(Lewis 1976). As the city prospered it 

 grew beyond its available high land into 

 the surrounding wetlands. 



The industrial corridor from New 

 Orleans to Baton Rouge on the Missis- 

 sippi River (taken together) is now one 

 of the most active port complexes in the 

 world. New Orleans' strategic location 

 fostered its development as a major port 

 city, but severe constraints on dry 

 land, combined with a shift from water 

 to rail and truck transport, reduced its 

 competitive advantage. The population 

 growth of New Orleans was thus slowed 

 relative to other major U.S. cities 

 (Figure 43) . 



Description of the Area 



Lakes Maurepas, Pontchartrain , and 

 Borgne are delta flank depressions, or 

 low areas at the edge of a formerly 

 active delta system. They are the rem- 

 nants of a large arm of the Gulf of 

 Mexico that was partially pinched off by 

 the progradation of the St. Bernard 

 Delta complex that was active from about 

 2,500 to 1,000 years B.P. (Kolb and Van 

 Lopik 1958). The area of these lakes 

 has increased somewhat in recent times 

 as a result of sea level rise, shoreline 

 erosion, land subsidence, and wetland 

 deterioration. For example, the north- 

 west shoreline of Lake Pontchartrain was 

 formerly bounded by swamp forest and 

 freshwater marsh that is now becoming 

 open water because of a combination of 

 the above processes. 



Water quality in the estuarine open 

 water habitat is a function of the com- 

 ponents of the basin watershed, which 

 extends inland to about 65 km (40 mi) 

 east of Natchez, Mississippi. This 

 watershed includes the large (300,000 

 population) and growing Louisiana State 

 Capital city of Baton Rouge and a 

 rapidly developing "bedroom community" 

 fringe along the north shore of Lake 

 Pontchartrain. 



Lake Pontchartrain is a large 

 (1,700 ha, 4,199 acres) shallow (5m, or 

 16 ft) well-mixed, oligohaline estuary 

 with a turnover, or flushing time from 

 60 to 100 days (Swenson 1980). Although 



the Mississippi River does not normally 

 flow directly into the lake, a natural 

 crevasse site on the west side of the 

 lake has been converted into a con- 

 trolled diversion structure (the Bonnet 

 Carre Floodway) through which excess 

 flood water from the river has been 

 released six times (1937, 1945, 1950, 

 1973, 1975, and 1979) since the con- 

 struction of the floodway in 1931. When 

 Mississippi waters are diverted into the 

 Bonnet Carre, enormous quantities of 

 sediment also enter, but there is no 

 sign that the lake has become shallower 

 during recent times (Stone 1980b). The 

 depth of the lake is apparently main- 

 tained by wind-driven erosion that 

 scours excess sediment from the bottom 

 (Price 1947) and exports it to the gulf 

 via Lake Borgne. 



Fresh water (other than precipita- 

 tion) enters the lake from Pass Manchac 

 to the west, the Tchefuncte and 

 Tangipahoa Rivers and some smaller 

 streams and bayous to the north, the New 

 Orleans drainage canals to the south, 

 some marsh bayous west of New Orleans, 

 and aperiodically from the Pearl River 

 basin under certain wind and flood con- 

 ditions . 



There are indications that Lake 

 Pontchartrain water has declined in 

 quality in recent times (Stone 1980a and 

 b; Turner et al. 1980; Sikora and Sikora 

 1982). These signs are nutrient in- 

 creases without corresponding increases 

 in primary production, population 

 changes in nekton and zooplankton, 

 benthic community changes, a decline in 

 organic matter in the sediments, a pos- 

 sible increase in turbidity, marsh loss 

 around the lake edge, and alarming con- 

 centrations of toxic chemicals (Stone 

 1980a and b; Sikora and Sikora 1982). 



Nutrient concentrations in Lake 

 Pontchartrain are not particularly high 

 at present (0.05 mg/1 P0 4 ; 0.41 mg/1 

 total N) , despite high phosphorus and 

 nitrogen loading rates. Excess nutrients 

 enter the lake via rain water from New 

 Orleans pumped into the lake along the 

 south shore, from the Baton Rouge area 

 via the Amite-Comite River system and 

 Pass Manchac, and from the communities 



97 



