Vegetation commonly advances in centripetal belts which spread from essen- 

 tially inorganic natural levee or other "rock" surfaces that flank the basins. 

 But there is considerable inorganic sediment contributed by crevasse over- 

 flows, through streams, and basin scour. Where local subsidence is domin- 

 ant, however, the water surface expands at the expense of marsh, and the 

 progress of vegetational belts is reversed, for the reason that increasing 

 salinities drive plant associations back toward higher ground. 



There are many complications in salt marsh development. Regional or 

 local subsidence opposes basin filling. Compaction of underlying sediment 

 has a similar effect. Tidal channel systems develop and become increasingly 

 complex as marsh area grows. Lakes and ponds originate and grow in area, 

 developing beaches which complicate or even subdivide basins. The worst 

 complications of all, however, are man-made; canals, spoil banks, and other 

 "improvements" that upset salinities, interfere with drainage, and change 

 depositional patterns and environmental conditions generally. 



As a marsh widens, typical belts of vegetation normally migrate seaward, 

 with fresh , brackish , and salt marsh zones pushing ahead in the wake of an 

 advancing beach. Where tidal range is large the various contrasting belts of 

 vegetation may be identified readily in the field or on aerial photographs. But 

 in Louisiana, where the tidal range is quite insignificant, plant associations 

 may be so complexly distributed that for the identification of marsh zones it 

 is helpful to use other criteria, such as animal life or the chemistry of water- 

 ways. The ratio of total dissolved solids to chlorinity, for example, drops 

 abruptly in the salt marsh. As a rule, the notable increase in marsh firmness 

 in proximity to the shore appears to be related to flocculation of colloids. 



Marshes exhibit interesting hydrographic features. Straight waterways, 

 if inactive, are likely to shoal to insignificant depths. These are ordinarily 

 the remnants of river courses or of distributaries between marsh basins. 

 Winding tidal channels, on the other hand, are typically deeper and may be 

 increasing in section, for the reason that they are functional and must be able 

 to meet the demands of a discharge that increases as marsh area broadens. 

 Lakes and ponds tend to develop ovate outlines and deepen as diameters in- 

 crease, in response to more effective wave erosion. 



Marsh may grow along the seaward front of the outer beach, provided a 

 surplus of sediment accumulates there as a result of nearshore drift. Pioneer 

 salt -tolerant plants, such as some Spartinas and Salic or nias , are followed, in 

 succession, by assemblages of sedges and grasses characteristic of the salt 

 marsh. 



Alternations between nnarsh advance and removal have occurred several 

 times along the comparatively smooth coast of western Louisiana, where the 

 supply of sediment has varied appreciably from time to time during the last 

 5-6000 years. When the supply was deficient, wave erosion removed marsh 

 and drove the beach landward at a rapid rate. But such periods have alter- 

 nated with times when sediment was so abundant that marsh grew Gulfward, 



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