The areal extent, the recent geologic history, and the landforms that have developed in the Mekong and Mississippi deltas generally are 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

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markedly dissimilar. A partial parallel in the 2 river systems is the upstream diversion of a sizable part 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 mi upstream from the mouth of the Mekong 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 river systems. The Tonle Sap also may 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 mount and nature of sediment load reaching the sea are the marked difference in tidal variation affecting the river systems and the fact that the Mississippi essentially is confined between artificial levees. The Mekong largely is unleveed and a large proportion of its suspended fine sediments is deposited overbank before they reach the sea.

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