Hydrologic unit III includes the 

 area enclosed by the artificial levees 

 on either side of the lower river (below 

 New Orleans). Urban-industrial, cypress- 

 tupelo, and other terrestrial habitats 

 are therefore included as part of this 

 unit. The areas of all habitats in the 

 Mississippi River Delta hydrologic unit 

 in 1955 and in 1978 are compared in 

 Table 11. 



The largest non-water portion of 

 the Mississippi River Delta hydrologic 

 unit is freshwater marsh habitat, com- 

 prising only 6% of the entire unit. 

 Fresh and brackish marsh together com- 

 prise about 9% of the total area. Ex- 

 cept for the two narrow levee areas on 

 either side of the river, where most 

 development has occurred, hydrologic 

 unit III is mainly a network of fresh 

 and brackish water marsh habitat heavily 

 laced with distributary streams and 

 bayous, and surrounded by estuarine open 

 water. The influence of the Mississippi 

 River on this unit is shown by the dom- 

 inance of fresh and brackish marsh that 

 occupies an area of formerly open gulf 

 water. 



At the local level, the greatest 

 cultural influence on the modern delta 

 is the artificial confinement of the 

 Mississippi River through much of the 

 hydrologic unit. The primary influence 

 on this area, however, is the Old River 

 Control Structure 500 km upstream which 

 prevents the river from abandoning its 

 present course. If the river were to 

 change course and move to the Atchafa- 

 laya basin, the modern delta would 

 quickly erode and the shoreline would 

 retreat landward. 



The main navigation channel from 

 the open Gulf of Mexico through the sill 

 of sediment that is deposited in the 

 mouth of Southwest Pass is continually 

 dredged. A navigable channel 12 m (40 

 ft) deep and 159 m (500 ft) wide is 

 presently maintained from the open gulf 

 to Baton Rouge, 370 km (230 mi) upstream 

 from the Head of Passes. The feasi- 

 bility of increasing the depth of this 

 channel to 55 ft is currently under 

 consideration (at an estimated original 



cost of 0.5 billion dollars), (USACE 

 1981). Spoil banks from channel dredg- 

 ing in the modern delta presently equal 

 about 1% of the total area of the 

 hydrologic unit. (See SPOIL BANKS Sec- 

 tion of this report.) 



BARATARIA HYDROLOGIC UNIT (IV) 



The Barataria hydrologic unit is a 

 classic interdistributary delta basin. 

 It is bound by natural levees of Mis- 

 sissippi River distributaries that grade 

 from high land down into a central delta 

 flank depression occupied by a series of 

 lakes. All of the habitat types are 

 represented, with a fairly even distri- 

 bution of the major types (Table 12). 

 The basin is complete, i.e., there are 

 no natural water bodies that connect it 

 to other coastal hydrologic units, with 

 the exception of the Gulf of Mexico. 



Because the Barataria hydrologic 

 unit is the most intensively studied in 

 the MDPR (e.g., Day et al. 1973; Conner 

 and Day 1976; Hopkinson 1978) this sec- 

 tion includes the most detailed of the 

 hydrologic unit descriptions. 



Description of the Area 



This unit is located between the 

 natural levees of the Mississippi River 

 and Bayou Lafourche (Figure 46). The 

 artificial closing of the Bayou La- 

 fourche distributary in 1904 by the 

 USACE and the completion of the manmade 

 levee system along the Mississippi River 

 have effectively eliminated overbank 

 flooding of river water and sediment 

 into the basin. Presently, most fresh 

 water that comes into the system is 

 precipitation, which averages 156 cm/yr 

 (61 inches/yr) (Sklar 1980). There is 

 an extensive network of interconnecting 

 water bodies that allows transport of 

 water, materials, and migrating organ- 

 isms throughout the basin. 



The rate of water movement through 

 the basin is a function of tidal range 

 (approximately 0.3 m, or 1 ft, at the 

 coast), wind, precipitation, and the 

 mild slope of the topography from the 



104 



