erosional stress. A general principle 

 of delta life history is that the 

 coastline of the most recently abandoned 

 delta lobe regresses most rapidly 

 (Becker 1972). The shoreline due east 

 of the mouth of Bayou Lafourche (Bara- 

 taria hydrologic unit) is eroding at a 

 rate of 15 m/yr (Sasser et al. 1981). 

 The typically low wave energy pattern 

 for the MDPR changes drastically during 

 the passage of a hurricane. 



In addition to the erosional ef- 

 fects of wave energy, there is a pattern 

 of littoral drift: longshore currents 

 move sediments and organisms to the west 

 toward Texas, as shown in Figure 8. 

 This figure indicates that although 

 current direction is variable near shore 

 along the MDPR, the overall pattern is 

 for westward-trending currents. This 

 pattern is partly due to the general 

 absence of westerly winds in the area, 

 as shown below in Figure 13. 



The mean tidal range along the 

 coast is about 0.3 m (1 ft), an empir- 

 ical estimate that includes both meteo- 

 rological (wind driven) and astronomical 

 tidal components. The low tidal ampli- 

 tude in the MDPR results in the devel- 

 opment of tidal flats and tidal creek 

 networks that are smaller than those of 

 deltas in regions having greater tides. 

 Tidal amplitude is attenuated inland 

 from the coast, as shown by the USACE 

 gauge records from Barataria Bay. Two 

 tidal components affect the inundation 

 regime. A biweekly range cycles between 

 tides that are equatorial (equivalent to 

 neaptide) and those that are tropical 

 (equivalent to springtide). A seasonal 

 maximum tidal range occurs during sol- 

 stices, and a seasonal minimum range 

 during equinoxes. 



Meteorological processes overshadow 

 tides in the control of water level 

 variation in the MDPR. Water-level 

 variations lasting from less than one to 

 several days are governed by the com- 

 bined effects of tidal and climatic 

 factors. Winter cold fronts are usually 

 preceded by several days of strong 

 southerly winds, which resuspend bottom 

 sediments and flood the marshes. When 

 the front passes, the wind shifts to the 



north, and the marshes drain rapidly, 

 flushing out organic detritus. 



Annual water levels show a bimodal 

 distribution, with low winter levels 

 reflecting northerly winds and contrac- 

 tion of water volume due to low temper- 

 ature. The spring peak is caused 

 primarily by a shift to strong onshore 

 winds. The summer minimum reflects a 

 time of weak winds and high evaporation. 

 The peak in September is the result of 

 strong onshore winds and some expansion 

 from summer heating (Baumann 1980). 



Variations in tidal range and 

 wind-related water level combine to 

 produce changes in the inundation of 

 wetlands in the Barataria basin, which 

 are fairly representative of the pattern 

 across the MDPR (Figure 9). 



The duration of inundation in both 

 coastal and inland sites (in the Bara- 

 taria basin for example) is largely a 

 function of the annual water-level cycle 

 (Figure 10) . The frequency of flooding 

 exhibits less variability than does 

 duration of flooding. When water level 

 is at a maximum in September, tidal 

 range is low, resulting in the deep 

 flooding of marshes for long periods of 

 time (Baumann 1980). The seasonal low in 

 frequency of flooding results in high 

 water levels and low tide range. Flood- 

 ing is more frequent in the saline marsh 

 during December because of both a high 

 tide range and frequent frontal pas- 

 sages. Frequent flooding during the 

 summer results from the seasonal high in 

 tide range and a low mean water level, 

 the opposite of the conditions that 

 prevail during September and October. 

 Swamps remain flooded for much of the 

 year, but they have a low flooding fre- 

 quency because of the absence of tidal 

 influence . 



In addition to tides and winds, 

 marine processes regulate salinity. The 

 salinity regime in estuaries is con- 

 stantly changing, causing osmotic 

 stresses that exclude many marine and 

 freshwater organisms and reduce the 

 number of species (Hedgpeth 1967; Remane 

 and Schlieper (1971). As land loss ac- 

 celerates in the region, saline water 



17 



