of a delta depends, most importantly, on the annual sediment discharge of the river, but the most extensive deltas also tend to be developed where wide, gently sloping continental shelves provide a platform for prolonged organic and inorganic sediment accumulation and morphological progradation. The processes that disperse, transport, and deposit the sediment discharged by a river determine where, and at what rate, sediments accumulate or erode and thereby control the configuration of the resulting delta. This is true for both the subaqueous (underwater) component and the subaerial (terrestrial) delta, which must surmount the subaqueous deposits in order to prograde. Wright and Nittrouer (1995) argue that the fate of sediment seaward of river mouths involves at least four stages: (1) supply via river plumes, (2) initial deposition, (3) resuspension and onward transport by marine forces (e.g., waves and currents), and (4) long-term net accumulation. Different suites of processes dominate each stage. Interruption or alteration of any of these stages can impact the essential continual sediment nourishment of the delta.

The Mississippi River Delta, one of the world’s most extensively studied deltas, is composed of sediments from a catchment that covers much of the continental United States (3.2 million square kilometers [km2] or 1.2 million square miles [mi2]) or 41 percent of the lower 48 land area of the United States (Environmental Protection Agency, 2005). Unlike some deltas, the Mississippi River Delta contains more subaerial material than subaqueous deposits. Mississippi River sediments generally prevail over the central portion of the wide, passive Gulf Coast continental margin (Uchupi, 1975). These sediments have accumulated on the shelf and at the base of the continental slope as the Mississippi River cone.

Past and present subsidence of the shelf and coastal plain reflects the large-scale response to the loading of these sediments. Isopach maps, which depict the thickness of sedimentary deposits based on syntheses of cores and seismic data (Coleman and Roberts, 1988a,b), show that late Quaternary deposits exceed 0.1 kilometers (km) (0.06 miles [mi]) in thickness over most of the shelf, and Holocene thicknesses are as great as 0.5 km (0.3 mi) in locations of maximum deposition.

In a recent analysis of deltaic systems of the world, Walsh et al. (in press) characterize the Mississippi River Delta as “proximal-deposition dominated,” meaning that the bulk of the sediments discharged by the river were deposited close to the river mouth. In recent geologic history, a series of lobate deltaic projections were followed by avulsions, or channel switches. According to Kolb and Van Lopik (1966), at least 16 such lobes were created and abandoned in late Quaternary time (the last 20,000 years). More recent data indicate that since sea level reached its present, postglacial maximum approximately 7,000 years ago, six major lobes, including an incipient new one at the mouth of the Atchafalaya River dis-



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