ALLUVIAL MORPHOLOGY OF LOUISIANA SALT MARSHES 



by 



Richard J. Russell 

 Louisiana State University 



Salt marshes are coastal alluvial flats that develop at levels related to 

 the existing "stillstand" of the seas which was established some 5-6000 years 

 ago. Many occupy the basins that occur between river -deposited natural 

 levees. In most cases they grade inland into brackish- or fresh-water marshes, 

 or even into tree -covered swamps. 



Natural levees ordinarily rise somewhat above the basins in which Lou- 

 isiana marshes occur. Natural levee patterns depend on many factors, impor- 

 tant among which are river diversions that commonly originate well inland. 

 Their distal bifurcations are likely to have submarine origins. Typical marsh 

 basins are V-shaped, with acute angles pointing inland. Initially the V's may 

 contain bays open to the sea, such as now lie between the "passes" of the bird- 

 foot delta of the Lower Mississippi River. The largest and most irregular 

 basins, however, lie between successive river courses, between distributaries 

 of a river, or between adjacent rivers. 



Along coasts with more relief, salt marshes may be confined to estuar- 

 ine mouths of river valleys. On low coasts not so dominated by a single river 

 as the coast of eastern Louisiana, marshes may lie in pockets or embayments 

 which relate to the structural irregularities of the terrain. In western Louis- 

 iana, they occupy local downwarps in a gently inclined terrace deposit of late 

 Pleistocene age. 



If salt marshes were developed during the latest stage of widespread 

 continental ice cover, they were located over 400 feet below the present stand 

 of the seas. During possibly 130 centuries, seas rose rather rapidly, so that 

 little chance existed for extensive development of salt marshes; even the last 

 50 or 60 centuries is an interval too short for growth of salt marshes compar- 

 able to those which must have existed during much of geologic time. 



Wave action and near -shore current drift not only account for filling of 

 many old river troughs that crossed continental shelves but also for the devel- 

 opment of linear beaches that flank the seaward sides of many salt marshes. 

 In Louisiana, several marsh basins contain large and irregular systems of 

 lakes and waterways that result either from inability of the process of basin 

 filling to keep pace with rising sea level or that result from local or regional 

 subsidence of the land. Over extensive areas, vegetation has grown with suf- 

 ficient luxuriance to form "flotant, " or mats that more or less permanently 

 overlie water, incipient peat, or saturated ooze. 



Organic processes are likely to dominate the filling of marsh basins. 



29 



