water regime displays two highs, in spring and in late 

 fall (fig. 3-18). The spring high may be associated with 

 river stages and abundant local rainfall. The fall peak 

 seems to be a result of the predominant southerly 

 summer winds that gradually build up the water level 

 along the northern Gulf coast. This seasonal cycle 

 means that wetland inundation does not occur equally 

 throughout the year. 



Table 3.46. Mean coastal tidal fluctuations for Chenier 

 Plain basins. 



Has no tide. 



Figure 3-18. Monthly water-level fluctuations for 

 1963-66, and montlily mean and stan- 

 dard deviation, 1963-74, in the Calcasieu 

 Basin (U.S. Army Corps of Engineers). 



The importance of tides varies widely in the dif- 

 ferent Chenier Plain basins. Vermilion and East Bay 

 are open estuarine systems with significant tidal action. 

 Although the pass to the Gulf in the Chenier Basin is 

 small, the basin lies parallel to the coast, and the wet- 

 lands that are not impounded are strongly influenced 

 by tidal action. Both the Calcasieu and Sabine basins 

 were fonnerly much fresher bodies of water than 

 they are now, but they currently sustain significant 

 tidal action because of the deep ship channels con- 

 necting the Gulf to inland waters. The Sabine ship 

 channel has resulted in strong tides and salt intrusion 

 directly into tlie northern end of Sabine Lake (part 

 3.6.6). In contrast, the Mermentau Basin is effectively 

 cut off from any tidal action by control structures 

 that impound the entire basin (part 3.6.3). 



For the past 40 years, the water level has been in- 

 creasing along the northern Gulf coast (Hicks and 

 Crosby 1974). Tide gage records for most of the 

 United States show the same phenomenon because of 

 a combination of sea level rise and land subsidence. 

 In the past 10 years, the rate of sea level rise seems to 

 have accelerated. Rates on the Gulf coast are among 

 the highest in the nation (Hicks and Crosby 1974). 

 On the Chenier Plain, tide gages in all basins agree 

 qualitatively. The average rate of apparent sea level 

 rise (i.e., change in gage readings) is about 1 .7 cm (0.7 

 in)/yr (table 3.47). Therefore, inter tidal zone wetlands 

 must aggrade at a rate equal to their subsidence; 

 changes in the rate of net subsidence (due to all causes) 

 signal major shifts in marsh formation or erosion. 



Table 3.47. Mean annual rise in water level by 

 basin. ^ 



Basin 



Annual rise 

 (cm/year) 



Vermilion 

 Mermentau 

 Chenier 

 Calcasieu 

 Sabine 

 East Bay 

 Average 



0.94 

 2.13 

 2.10 

 2.00 

 No record 

 1.50 

 1.73 



Mean of one to five gages per basin. See part 3.6. 



3.3.4 SUBUNITS INFLUENCED BY FRESHWATER 



Both local rainfall and river discharge into the 

 Chenier Plain significantly infiuence the hydrology of 

 the basins. The region influenced by river runoff de- 

 pends on the discharge volume of the river. If the 

 volume of discharge is small in comparison to the tidal 

 prism, then the river influence is secondary to tides. 

 This is the case for most shallow embayments along 

 the southeastern Atlantic coast and the northern Gulf 

 of Mexico. However, the ridges that give the Chenier 

 Plain its name are effective barriers to the Gulf, and 



66 



