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morphology, sediment distribution, and stratigraphy are the result of numerous 

 factors related to the action and interaction of fluvial and marine or lacustrine 

 processes and structural controls. Deltas develop when streams deliver more 

 sediment to the coast than coastal processes can erode and transport away. 



Deltas form under a wide variety of environmental conditions. A major 

 prerequisite is the existence of a major river drainage system that carries sub- 

 stantial quantities of clastic sediment. Drainage basin climate, geology, relief, 

 and area are all critical determinants of river discharge and sediment load. In 

 addition to fluvial processes, marine processes (waves, tides, and currents) 

 and tectonic and deformational forces influence delta formation and form. 

 Though numerous factors tend to produce great variability in active deltas, it 

 is generally agreed that the interaction of river, wave, and tide regimes is the 

 major factor influencing delta morphology and sediment types (Galloway 

 1975). Littoral drift and offshore slope also control sand body geometry. 



Subaqueous zones of deltas include the prodelta, the seawardmost element 

 consisting of an apron of fine-grained sediments, and the more landward delta 

 front, usually consisting of coarser silt and sand. The subaerial delta plain lies 

 above the low tide level. It can be divided into a lower delta plain, where 

 marine or lacustrine influence is active, and an upper delta plain, which lies 

 above tidal or other marine or lacustrine influence and is dominated by 

 riverine processes and features. 



Active and abandoned zones occur on both the subaqueous and subaerial 

 deltas. Active delta zones are accreting, and on the delta plain are occupied 

 by functioning distributary channels, which develop from the deposition of 

 bed material at channel mouths. Abandoned deltas occur where distributary 

 chaimels became blocked or lost flow to other channels, causing them to fill 

 with silts and clays. Abandonment can also result from upstream channel 

 avulsion, in which the river occupies an alternative course with steeper 

 gradient. Such events are attributed to seaward growth of the delta and the 

 associated reduction in hydraulic head, a process which occurs about every 

 1,000 to 1,500 years in the Mississippi Delta Plain (Kolb and van Lopik 

 1966). 



Depositional environments in deltaic areas can thus be distinguished 

 according to which processes influence sedimentation. Although the nature of 

 sediment supply may vary, the coarsest materials are deposited near the 

 channel and mouth, while deposition of fine sediments occurs in environments 

 of lower energy. The prodelta consists of fine-grained sediments deposited 

 from suspension. On the delta front, features may include distributary mouth 

 sands, distal-bar silts, tidal ridges, and shoreface beach deposits. On the delta 

 plain, deposits associated with the distributary channel and margins include 

 channel bed, charmel fill, natural levee, and crevasse splay deposits. Inter- 

 distributary sediments are deposited in swamps, marshes, lakes, bays and 

 lagoons, and are primarily laminated or bioturbated fine sediments. At the 

 seaward edge, delta plain sediments of various kinds may be reworked into a 



Chapter 3 Variable Coastal Features 



