Hydrology 



The hydrology of Jean Lafitte National Park has been drastically altered by the construction of 

 canals through the wetlands during the past 280 years (Figure 11). Under natural conditions, water 

 flowed across the wetlands, the rate depending on the freshwater runoff, tidal exchange, and winds. 

 Currently, however, altered hydrology in the park includes (1) upland runoff which quickly flows 

 into the canals and is shunted out of the park, (2) regional water level fluctuations, especially 

 during frontal passages, which lead to both rapid water level changes and water flowing quickly into 

 and out of the park, and (3) semi-impoundment of wetlands caused by spoil banks. Most upland 

 runoff and tidal exchange are primarily shunted directly in canals, thereby influencing relatively 

 small areas of wetlands in the park. For example, during flood tide, water collects at the 

 intersection of Kenta and Pipeline Canals and quickly leaves the park during ebb tide. Semi- 

 impoundment also causes relatively low net flows through the park. Overland sheet flow through 

 the wetlands is minimal. 



In addition to the gross hydrologic flows which have been altered, spoil placement along the 

 canals has impounded some areas of the park. The impounded cypress swamp, (site 18) was 

 flooded by at least 0.6 m of water during the entire study because spoil placement altered the 

 natural hydrologic gradient of flow from the swamp into the marshes. Impoundment of this swamp 

 undoubtedly has led to altered nutrient export and lowered productivity and seeding regeneration 

 (Conner et al. 1981). Brown and Lugo (1982) report that cypress tupelo communities show 

 severely reduced growth and productivity if the mean depth of flooding exceeds 60 cm. The 

 impounded site in the park is currently experiencing this critical water depth and thus may be 

 experiencing stress. 



Altered hydrology such as channelization and impoundment can lead to both increased and 

 decreased retention times of water in wetlands. The channels themselves may lead to more rapid 

 drainage of some areas while spoil banks may retard drainage from other areas. For example, 

 Swenson and Turner (1987) reported that partially impounded marsh sites in Louisiana were 

 characterized by both longer flooding events and reduced water exchange both above and below 

 ground. In addition, wetland ecosystems with altered hydrology, which include deeper water levels, 

 longer retention times, and slow flushing rates, often have lower productivity and symptoms of 

 stress. Conner et al. (1981) showed that a permanently flooded impounded swamp had fewer trees 

 and those trees had lower basal areas, reduced recruitment, and lower productivity compared to 

 a healthy control swamp. Prolonged periods of deep inundation often reduced vegetative 

 productivity and regeneration. 



Floating marsh formation hypothesis 



Floating marshes are widespread in coastal Louisiana, yet the process of formation is uncertain. 

 Two different theories have emerged: (1) Russell (1942) concluded that flotant resulted from the 

 encroachment and expansion of emergent vascular aquatics into previously open water area, and 

 (2) O'Neil (1949) proposed that floating marshes were formed by buoyant detachment of marsh 

 from the subsiding soil substrate as a response to marsh flooding and an absence of mineral 

 sediment. Swarzenski (1987) studied floating marshes in coastal Louisiana and determined that 

 buoyant detachment was probably the most important mode of formation. 



Aerial imagery of the park indicates that 322 ha of wax myrtle stands emerged in the area during 

 the past 30 years. Field observations, soil characteristics, and sediment accretion rates all suggest 

 that this area has undergone an accelerated rate of flotant formation catalyzed by both a sediment 



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