The MDPR exists as a geologically 

 ephemeral balance of high land, wetland, 

 and open water. Water balance, flooding 

 patterns, and water quality (salinity, 

 turbidity, nutrient concentration, and 

 pollutant levels) determine the makeup 

 of a given habitat at a given time. 



Freshwater inputs come from precip- 

 itation, surface runoff, stream and 

 river discharge, and subsurface water. 

 Water is lost through runoff, evapora- 

 tion from open water, evapo transpiration 

 from vegetated areas, stream drainage, 

 and percolation (infiltration) into the 

 soil. These interrelated activities are 

 controlled by slope, soil type, climatic 

 factors (precipitation, temperature, and 

 wind regime), biological processes, ma- 

 rine influences (tidal patterns and 

 storm surges), riverine processes, and 

 human factors. 



Hydrology can be divided into four 

 main categories: riverine processes, 

 marine processes, geomorphology and soil 

 characteristics, and climatic factors. 

 The influence of these is illustrated in 

 Figure 7. 



The MDPR, which contains the eighth 

 largest deltaic plain in the world 

 (Wright et al. 1974), is a classic 

 river-dominated delta system. The pres- 

 ence of a delta at a river mouth indi- 

 cates a net deposition of alluvial 

 sediment in excess of erosion by marine 

 energy. Compared with 34 major delta 

 systems around the world, the MDPR ranks 

 first in its degree of riverine domi- 

 nance over marine processes (Wright et 

 al. 1974). 



Total annual freshwater discharge 

 into the lower Mississippi and Atchafa- 

 laya Rivers from the drainage basin up- 

 stream is strongly seasonal, usually 

 reaching a peak between February and 

 April. The pattern is variable, how- 

 ever, generally having several smaller 

 peaks throughout the year (Wright et al. 

 1974). The two major distributaries of 

 the Mississippi drainage basin together 

 discharge an average of 51,800 m 3 /sec 

 (1,830,000 ft 3 /sec) of water into the 

 MDPR, or about 2 trillion m 3 /yr (58 

 trillion ft 3 /yr) (USACE annual data). 

 This water is discharged into the gulf 



at two primary sites, the modern delta 

 (70%) and the Atchafalaya Bay complex 

 (30%) . 



River water accounts for over 90% 

 of the total input to the region. About 

 95% of this water and its suspended load 

 is shunted straight to the Gulf of 

 Mexico (USACE Annual data) . Before the 

 1930's, overbank flooding and crevasses 

 deposited millions of tons of silt into 

 the Barataria and Pontchartrain basins. 

 Levees in the lower Mississippi River 

 system now entrain flood waters. Thus 

 the river directly affects only the 

 Atchafalaya and the modern delta, except 

 when the Bonnet Carre spillway is opened 

 to divert floodwater into Lake Pont- 

 chartrain. The spillway, built in 1931, 

 has been opened six times to ease the 

 pressure on the levees protecting New 

 Orleans. During these times the river 

 has affected both the Pontchartrain and 

 Mississippi Sound systems. 



Because of the construction of 

 levees on the lower Mississippi River, 

 hydrologic processes in the Barataria, 

 the eastern portion of the Terrebonne, 

 and the Pontchartrain hydrologic units, 

 are dominated by marine, climatic, bio- 

 logical, and cultural factors. Local 

 precipitation accounts for only 10% of 

 freshwater input into the total MDPR, 

 but it is the only significant source 

 for the Barataria basin. 



Marine processes are overshadowed 

 by riverine processes in portions of the 

 MDPR, but the Gulf of Mexico erodes and 

 redistributes river-supplied sediments, 

 especially in the formation of barrier 

 islands. Compared with the other great 

 deltas of the world, nearshore wave 

 power along this coast is minimal, as 

 shown in Table 1 (Wright et al. 1974). 

 Note also that the attenuation ratio (a 

 measure of the amount of wave energy 

 that is dissipated offshore by the slope 

 of the bottom) for the Mississippi Delta 

 is much higher than that of other 

 deltas. 



Wave energy along the entire Lou- 

 isiana coastline is variable with areas 

 closest to the Continental Shelf re- 

 ceiving the highest wave energy; hence 

 the modern delta receives the greatest 



14 



