Mississippi River Delta: an Overview

James M. Coleman, Harry H. Roberts, Gregory W. Stone

Abstract


Over the last century, the river-dominated Mississippi delta has received increasing attention from geoscientists, biologists, engineers, and environmental planners because of the importance of the river and its deltaic environments to the economic well-being of the state of Louisiana and the nation. Population growth, subsurface re source extraction, and increased land-water use have placed demands on the delta's natural 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 and human- induced processes, such as subsidence, eustatic sea level rise, salt water intrusion, and wet land loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta.

Under natural conditions, the fundament al changes that result in land-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associated with major shifts in the river 's course-the process of delta switching. Research over the last half century has shown that major relocations of the Mississippi's course have resulted in five Holocene delta complexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holocene deltas have produced a delta plain that covers an area of ~30,000 km2 and accounts for 41% of the coastal wetlands in the United States. After a river diversion takes place, the resulting delta evolves through a systematic and semipredictable set of stages generally characterized by: (a) rapid progradation with increasing-to-stable discharge, (b) relative stability during initial stages of waning discharge, (c) abandonment by the river in favor of a higher gradient course to the receiving basin , and (d) marine reworking of a sediment-starved delta as it under goes progressive submergence by the combined processes of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thickness of approximately 35 m. Within a single delta there are subdeltas, bayfills, and crevasse-splays that have higher frequency delta cycles ranging from several hundred years to a few decades. These depositional features are usually less than 10 m thick, and some have produced marshland areas of over 300 km2. The net result of these delta-building events is a low-lying landscape with components that are changing (building and deteriorating) at different rates. Geologically, these depositional cycles produce a thick accumulation of coarsening, upward deltaic deposits that have various thicknesses in response to development on a variety of temporal and spatial scales.

In this river-dominated delta system, distributaries can prograde seaward at rates of over 100 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 displacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the outer continental she lf, is characterized by rapid deposition of silt - and clay- rich sediments and slope instability, which results in sea ward displacement of sediments by a variety of mass-movement processes.

Superimposed on the natural processes and forms of the Mississippi deltaic plain and its associated estuarine environments, are human impacts, most of which have been imposed in this century. The most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees. Measures are now being taken to reinstate some of the delta's natural processes, thereby mitigating landloss so that decline in animal and plant productivity can be mitigated.


Keywords


Mississippi River delta; delta cycle; progradation; subsidence; shoreline change

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