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20-526: The strait begins at 30°10′40″N 89°44′40″W  /  30.17778°N 89.74444°W  / 30.17778; -89.74444 and follows a generally eastward course to Lake Borgne , a lagoon in the Gulf of Mexico , and finally to the Gulf of Mexico, where it ends at 30°09′16″N 89°37′31″W  /  30.15444°N 89.62528°W  / 30.15444; -89.62528 . Along with nearby Chef Menteur Pass ,

40-662: A role in the critical problem of the Pontchartrain Basin—increased salinity . The construction of the MRGO, which breaches the natural barrier of the Bayou La Loutre ridge and the Borgne land bridge, has allowed sea water to push farther into the basin. Relative sea level rise of up to 0.96 feet per century also gives saltier waters greater access to surrounding wetlands. As a result, mean monthly salinities have increased since

60-517: Is a lagoon of the Gulf of Mexico in southeastern Louisiana . Although early maps show it as a lake surrounded by land, coastal erosion has made it an arm of the Gulf of Mexico. In southern Louisiana, three large lakes— Maurepas , Pontchartrain , and Borgne—cover 55% of the Pontchartrain Basin . A brackish marsh land bridge and Lake St. Catherine separate Lake Pontchartrain from Lake Borgne. The Rigolets and Chef Menteur Pass are

80-468: Is a significant global problem. The large delta plains of the world, including the Danube , Ganges , Brahmaputra , Indus , Mahanadi , Mangoky , McKenzie , Mississippi , Niger , Nile , Shatt el Arab , Volga , Yellow , Yukon , and Zambezi deltas, have all suffered significant land loss as the result of either coastal erosion, internal conversion of wetlands to open water, or a combination of both. For

100-695: Is spanned by two bridges. The western terminus of the U.S. Route 90 Rigolets Bridge is located immediately north of Fort Pike. It was damaged by Hurricane Katrina in August 2005, and required major repairs. Farther south, CSX Transportation crosses the Rigolets on a 1,388-meter (4,555-ft) railroad bridge. Hurricane damage there from Katrina included shifted spans and the loss of timber decking. Lake Borgne Lake Borgne ( / b ɔːr n / BORN ; French : Lac Borgne [lak bɔʁɲ] , lit.   ' One-Eyed Lake ' ; Spanish : Lago Borgne )

120-606: Is the term typically used to refer to the conversion of coastal land to open water by natural processes and human activities. The term land loss includes coastal erosion . It is a much broader term than coastal erosion because land loss also includes land converted to open water around the edges of estuaries and interior bays and lakes and by subsidence of coastal plain wetlands. The most important causes of land loss in coastal plains are erosion , inadequate sediment supply to beaches and wetlands , subsidence , and global sea level rise . The mixture of processes responsible for most of

140-586: The Mississippi River Gulf Outlet (MRGO). It is the largest design-build civil works project in the history of the Corps. It was first used in 2012 to protect against storm surge from Hurricane Isaac and was fully completed in 2013. The Mississippi River levees significantly limit the input of fresh water, sediment, and nutrients into the Pontchartrain basin. This reduction in freshwater input plays

160-541: The 15 deltas studied by Coleman and others, these deltas experienced a total irreversible land loss of 5,104 km (1,971 sq mi) of wetlands between the early 1980s and 2002. During this period, the total average land loss for all these deltas was about 41 km (16 sq mi) per year. In the case of the Mississippi River Delta, they found that in 12 years, some 253 km (98 sq mi) of wetlands had been converted to new open water at

180-604: The Pontchartrain Basin—over 22% of the marsh that existed in 1932. The primary causes of wetland loss in the basin are the effects of natural changes to the topography over time and can and will change in the future, the hydrological isolation of the Mississippi River from its floodplains (which deprives the coast of sediment needed to build and sustain land), and the extensive cutting and channelization of coastal wetlands (which destabilizes existing land, hastening

200-465: The Rigolets connects Lake Pontchartrain and Lake St. Catherine in Louisiana to Lake Borgne, and then to the Gulf of Mexico. It forms the boundary between New Orleans (Orleans Parish) and St. Tammany Parish . As a deepwater tidal pass, the Rigolets helps supply salt water from the Gulf to Lake Pontchartrain. Tidal scouring has produced a deep pit in the lake at the western mouth of the strait. Since

220-547: The Rigolets is a channel through which Gulf storm surges can approach the New Orleans area, there have been proposals to construct floodgates to try to protect the city, especially since the destructiveness of hurricanes in the early 21st century. The United States constructed Fort Pike following the War of 1812 to protect passage on the Rigolets. The fort was abandoned in 1890 when it was no longer considered necessary. The Rigolets

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240-668: The State of Mississippi argued that at the time of Louisiana's admission into the Union (1812), there probably was no such "Lake Borgne" or "Mississippi Sound". The basin contains 483,390 acres (1956 km ) of wetlands, consisting of nearly 38,500 acres (156 km ) of fresh marsh, 28,600 acres (116 km ) of intermediate marsh, 116,800 acres (473 km ) of brackish marsh, 83,900 acres (340 km ) of saline marsh, and 215,600 acres (873 km ) of cypress swamp. Since 1932, more than 66,000 acres (267 km ) of marsh have converted to water in

260-717: The construction of the MRGO and other canals. In recent years, salinities have stabilized. The heightened salinity has stressed wetlands, especially freshwater marshes and swamps. Since 1932, approximately 24% of the Borgne Land Bridge has been lost to 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 because of subsidence and spikes in lake salinity. These land bridges prevent estuarine processes, such as increased salinity and tidal scour , from pushing further into

280-580: The dam prevent sediment from replenishing the delta. Subsidence is the compaction of soil resulting in a lower elevation. Subsidence can occur when oil, gas, or groundwater are extracted. These substances hold the land up until they are removed. Compaction due to heavy urban infrastructure also occurs. Sea level rise due to climate change is another threat to coastal land. Because of a highly variable combination of sea level rise , sediment starvation, coastal erosion, wetland deterioration, subsidence , and various human activities, land loss within delta plains

300-682: The land loss will vary according to the specific part of a coastal plain being examined. The definition of land loss does not include the loss of coastal lands to agricultural use, urbanization, or other development. Although seemingly related, wetland loss, is defined differently than land loss. Commonly, wetland loss is defined as the conversion of vegetated wetlands into either uplands or drained areas, unvegetated wetlands (e.g., mudflats), or (submerged habitats (open water). According to this, and similar definitions, wetland loss includes both land loss and land consumption as components of it. In historic times, both wetland and land loss typically are

320-422: The middle and upper basins. Additionally, from 1968 to 1988, 32% of the cypress swamp on the land bridge had either converted to marsh or became open water. If these buffers are not preserved, the land loss rates around Lakes Pontchartrain and Maurepas will increase dramatically. Several marshes in the basin are vulnerable to rapid loss if adequate protection is not quickly provided. Land loss Land loss

340-431: The rate of sediment deposition is slower than the rate of sediment removal by coastal currents. The most important cause of decreased rates of sediment deposition is the construction of dams and reservoirs although sediment control and conservation programs can also play a role. Once a dam is constructed, sediment that previously traveled freely in the river is trapped in the reservoir. Decreased sediment loads downstream of

360-438: The result of a varying, often controversial mixture of natural and anthropogenic factors. There are other definitions of wetland loss commonly used. For example, some researchers defined wetland loss as "the substantial removal of wetland from its ecologic role under natural conditions." The main causes of land loss are coastal erosion , inadequate sediment supply, subsidence , and sea level rise . Coastal erosion occurs when

380-596: The transition to open water). In response to the extensive damage from Hurricane Katrina in 2005, the Army Corps of Engineers constructed the 1.8-mile-long (2.9 km) IHNC Lake Borgne Surge Barrier , as part of the Hurricane & Storm Damage Risk Reduction System for southeast Louisiana. The project cost approximately $ 1.1 billion and was built at the confluence of the Gulf Intracoastal Waterway and

400-507: The two open water connections between Pontchartrain and Borgne. Coastal erosion has transformed Borgne into a lagoon connecting to the Gulf of Mexico. Early 18th-century maps show Borgne as a true lake, largely separated from the gulf by a considerable extent of wetlands that have since disappeared. In a 1902 case before the United States Supreme Court over the oyster banks at the boundary between Louisiana and Mississippi,

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