168 



Tidal freshwater marshes are located in the upper reaches of estuaries 

 and other areas where ambient salinities are less than about 5 ppt. The 

 effects of rising sea levels will be saltwater intrusion and the eventual 

 dominance of higher salt -tolerant plants. However, the effects of 

 canalization on tidal freshwater marshes in the Mississippi delta 

 demonstrate that dramatic increases in salinity over a comparatively short 

 period exceed the capability of these marshes to adjust so that rapid 

 losses ensue (National Research Council, 1987b). 



The complexity of marsh response to water level and associated factors 

 has meant that predictive modeling effort has been limited. Most modeling 

 to date has been of descriptive nature, as illustrated in Fig. 10.6 (Titus, 

 1986) . Coastal marshes have kept pace with the slow rate of sea level rise 

 that has characterized the last several thousand years. Thus, the area of 

 marsh expanded over time as new lands were inundated. If in the future, 

 sea level rises faster than the ability of the marsh to keep pace, the 

 marsh area will contract. Construction of bulkheads along lagoonal banks 

 may prevent new marsh from forming and result in a total loss of marsh in 

 some areas . 



Krone (1985) used a simple deposition model to predict the response of 

 marshes to sea level rise in the San Francisco Bay area. As water leaves a 

 channel and flows onto the marsh surface during a rising tide, the 

 concentration, C, of suspended sediment diminishes as sediment aggregates 

 settle to the marsh surface. As new sediment- laden water from the channel 

 mixes with the previously flooding water, however, the added sediment 

 increases the concentration. This process continues until the tide reaches 

 its maximum elevation after which only deposition occurs while the water 

 drains from the marsh. A mass balance of this process leads to 



(yw-ym)i-W^C-C^g^=0 (10.4) 



where y^ and yjj, are the elevations of the water surface and marsh surface 

 relative to a selected datum, respectively, Wg is the settling velocity of 



