MISSISSIPPI RIVER DELTA - AN OVERVIEW

Over the last century, the river-dominated Mississippi delta has recei ved increasing attention from geoscientists, biologists, engineers, an d environmental planners because of the importance of the river and it s deltaic environments to the economic well-being of the state of Loui siana and the nation. Population growth, subsurface resource extractio n, and increased land-water use have placed demands on the delta's nat ural geologic, biologic, and chemical systems, therefore modifying the time and spatial scales of natural processes within the delta and its lower alluvial valley. As a result, the combined effects of natural a nd human-induced processes, such as subsidence, eustatic sea level ris e, salt water intrusion, and wetland loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta . Under natural conditions, the fundamental changes that result in lan d-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associat ed with major shifts in the river's course-the process of delta switch ing. Research over the last half century has shown that major relocati ons of the Mississippi's course have resulted in five Holocene delta c omplexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holoce ne deltas have produced a delta plain that covers an area of similar t o 30,000 km(2) and accounts for 41% of the coastal wetlands in the Uni ted States. After a river diversion takes place, the resulting delta e volves through a systematic and semipredictable set of stages generall y characterized by: (a) rapid progradation with increasing-to-stable d ischarge, (b) relative stability during initial stages of waning disch arge, (c) abandonment by the river in favor of a higher gradient cours e to the receiving basin, and (d) marine reworking of a sediment-starv ed delta as it undergoes progressive submergence by the combined proce sses of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thi ckness of approximately 35 m. Within a single delta there are subdelta s, bayfills, and crevasse-splays that have higher frequency delta cycl es ranging from several hundred years to a few decades. These depositi onal features are usually less than 10 m thick, and some have produced marshland areas of over 300 km2. The net result of these delta-buildi ng events is a low-lying landscape with components that are changing(b uilding and deteriorating) at different rates. Geologically, these dep ositional cycles produce a thick accumulation of coarsening, upward de ltaic deposits that have various thicknesses in response to developmen t on a variety of temporal and spatial scales. In this river-dominated delta system, distributaries can prograde seaward at rates of over 10 0 m/year. The cumulative effect of the Holocene depository has been to depress the underlying Pleistocene surface. In a local setting, e.g., the modern Balize Lobe, differential loading causes the vertical disp lacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the o uter continental shelf, is characterized by rapid deposition of silt- and clay-rich sediments and slope instability, which results in seawar d displacement of sediments by a variety of mass-movement processes. S uperimposed on the natural processes and forms of the Mississippi delt aic plain and its associated estuarine environments, are human impacts , most of which have been imposed in this century. The most significan t impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sedime nt dispersal processes through the construction of levees. Measures ar e now being taken to reinstate some of the delta's natural processes, thereby mitigating landless so that decline in animal and plant produc tivity can be mitigated.