CHAPTER TWO 

 TEMPORAL AND SPATIAL GRADIENTS IN DELTA MARSHES 



The ecology of a marsh is determined 

 by the biota as constrained by the 

 regional geologic platform on which it 

 develops, and by the water regime. These 

 create physical gradients that are closely 

 related to variations across the delta in 

 marsh vegetation, fauna and ecological 

 processes. Furthermore, in the Mississippi 

 Delta geologic processes are so rapid that 

 the platform cannot be assumed to be 

 constant in the time scale of human 

 generations . 



As we have seen, a typical delta lobe 

 has a life cycle of about 5,000 years. 

 But the accretionary phase is \/ery rapid. 

 Wells et al. (1982) showed subdelta 

 cycles in the modern birds foot delta of 

 115 - 175 years. In the Atchafalaya Delta 

 about 20 km^ of new land has appeared 

 since 1973. And with current subsidence 

 rates of about 1 cm/yr even the 

 destrjctional phase of a delta is rapid; 

 marsh degradation to open water is 

 occurring at a net rate of about 75 km^/yr 

 for the deltaic plain as a whole. As a 

 result, the spatial gradients are not 

 constant but vary with the age of the 

 delta lobe. In this chapter we will 

 consider the spatial and temporal 

 gradients of Mississippi delta marshes, 

 particularly as they control the physical 

 substrate, water and water chemistry, and 

 vegetation. 



TEMPORAL GRADIENTS 



Gagliano and Van Beek (1975) 

 suggested that the geologic cycle of delta 

 growth, abandonment, and destruction is 

 paralleled by a cycle of biological 

 productivity. The biotic cycle lags the 



geologic one so that peak productivity 

 occurs during the delta lobe's 

 destructional phase (Figure 25). In order 

 to throw some further light on this 

 interesting hypothesis, i t is pertinent to 

 describe the way marshes develop in the 

 context of whole basin systems. 



To do this, ! have used data from the 

 delta hydrologic units, arranged by age to 

 get an instant snapshot of a basin's 

 development over time. This approach is 

 not ideal. The hydrologic units are 

 i nterdistributary, except for the active 

 deltas, and thus represent the active 

 sedimentation of more than one river 

 distributary. For exa^nple, the west side 

 of the Sarataria basin was formed when the 

 Lafourche distributary was active; the 

 east side is strongly influenced by recent 

 Mississippi River sediments. However, 

 biological data have, in general, been 

 collected by hydrologic unit, and a rough 

 tine sequence of six units can be 

 identified, ranging from modern to about 

 5,000 years old. 



When a delta lobe first begins to 

 form, it is overwhelmingly riverine. The 

 mineral sediment load is high, and water 

 is fresh. As a result, the newly emerged 

 sediments are mineral, and the first 

 marshes to appear are fresh (Figures 25 

 and 27). 



As the delta grows, the fresh marshes 

 expand. As described in Chapter 1, the 

 expansion is not uniform; as subdeltas are 

 cut off from stream flow, they become more 

 and more influenced by marine tidal 

 waters. Consequently, sal ini ty increases, 

 and brackish and saline marshes begin to 

 appear. 



When the river diverts to another 

 delta site, the periphery of the abandoned 



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