g400 



z 



i 300 



X 



2 



< 



I 200 



100- 



M J J 

 Month 



Figure 3-22. The hours per month that the water 

 levels are above mean high water in 

 marshes at Hackberry, Louisiana (U.S. 

 Army Corps of Engineers). 



The pattern of inundation is important for an- 

 other reason (fig. 3-22). During the months of June, 

 July, and August, salinities are highest in the estuaries 

 because of a combination of high sea stage (fig. 3-18) 

 and low rain surplus (fig. 3-20). During this period, 

 marshes are infrequently flooded with water more sa- 

 line than normal. Wlien such floods occur during a pe- 

 riod without rain and with high evapotranspiration 

 rates, salts accumulate in marsh sediments. This may 

 have serous consequences in fresh, intermediate, 

 brackish, and salt marshes. For instance, there is some 

 indication that periodic salt accumulation has killed 

 stands of saltmeadow cordgrass in brackish marshes 

 along Calcasieu Lake. The salt accumulation is a re- 

 sult of the overall salinity increase accompanying 

 dredging of the Calcasieu Ship Channel (J. Valentine, 

 Pers. Comm., U.S. Fish andWildlife Service, Lafayette, 

 La.). 



In the Chenier Plain, the amount of wetland that 

 is freely flooded by estuarine waters has been dras- 

 tically reduced by human activities. Much of the wet- 

 land (162,000 ha, 400,311 a) is impounded, e.g., 

 over one-half of the wetlands in the Chenier Basin are 

 behind some type of levee. In addition, many marshes 

 are semi-impounded by canal spoU banks and other 

 levees that restrict or redirect water flows. The entire 

 Memientau Basin can be considered an impoundment 

 in which water levels are controUed by structures in 

 all the major channels draining the basin. Only the 

 Calcasieu and Sabine basins have large areas of unim- 

 pounded wetlands. In both of these basins, hydrologic 

 modifications have changed water flow and saUnity 

 patterns, which have resulted in high marsh loss rates 

 (part 3.4). 



3.3.7 SUBUNITS WITH LITTLE OR NO WATER 

 EXCHANGE-UPLANDS AND IMPOUND- 

 MENTS 



Uplands are important to the hydrologic processes 

 of a basin as a source of local runoff water and as na- 

 tural barriers to water flow. Since much of the upland 

 area in the Chenier Plain is impounded for rice, the 

 quantity of free runoff is probably not as important 

 as the quality of this water since it carries sUt, nu- 

 trients, and toxic chemicals into the estuary . The drain- 

 ing of rice fields is controOed, so runoff from them 

 does not correspond with heavy rainfalls. Impounded 

 wetlands normally have very Utile exchange with sur- 

 rounding waters, although undoubtedly there is seep- 

 age through levees, and overflows during high water 

 conditions. Water-level management practices also re- 

 sult in some water exchange. However, in terms of 

 the estuarine system, these impoundments are effec- 

 tively cut off and no longer contribute to the normal 

 hydrology of the basin. Also, sheet flow across wet- 

 lands is disrupted by impoundments, and continuous 

 canals associated with leveee construction act as con- 

 duits that speed drainage and allow water to bypass 

 marshes altogether. 



3.3.8 HUMAN IMPACTS ON THE HYDROLOGIC 

 REGIME 



Man has modified the hydrologic regime of the 

 Chenier Plain basins to such an extent that there are 

 now no basins on the plain untouched by human in- 

 tervention. These modifications can be classed as those 

 that affect (1) the upstream water flow into the basins, 

 (2) the circulation within the basin, or (3) the near- 

 shore Gulf circulation (table 3.51). The direct effects 

 can be measured in terms of a number of attributes of 

 the hydrologic regime: freshwater supply, salinity, 

 sediment input, sediment deposition and erosion, 

 water levels, overland flow, and circulation. Table 3.51 

 indicates the sections of this report that discuss those 

 effects. The primary hydrologic changes give rise to a 

 series of secondary effects. The concern at the basin 

 level is primarily for the secondary effects to habitat 

 type, area, and interactions. However, the functional 

 characteristics and the biota of habitats also respond 

 to the changes (part 3.4.3) and are discussed in parts 

 4.0 and 5.0. 



The quantitative evaluation of the effects of modi- 

 fication in the hydrologic regime of Chenier Plain 

 basins is severly hampered by the absence of good 

 hydrodynamic models. Good models should be a pri- 

 ority item for management because water flows are 

 the key to the productivity of the Chenier Plain. Ex- 

 isting models have demonstrated their usefulness. 

 Tracor, Inc. (1971) modeled water quahty parameters 

 in a two-dimensional model of Galvestion Bay that in- 

 cluded East Bay. The U.S. Army Corps Engineers 

 (1950) predicted saltwater intrusion from a model of 

 the Calcasieu River and connecting waterways. More 

 comprehensive hydrodynamic models have been de- 

 veloped for estuarine areas (Lauff 1967), but they 

 have not been applied to the Chenier Plain basins nor 

 have they been apphed in any systematic way to pre- 

 dict the hydrological modifications associated with 

 canals in general. However, certain large-scale water 



71 



