THE FORM OF THE MARSH 



The shape and appearance of each 

 marsh result from unique and complex 

 interactions of local topography and 

 bathymetry, sea level rise, tides, 

 sedin,ent supply, and vegetation. 

 While practical considerations force 

 us to consider each of these subjects 

 in turn, it is their interaction that 

 makes a marsh. For example, the salt- 

 marsh plants play a major role in 

 trapping and stabilizing inorganic 

 sediments as well as in producing the 

 organic matter that forms the marsh 

 peat. But marsh vegetation, espe- 

 cially its zonation and productivity, 

 has received much study and a separate 

 chapter will be devoted to this 

 problem after a brief discussion of 

 the development and form of the marsh 

 substrate. 



Marsh Development, Topography, and 

 Morphology 



Marshes usually develop behind 

 barrier spits or in the mouths of 

 tidal river estuaries where there is 

 some protection from waves. The major 

 problem with waves appears to be that 

 they prevent the sediments from 

 forming a stable substrate, rather 

 than mechanically damage the marsh 

 grasses (Redfield 1972). As tidal 

 currents carry water and sediments 

 into these areas, they become progres- 

 sively slower due to constrictions 

 and bottom friction. As a result, 

 their ability to keep particles in 

 suspension decreases, so that sands 

 become deposited near the mouth of 

 the embayment with silts and clays 

 toward the head of tidal creeks and 

 meanders (Figure 11). Redfield (1972) 

 distinguished between marshes that 

 developed on sloping foreshores, in 

 which the distribution of sediment 

 had been relatively uniform and the 

 drainage at low tide was reasonably 

 complete, and those that developed 

 across sand or mud flats where: 



"The pattern of development 

 appears to have depended on the 

 vagaries of the sedimentary 

 processes which built up the sand 

 flats to the critical level above 

 which S^. alternif lora can grow. 

 The drainage pattern of the high 

 marsh has been fixed by that of 

 the channels which finally 

 drained the flats in the broad 

 sounds enclosed by the developing 

 marsh. Such channels shift their 

 position continually until 

 stabilized by the turf of the 

 marsh, which then fixes their 

 final position." 



In the former case, the resulting 

 marsh has a more or less uniform 

 appearance, and the sloping surface of 

 the landform makes it possible for the 

 high marsh to develop independently of 

 the intertidal Spartina alterniflora . 

 If the marsh accretes and aggrades 

 across flats, however, its appearance 

 is more interesting and its develop- 

 ment follows the general pattern 

 described by the Shaler model. 



In spite of its appearance on 

 casual inspection, the surface of the 

 high marsh is not absolutely flat, but 

 is elevated slightly toward its inner 

 and older portions because of the 

 longer period over which this area has 

 been able to accumulate sediment and 

 peat (Figure 12). Low natural levees, 

 perhaps 5 to 15 cm (2 to 6 inches) 

 high and several meters wide, 

 sometimes occur along the major marsh 

 creeks; these levees develop because a 

 relatively larger amount of sediment 

 is deposited there when the rising 

 tidal water first overflows the creek 

 banks and slows down as it spreads out 

 across the marsh. 



The greater elevation of the 

 older marsh means that it will be less 

 frequently flooded by tides, that it 

 will be submerged for shorter periods 

 of time, and that less water will need 



15 



