Under conditions of slow sea level rise or short-term equilibrium, salt marsh establishment 

 and growth can occur. In fact, some observers conclude that marsh formation can occur only 

 under these conditions. However, others have noted that salt marshes generally, with the 

 exception of Gulf Coastal areas, have kept pace with sea level rise even in the past 35 years when 

 the rate of sea level rise has increased noticeably (Nixon 1982). Under favorable conditions, 

 young salt marshes can accrete at very high rates. Redfield (1972) found that Spartina 

 altemiflora sediments accreted at over 50 mm per year in Barnstable marsh (Massachusetts). 

 Generally, however, rates are far slower and may exceed measured sea level rise rates by only a 

 small amount (Table 4-2). According to McCaffrey (cited in Nixon 1982), salt marshes may 

 continue to accrete even during a short period of sea level decline. 



The factors principally responsible for determining accretion rates are sediment loads, 

 current velocity, and flooding frequency and duration. Local site differences in these factors 

 account for differences among and within marshes. Thus, in low, silty Oregon marshes, accretion 

 rates varied between 5 and 17 mm per year (Seliskar and Gallagher 1983). Five high marshes in 

 Connecticut varied in sediment accretion from 2.0-6.6 mm per year in correlation with tidal 

 range and therefore increased flooding (Harrison and Bloom 1977). Year-to-year differences were 

 attributed to storm frequency, with greater accretion during storm years. Conditions are similar 

 along the Pacific coast where studies in British Columbia and Oregon showed that most 

 deposition occurred during a few annual storms (Seliskar and Gallagher 1983). 



Based on Table 4-2, accretion rates do not appear to increase with decreasing latitude, 

 although marsh productivity does. However, Mississippi Delta marshes appear to accrete at 

 exceptionally high rates, suggesting that local sedimentation and sea level rise rates may be more 

 important than climate in determining accretion rates. Most studies indicate that low marsh 

 zones, in contrast to high marsh zones, have been accreting over the measurement period at a 

 rate clearly exceeding sea level rise rate. Conspicuous exceptions are found throughout the 

 Mississippi Deltaic Plain, at least in interdistributary back marshes (Table 4-2), although on levees 

 rapid accretion exceeds the sea level rise rate. Particularly in Louisiana, and to a lesser extent 

 elsewhere, measured sea level rise clearly is a net rate that includes a significant downwarping 

 effect from coastal overburden. Furthermore, the potential capacity of Louisiana salt marshes to 

 accrete cannot be determined from measured rates because of significant interruption of normal 

 fluvial sedimentation processes by human alteration of Mississippi River flowages (Hatton, 

 DeLaune, and Patrick 1983; Gosselink 1984). 



Accretion in high marshes has seldom been studied, but as found by Harrison and Bloom 

 (1977), rates are below those of low marshes probably because delivery of suspended sediment in 

 tidal waters is greatly reduced. Although increasing sea level rise might be expected to increase 

 sediment supplies and in situ productivity in high marsh, gradual conversion to low marsh might 

 occur when the threshold tolerance for exposure and flooding of Spartina altemiflora and S. 

 patens, respectively, is exceeded. 



Few data are available on sedimentation rates in coastal brackish and freshwater marshes. In 

 Louisiana, accretion in freshwater marshes appears to be only marginally less than in salt 

 marshes, indicating the continuing (although reduced) importance of fluvial sediment sources as 

 well as high productivity rates (Table 2, Hatton, DeLaune, and Patrick 1983). In other areas, 

 sedimentation in coastal freshwater marshes can be inferred for sites of considerable age, given 

 the influence of rising sea levels. However, further data must be sought on fresh and brackish sites 

 before conclusions can be drawn as to their capacity for responding to accelerated sea level rise. 



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