Geomorphology

Geomorphic processes involved in the formation of the Mississippi River delta, range in length from the alternating marine environments of the Tertiary Period (65-2.5 Ma) to the glacial and interglacial intervals of the Pleistocene (1.8-8,000 Ma) to the cycles of delta lobe formation and loss in the past 10,000 years. Sediment deposition associated with the alternating marine environments during the Miocene (20 Ma) is responsible for the subsurface oil and gas reserves exploited in Louisiana today. Global cooling began during the Tertiary Period causing global climates to see-saw. This see-sawing of global temperatures resulted in the expansion and contraction of the polar ice caps. The Pleistocene ice ages greatly influenced sedimentary patterns along the coast and led to the cycles of delta lobe formation (Gosselink, 1984).
 
Mississippi River delta lobes numbered in chronological order.
Modified from Kolb and Van Lopik, 1958



Variations in the volume of water flowing down through the Mississippi River Basin caused the river to meander its way from its source to the ocean trying to find the path of least resistance. This led to the building and subsequent abandonment of at least seven delta lobes. Each time the river changed its course a new Delta was formed. Once the river abandons its deposition site, deltaic sediment is subject to compaction and subsidence (Coleman, 1976).
After the distributary is abandoned erosional headlands and barrier deposits begin to develop (Penland et al. 1988). As the delta lobe gradually submerges it causes the headland to separate from the shoreline which then creates a lagoon behind the barrier island. The barrier island then moves toward land but not at the same rate that the shoreline retreats toward the mainland. Eventually, the island is left far from the mainland and becomes a shoal (Reed, 1995). It takes 4,000-5,000 years for a delta lobe to complete this cycle and is dependant upon the forces responsible for the constructional and destruct ional phases (Scruton, 1960). This results in flooding of marshes and within a few thousand years the delta lobe sinks beneath the sea. Relative sea level rise occurs because the rate at which coastal land is lowering is very high.
 
Fig 4. Conceptual diagram illustrating the growth (a and b)
and abandonment (c and d) of delta marshes
(modified from Sasser 1994).

.Over the last 600 years, the most recent Deltaic Cycle formed the Belize Delta. This delta has completed its constructional phase and instead the Atchafalaya Delta is now the delta that is expanding. This is because the Atchafalaya River has drawn off a portion of the Mississippi River’s water and sediment discharge (Van Heerden and Roberts, 1980; Wells et al. 1982). Rapid alteration of these processes has caused changes in rates of sediment deposition along the Gulf Coast (Kesel et al. 1992). Since 1850 the sediment supplied by the Mississippi river and its tributaries has decreased by almost 80 percent.
  Graph showing suspended sediment load of the Mississippi River at New Orleans. Modified from Kesel, 1987. Graph shows three historical periods 1) before 1900, 2)pre-dam period (1930¬-1952), 3) and post-dam period (1963¬-1982). Suspended sediment load decreases as dams on river begin to show up.
 
Sediments deposited by the river during the last several million years have caused sediment loading resulting in subsidence throughout the Mississippi River Deltaic Plain. The rate of coastal habitat addition and loss has been controlled by sediment loading throughout much of delta lobe formation (Gosselink, 1984). Sediment deposition is the limiting factor in coastal deltaic environments and is essential to the accretion processes that counter the effects of subsidence. This is because subsidence involves the lowering of land area in relation to sea level (Saucier, 1994). If land cannot be built up as fast as it is being lowered then certain regions of the coast will experience subsidence and relative sea level rise