Lake Borgne is a lagoon in eastern Louisiana of the Gulf of Mexico. A lagoon is a body of comparatively shallow salt water separated from the deeper sea by a shallow or exposed sandbank, coral reef, or similar feature. ... State nickname: Pelican State Other U.S. States Capital Baton Rouge Largest city New Orleans Governor Kathleen Blanco (D) Official languages None; English and French de facto Area 134,382 km² (31st)  - Land 112,927 km²  - Water 21,455 km² (16%) Population (2000)  - Population 4,468,976 (22nd)  - Density 39. ...

The three large lakes, Maurepas, Pontchartrain, and Borgne (actually a lagoon) cover 55% of the basin. Lakes Maurepas and Pontchartrain are separated by land bridges of cypress swamp and fresh/intermediate marsh. A brackish marsh land bridge and Lake St. Catherine separate Lake Pontchartrain from Lake Borgne. Lake Maurepas is a saltwater lake in southeastern Louisiana. ... Lake Pontchartrain (local pronunciation ) is a brackish-water lake in southeastern Louisiana. ... Lake St. ...

The basin contains 483,390 acres (1956 km²) of wetlands, consisting of nearly 38,500 acres (156 km²) of fresh marsh, 28,600 acres (116 km²) of intermediate marsh, 116,800 acres (473 km²) of brackish marsh, 83,900 acres (340 km²) of saline marsh, and 215,600 acres (873 km²) of cypress swamp. Since 1932, more than 66,000 acres (267 km²) of marsh have converted to water in the Pontchartrain Basin — over 22% of the marsh that existed in 1932. The primary causes of wetland loss in the basin are the interrelated effects of human activities and the estuarine processes that began to predominate many hundreds of years ago, as the delta was abandoned.

The Mississippi River levees significantly limit the input of fresh water, sediment, and nutrients into the basin. This reduction in riverine input plays a part in the major critical problem in the Pontchartrain Basin — increased salinity. Construction of the MRGO, which breaches the natural barrier of the Bayou La Loutre ridge and the Borgne land bridge, allowed saline waters to push farther into the basin. Relative sea level rise of up to 0.96 feet per century (3 mm per year) gives saltier waters greater access to basin wetlands. Mean monthly salinities have increased since the construction of the MRGO and other canals. However, these mean increases are less than the overall variability in salinity. In recent years, salinities have stabilized. The heightened salinity, caused mainly by subsidence, stresses wetlands, especially fresh marsh and swamp. Length 6,270 km Elevation of the source 450 m Average discharge Saint Louis¹: 5,500 m³/s Vicksburg²: 16,800 m³/s Baton Rouge³: 12,800 m³/s Area watershed 2,980,000 km² Origin  Lake Itasca Mouth  Gulf of Mexico Basin countries United States (98. ...

A second critical problem, occurring in the lower basin, is the erosion along the MRGO caused by ship-induced waves. The channel's north bank continues to eroding at a rate of 15 feet per year. This mechanism has resulted in the direct loss of over 1,700 acres (6.9 km²) of marsh since 1968.

The third critical problem is the potential loss of the Borgne and the Maurepas land bridges where wetland soils are especially vulnerable to erosion. Since 1932, approximately 24% of the Borgne Land Bridge has been lost to estuarine processes such as severe shoreline retreat and rapid tidal fluctuations, and the loss rate is increasing. During the same time, 17% of the Maurepas Land Bridge marshes disappeared due to subsidence and spikes in lake salinity. In addition, from 1968 to 1988, 32% of the cypress swamp on this land bridge either converted to marsh or became open water. These land bridges prevent estuarine processes, such as increased salinities and tidal scour, from pushing further into the middle and upper basins. If these buffers are not preserved, the land loss rates around Lakes Pontchartrain and Maurepas will increase dramatically.

The fourth critical problem is that several marshes in the basin are vulnerable to rapid loss if adequate protection is not provided soon. Examples of theses areas are: marshes adjacent to lakes and bays where if the narrow rim of shore is lost, interior erosion will increase dramatically; the perched fresh marsh on the MRGO disposal area which will drain and revegetate with shrub unless the back levee dikes are repaired; and near Bayou St. Malo, where unless canals are plugged, rapid water level fluctuations and salinity intrusion into adjacent marshes will continue.

Site specific problems of shoreline erosion, poor drainage, salinity stress, and herbivory are apparent throughout the basin. Solving these problems is important, but less urgent than solving the four critical problems described above.

Reference

• United States Geological Survey