Mississippi-Alabama State line (Figure 

 41). 



The areas of each of the 20 compo- 

 nent habitats in this hydrologic unit 

 and changes in habitat areas from 1955 

 to 1978 are shown in Table 9. 



The coastal plain is narrow in 

 Mississippi, compared to its width in 

 the remainder of the MDPR to the west. 

 The rolling, upland DeSoto National 

 Forest occupies much of the land north 

 of the basin. Upland forest (habitat 

 19) makes up a significant proportion of 

 the Mississippi Sound hydrologic unit, 

 primarily because the inland boundary is 

 defined by a relatively high elevation 

 contour rather than by a political 

 coastal zone boundary as in Louisiana. 



The string of barrier islands in 

 the Mississippi Sound hydrologic unit 

 are unusually far from the mainland 

 compared to other islands in the MDPR 

 (Otvos 1981). These islands demarcate 

 Mississippi Sound, the single largest 

 habitat component of the basin (habitat 

 16, estuarine open water). The open 

 water area beyond the barrier islands to 

 the three mile limit is considered 

 nearshore gulf habitat (habitat 13), and 

 the Mississippi Sound hydrologic unit 

 contains the largest portion of this 

 habitat of the seven hydrologic units in 

 the MDPR. 



Wetland habitats consist of the 

 marshes and swamp forests that occupy 

 the lower Pearl River valley and sur- 

 round the four bays that interrupt the 

 shoreline of Mississippi: Bay St. 

 Louis, Biloxi Bay, Pascagoula Bay, and 

 Point aux Chenes Bay. 



The eastern boundary of the hydro- 

 logic unit (the Mississippi-Alabama 

 State Line) has no geological or eco- 

 logical significance. The Alabama por- 

 tion of Mississippi Sound (including 

 Mobile Bay) is functionally continuous 

 with the drainage basin described here. 

 The western boundary of the basin is 

 continuous with the Pontchartrain 

 hydrologic unit, and waters from the two 

 hydrologic units mix at the interface 

 between Lake Borgne and Mississippi 

 Sound. 



Because of the rapid urban growth 

 of the Mississippi gulf coast area, the 

 urban-industrial habitat (habitat 20) is 

 now relatively prominent in this unit. 

 The agricultural habitat (habitat 1) is 

 also relatively large, but it has been 

 rapidly losing ground to urban-indus- 

 trial uses in the last two decades, as 

 shown in Table 9. 



Freshwater input to the basin is 

 normally limited to rainfall and drain- 

 age from coastal rivers. Average river 

 inputs in cubic meters per second are as 

 follow: Pascagoula, 378.35; Pearl, 

 327.72; Jourdan, 23.47; Wolf, 19.98; 

 Biloxi, 13.97; and Tchoutacabouf f a, 

 12.36 (Eleuterius 1978). During extreme 

 high water stages on the Mississippi 

 River the Bonnet Carre Spillway is 

 opened, and Mississippi River water 

 drains through Lake Pontchartrain and 

 Lake Borgne into Mississippi Sound. The 

 effect of opening the spillway may be 

 quite significant, as shown by the 

 widespread oyster mortality that accom- 

 panied the last opening of the spillway 

 during the spring of 1979. Oyster pro- 

 duction in Mississippi Sound in 1980 was 

 lower than it had been for at least 3 

 decades (Deegen et al. 1981). 



Geological History 



The following brief description of 

 the geological history of the Missis- 

 sippi Sound hydrologic unit is summa- 

 rized from Otvos (1981). The developed 

 part of the coastline (the so-called 

 Mississippi Gold Coast) is located on 

 ancient beach ridges, the Gulf port For- 

 mation overlying the Biloxi Formation, 

 that formed about 1.25 million years 

 B.P., during the Sangomon interglacial 

 period when sea level stood higher than 

 at present. More recently during the 

 late Pleistocene period of sea level 

 decline, river trenches were eroded into 

 the prairie terrace in the Gulf of 

 Mexico seaward of the present location 

 of the barrier islands. These entrench- 

 ments filled with sediments as sea level 

 rose again in late Wisconsin and early 

 Holocene times until it stabilized again 

 about 4,500 years B.P. The bays along 

 the Mississippi coast formed at this 

 time, as sea water filled the coastal 

 depressions, and most of the area now 



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