief afforded by the Tonle Sap, a massive sump more than 200 miles upstream from the Mekong's mouth, has no counterpart in the Mississippi system. The effect of this sump in smoothing stage differences in the river downstream from the point of diversion may be one important reason for the differences between the two river systems. The Tonle Sap may also affect sediment concentrations in the Mekong, concentrations which are not only lower than on the Mississippi but are also strikingly different from the standpoint of coarse-to-fine ratios. Two additional reasons for the dissimilarity in the amount and nature of sediment load reaching the sea are: (1) the marked difference in tidal variation affecting the two river systems, and (2) the fact that the Mississippi is now essentially confined between artificial levees. The Mekong is largely unleveed and a large proportion of its suspended fines are deposited overbank before they reach the sea.

AREAL EXTENT

Comparison of the Mississippi and Mekong deltas reveals interesting similarities and dissimilarities. Defining the limits of deltaic plains has never been precise, but in this case, a fortunate parallel exists in the lower systems of the two rivers. Some 300 miles upstream from the mouth of the Mississippi, its farthest upstream distributary, the Atchafalaya, diverts about one-quarter of the flow of the main stream. Similarly, about 180 miles upstream from the mouth of the Mekong, the Bassac distributary diverts a comparable volume of flow. If the deltaic plains of the two rivers are considered limited by these upstream distributaries, their areal extends are comparable, with the deltaic plain of the Mekong being somewhat larger. Figure 1 shows the areas involved and Figure 2 compares the areal extents.

Deltaic plains are loosely defined as lowlands created by growth of a single delta or growth and abandonment of a series of overlapping, touching, or nearly contiguous deltas. As such, marine deposits not associated with deltaic advance or retreat are excluded, as are those deposits that are essentially fluviatile. Using these criteria, dotted areas are shown on Figure 1 that are identified as marginal plains associated with, but not necessarily part of, the mapped deltaic plains. The dotted area shown with the Mississippi deltaic plain is based on a large amount of boring data that identifies it as essentially fluviatile backswamp deposits. The marginal plains adjoining the Mekong are based on much less data and their classification is far from certain.

HYDRAULIC AND RELATED FACTORS

Average and low water discharge of the Mekong is significantly less than that of the Mississippi but both carry a maximum discharge of about 2,000,000 cu ft/sec. Gradients of both rivers are comparable. Sinuosity (river distance/straight-line distance) of the Mississippi, however, is considerably greater than that of the Mekong, the Mekong being nearly straight in its lower reaches, with islands or towheads characteristic. The Mississippi is decidedly sinuous upstream from New Orleans and only two small islands exist in the stream in the 300 miles from Old River to its mouth. Depths of the Mississippi (Fig. 1) are nearly twice those of the Mekong.

As mentioned above, a partial parallel in the two river systems is the upstream diversion of a sizable portion of the Mekong's flow by the Bassac and of the Mississippi's flow by the Atchafalaya. Flood relief afforded by the Tonle Sap, a massive sump more than 200 miles upstream from the Mekong's mouth, has no counterpart in the Mississippi system. The effect of this sump in smoothing stage differences in the river downstream from the point of diversion may be one important reason for the differences between the two river systems. The Tonle Sap may also affect sediment concentrations in the Mekong. Suspended sediment concentrations of the Mississippi are considerably greater than those of the Mekong. During low water flow, the Mekong often runs essentially clear (5 to 10 ppm). Because of this and the larger annual discharge of the Mississippi, the annual sediment load of the Mississippi is estimated at some six times greater than its Asian counterpart. Two additional reasons for the dissimilarity in the amount and nature of sediment load reaching the sea are: (1) the marked difference in tidal variation affecting the two rivers, the range on the Mekong being eight to ten times greater; and (2) the fact that the Mississippi is now essentially confined between artificial levees. The Mekong is largely unleveed and a large proportion of its suspended fines are deposited overbank before they reach the sea.

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FIGURE 1. Mekong-Mississippi River Comparisons

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PHYSIOGRAPHY

Landforms associated with the deltaic plains of the two rivers have much in common--so much so that air-photo interpretive techniques based on Mississippi delta experience can be used with considerable confidence; however, there are some differences. The Mississippi has built and abandoned at least five deltas, each with its own intricate network of distributaries, in the past several thousand years. As a result centers of deposition have shifted as the deltas shifted along the coast, and areas distant from the active delta have retreated under the impact of wave wash and marine currents. Beaches, built by these processes and by littoral drift, have in some instances been trapped by marine and renewed deltaic sedimentation, leaving behind stranded beaches (cheniers) along the Louisiana coast. No cheniers are found along the coast more than 10 miles inland. The Mekong, on the other hand, is characterized by hundreds of well-developed stranded beaches extending as far as 50 miles inland. The as-yet poorly developed geomorphic history of the Mekong suggests that only one large delta--its present delta--has remained active during the past several thousand years. Various distributaries have probably waxed and waned carrying the majority of the sedimentary load to various portions of the coastline, but there have been no major shifts of discrete deltas from one area to another.

Evidence indicates that despite the relatively small sedimentary load carried by the Mekong, its deltaic plain has grown consistently seaward. There are few areas where wave attack or subsidence has kept pace with or entirely destroyed former land areas as is the case along the present Louisiana shore.

GEOLOGIC HISTORY

Much is known concerning the history and nature of Holocene deposition in the Mississippi deltaic plain. Figure 1 shows a transdelta profile (B-B) across southern Louisiana from Franklin to Donaldsonville to Slidell. Note the deep entrenchment in the western portion of the deltaic plain and the high shelf of Pleistocene deposits beneath the eastern portion. The deepest part of the entrenchment is about 425 ft msl, a depth that corresponds fairly well with the maximum drop in sea level during the Late Wisconsin. Entrenchment of the Mekong, in comparison, is considerably more--about 600 ft.--if the data produced on Figure 1 (Profile A-A) have been correctly interpreted. This interpretation was based on the occurrence of lateritic or oxidized zones beneath essentially unoxidized soils, much the same criterion as is used in similar interpretations in the Mississippi delta. The implication here is that the Mekong is depositing its sediment load into a trough that is subsiding more rapidly than is the area beneath the Mississippi deltaic plain. The essential absence of coarse material in the Mekong Holocene is puzzling. It poses the possibility that the coarse material identified in the Mekong profile as Pleistocene may be analogous to the Holocene substratum in the Mississippi. If so, the amount of subsidence in the Mekong may be even greater than hypothesized above.