Natural processes also erode marshes. These pro- 

 cesses involve the relationship between the elevation 

 of the marsh and of the sea. The northern Gulf coast 

 is subsiding. The elevation of the marsh can never ex- 

 ceed the highest elevation of ambient water, for water 

 carries and deposits sediment onto the marsh surface. 

 The deposition of waterbome sediment coupled with 

 the fonnation of peat elevate the marsh surface. Re- 

 gional and local subsidence, along with a rise in sea 

 level during this century, tends to lower the elevation 

 of the marsh with respect to the sea. The net subsid- 

 ence rate of the land in the Chenier Plain is about 1 .7 

 cm (0.67 in) per year (equal to the net rise in water 

 level, table 3.47). Unless marshes are elevated by sedi- 

 mentation and peat deposition at a rate equivalent to 

 the net subsidence rate, they eventually drovm and 

 become shallow, open water. 



Table 3.57. Percentage of onshore area occupied 

 by canals in each basin in the Chenier 

 Plain, excluding spoil bank area. 



Basin 



Percentage 



(%) 



Vermilion 



Chenier 



Mermentau 



Calcasieu 



Sabine 



East Bay 



2.2 

 1.9 

 1.7 

 1.8 

 1.2 

 1.0 



The present distribution of habitats within indi- 

 vidual basins and habitat area changes are provided in 

 appendix 6.4. Over the entire Chenier Plain the unex- 

 plained natural marsh loss rate (that is, the conversion 

 to open water, table 3.58) is 6.4%. However, in four 

 of the basins (excluding Calcasieu and Sabine), this 

 unexplained loss rate is only about 2%. Since the geo- 

 logical history of all basins is similar, and since all show 

 about the same rates of net subsidence (or sea level 



Table 3.58. Percentage of land loss" 

 Chenier Plain 



in the 



Chenier Plain 

 Basin 



Calcasieu 



Sabine 



Chenier 



Mermentau 



Vermilion 



East Bay 



All basins 



Land loss 



(%) 



17.2 

 6.5 

 2.2 

 3.3 

 2.0 

 1.9 

 6.4 



Land loss is defined as that area of natural marsh that has 

 transformed into open water during the specified period of 

 time, including shoreline retreat and direct conversion to 

 canals. 



From 1952 to 1954 except for East Bay, which was com- 

 puted for 1954 to 1974 and adjusted. 



rise), the estimated rate of marsh loss due to natural 

 processes of 2.3% in 23 years, or 0.1% per year, ap- 

 pears reasonable. This suggests that the extraordinarily 

 high rates of loss of natural marsh in the Calcasieu 

 and Sabine basins are the result of the many indirect 

 and cumulative stresses that locally upset the balance 

 between aggradation and subsidence (part 3.6). This 

 conclusion is supported by a recent study of marsh 

 loss within the Louisiana coastal zone by Craig et al. 

 (1979). They found that after a canal is dredged, it 

 tends to widen at a rate of 4 to 15%/yr. The Humble 

 Canal system and the Superior Canal, both in the 

 Rockefeller Wildlife Refuge in the Chenier Basin, are 

 widening at rates of 7 and 13%/yr, respectively 

 (Nichols 1958, Craig etal. 1979). 



Craig et al. (1979) also found a direct relation- 

 ship between land loss rates and canal density for the 

 entire Louisiana coast and for sections of Barataria 

 Bay (fig. 3-24). The regression lines for the two graphs 

 cross the ordinate somewhere around 0.1% marsh loss 

 per year. The 0.1% represents losses to processes other 

 than those caused by man. 



Equation of best fit 

 • y . 074 + 0. Ix 



'^ 0.69 



Canal Aiaa Paicani ol Totat Marsh Araa j 



Figure 3-24. Relationsiiip between canal density and 

 wetland loss rates (A) in coastal Louisi- 

 ana, and (B) in the Barataria Basin, Loui- 

 siana (Craig et al. 1979). 



80 



