Lake Pontchartrain is roughly oval in shape, about 40 miles wide from east to west, and measures about 25 miles 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, Louisiana; to the northeast of the lake is the city of Slidell. Lake Maurepas connects with Lake Pontchartrain to the west. To the east the Rigolets straits connect 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 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 of 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 old Indian 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.
A pair of automobile causeways were constructed in the 1950s and 1960s connecting New Orleans with Mandeville, bisecting Lake Pontchartrain from north to south.