Lake Pontchartrain

Lake Pontchartrain is a brackish-water lake in southeastern Louisiana. It is the second largest salt-water lake in the United States (after the Great Salt Lake in Utah) and the largest lake in Louisiana.

Lake Pontchartrain is roughly oval in shape, about 40 miles (64.4 km) wide from east to west, and measures about 24 miles (38.6 km) from north to south. The south shore of Lake Pontchartrain forms the northern boundary of the city of New Orleans. On the north shore of the lake are the towns of Mandeville and Madisonville; to the northeast of the lake is the city of Slidell. Lake Maurepas connects with Lake Pontchartrain to the west via the Pass Manchac. To the east the Rigolets Strait connects to Lake Borgne, which in turn connects to the Gulf of Mexico.

The lake is wide but for the most part not especially deep, averaging some 12 to 14 feet (3.65 to 4.26 m) in depth. Some channels for shipping are kept deeper through dredging. Geologically the lake is a fairly recent feature, believed to have been formed due to natural changes in the course of the Mississippi River within the last 5,000 years.

The Native American name for the lake was Okwata, "Wide Water". In 1699 French explorer Pierre La Moyne, Sieur d'Iberville, renamed it "Pontchartrain" after the Comte de Pontchartrain, the French Minister of the Marine at the time.

The city of New Orleans was established at an Native America portage between the Mississippi River and Lake Pontchartrain. In the 1920s the Industrial Canal in eastern New Orleans gave a direct navigable water connection (with locks) between the river and the lake.

The Lake Pontchartrain Causeway was constructed in the 1950s and 1960s connecting New Orleans with Mandeville, bisecting Lake Pontchartrain from north to south. It is also the longest bridge in the world, spanning about 24 miles (38.6 kilometers).

During hurricanes, a storm surge can build up on the large lake, just as with Florida's Lake Okeechobee. Therefore, levees have been built to protect the city of New Orleans, most of which is actually below lake level. These have so far been constructed for only moderate hurricanes. In addition to the surge from the lake, the lake can also get its own surge from the Gulf of Mexico, adding significantly to the problem. This is considered a worst-case scenario for the city, as a storm approaching from the south (like Katrina) first blows the surge into the lake, then into the city. Most of the levees were constructed after 1965, when Hurricane Betsy left much of the city under the lake's waters for weeks.

When Hurricane Katrina in 2005 reached Category 5 strength, experts expected the levee system to fail completely if it remained at that strength and passed close enough to New Orleans if not directly over. Although it weakened to Category 4, the levees suffered multiple breaks the next day, flooding the city of New Orleans almost entirely.

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