BIOLOGICAL PRODUCTIVITY AS A FUNCTION OF THE DELTA CYCLE 



SALINITY 



SUBAERIAL 

 DEVELOPMENT 



LENGTH OF LAND - 

 WATER INTERFACE 



BIOLOGICAL 

 PRODUCTIVITY 



© Maringouin /?\ ^ ^ . 



(3) St. Bernard 



(D Teche 



W Lafourche 

 ©Plaquemines 



High 



(0 



UJ z 



- 2 



UJ = 



a ± 

 o 



Low 



Marine 



^XD> 



Subaqueous 

 Growth 



NATURAL ENVIRONMENTS 



OPEN BAY 



SUBAQUEOUS LEVEES 



MUDFLATS 



FRESH MARSH 



BRACKISH MARSH 



SALINE MARSH 



SWAMP 



LAKES 



OYSTER REEFS 



MARGINAL BEACHES 

 BARRIER ISLANDS 



Rapid Subaerial 

 Growth 



Time Span 



Deterioration 



Figure 25. Environmental 

 Van Beek 1975). 



succession of an idealized delta cycle (fiagliano and 



delta becomes saline and is modified by 

 marine processes which typically rework 

 the delta edge into a series of barrier 

 reefs and islands that protect the inner 

 estuary. Riverine hydraulic energy is 

 much reduced and sediment loads decline. 



Further marsh development is 

 increasingly controlled by the 



productivity of the vegetation, which 

 forms peat. This is especially true at 

 the landward edge of the basin. Here, too 

 far from the coast to experience much 

 tidal activity and with the river's 

 sediment supply cut off, organic material 

 produced in situ is the only material 

 available for marsh accretion. Thus, as 

 Figure 26 shows, fresh marshes start out 

 as highly mineral, but as the delta lobe 

 ages become increasingly organic. Salt 



marsh sediments, subject 

 turbid tidal washes, are 

 high in mineral content. 



to frequent, 

 always fairly 



The general sequence is clear in the 

 figure, but some exceptions deserve com- 

 ment. Sediment mineral content decreases 

 with distance from the river source (that 

 is, from fresh toward salt marshes) in 

 active deltas (units II and V) but de- 

 creases with distance from the marine 

 sediment source in the abandoned basins. 

 This trend is consistent in all basins. 

 However, compared to the low mineral 

 contents in the recently abandoned basins 

 III and IV, marshes of the older basins I 

 and VI have relatively high mineral con- 

 centrations. This probably reflects the 

 continued sediment- laden freshwater input 

 into these systems. 



29 



