Lake Borgne

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Lake Borgne
A strip of marsh separates MRGO (left) from Lake Borgne (right).
Location	Louisiana
Coordinates	30°9′N 89°36′WCoordinates: 30°9′N 89°36′W
Lake type	lagoon
Primary sources	MRGO and Mississippi River
Primary outflows	Gulf of Mexico
Catchment area	38,000 km²^[1]
Basin countries	USA
Surface area	730 km²^[1]
Average depth	3 m^[1]
Surface elevation	0
Settlements	New Orleans

Lake Borgne is a lagoon in eastern Louisiana of the Gulf of Mexico. Its name comes from the French word, borgne, which means "one-eyed".

[edit] Geography

The three large lakes, Maurepas, Pontchartrain, and Borgne cover 55% of the Pontchartrain 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. The Rigolets and Chef Menteur Pass are the two open water connections between Pontchartrain and Borgne.

Due to coastal erosion Borgne is now a lagoon connecting to the Gulf of Mexico, but early 18th century maps show it as a lake largely separated from the Gulf by a considerable extent of wetlands which have since disappeared.

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.^{[citation needed]}

[edit] Ecology

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The Mississippi River levees significantly limit the input of fresh water, sediment, and nutrients into the Pontchartrain basin. This reduction in riverine input plays a part in the major critical problem in the Pontchartrain Basin — increased salinity. Construction of the Mississippi River-Gulf Outlet Canal (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.

This 1759 map based on the 1720 de la Tour survey shows Lake Borne seperated from the Gulf of Mexico by "Low and Marshy Meadows" (wetlands).

A second critical problem, occurring in the lower basin, is the erosion along the MRGO caused by ship-induced waves.{{fact} The channel's north bank continues to erode 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.^{[citation needed]}

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.