DRAINAGE PATTERNS IN SALT MARSHES ^) 



by 



Robert A. Ragotzkie 

 University of Georgia 



Introduction 



Drainage pattern development is determined by the interaction of morpho- 

 logic and dynamic factors. In salt marshes the dynamic factors are probably 

 the more important. Drainages in salt marshes develop simultaneously with 

 the structure of the marsh. The process is easily visualized by assuming a 

 level surface slightly below sea level which is being uniformly sedimented. 

 As the surface rises above the level of low tide it drains completely during each 

 tidal cycle. Friction between the water and the substrata delays the draining 

 process and permits hydraulic slopes to be built up resulting in a more rapid 

 movement of water during the ebbing tide than during the flooding tide. We see 

 elaborate drainage systems in the region of the Georgia sea islands on mud 

 flats which have not yet risen high enough to support the growth of Spartina or 

 other vegetation. Later when Spartina invades the area, the drainage channels 

 are somewhat stabilized in location by the root development of the plants. Even 

 then, however, erosion and differential sedimentation cause migration of creek 

 channels and continued development of the drainage pattern itself. 



B. Description 



In the salt marshes of the coast of Georgia and South Carolina a general- 

 ized drainage pattern appears repeatedly (Fig. 7). In the most headward part 

 of the drainage system there is an elaborate anastomosing pattern or network 

 of small creeks and channels. The drainage texture in this region is extremely 

 fine. Proceeding downstream a higher level of organization is apparent with 

 certain streams becoming master streams fed only by very small tributaries. 

 Texture is less fine and pattern is more stable. 



Adjacent to large creeks and rivers in the downstream regions are found 

 well developed levees. Progressing away from the larger water courses the 

 levees are less well developed and the level of the marsh itself somewhat 

 lower. That the headward regions are in an earlier stage of vertical develop- 

 ment than the downstream areas is apparent from the nature of the drainage 

 pattern itself. Elongation, elaboration and headward development are charac- 

 teristic of the headward region while integration and master creek dominance 

 are found in the downstream areas indicating a more mature stage of vertical 

 development. As the downstream marshes build up they are flooded less often 

 and hence have a smaller potential supply of sediment. The rate of growth 

 slows as their level approaches that of high water. Meanwhile the suspended 

 load is being conducted upstream to areas where flooding is occurring at a 

 lower tide stage and continued rapid growth is possible. 



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