Factors controlling depositional processes are analyzed for three of the world's major river delta systems: the Mississippi, Ganges-Brahmaputra, and Mekong. Despite variations in amount of factual data available it is apparent that these three river systems have developed different delta types which are of interest to the geologist attempting to compare ancient with modern deltaic environments.

Four basic factors control and influence delta formations: (1) sediment load--both quantity and particle sizes transported by the river to its delta; (2) river regime--in particular its sediment-carrying ability and seasonal variations in that ability; (3) coastal processes--essentially the influence of waves, currents, and tides in the deltaic environment, and (4) structural behavior of the depositional site--progradation across a stable platform or subsidence contemporaneously with sediment deposition.

Although the deltas which are considered here differ to some degree in most of the parameters involved, certain factors play dominant roles. The Mississippi delta area is characterized by rapid subsidence and its deposits are subject only to minor modification by coastal processes. Several overlapping deltas have formed during the Recent as sedimentation sites have shifted systematically during progradation. Rapid burial and minor modification by coastal processes preclude the development of laterally continuous sand bodies.

The Ganges-Brahmaputra delta is tectonically controlled. River courses and sedimentation sites have shifted erratically in response to structural change. Coastal processes are dominated by high tides which have created an extensive tidal plain of predominantly fine-grained sediments. Laterally continuous sands are not typical of this delta.

The Mekong has created a single delta system which has prograded across an essentially rigid platform. Contemporaneously, coastal processes (tides, waves, and currents) have reworked the delta front creating sand beaches which merge laterally to form sheet sands in the deltaic plain.

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