Tangipahoa Parish, Louisiana in the context of Manchac, Louisiana

Lake Pontchartrain (/ˈpɒntʃətreɪn/ PON-chə-trayn; French: Lac Pontchartrain) is an estuary located in southeastern Louisiana in the United States. It covers an area of 630 square miles (1,600 km) with an average depth of 12 to 14 feet (3.7 to 4.3 m). Some shipping channels are kept deeper through dredging. It is roughly oval in shape, about 40 miles (64 km) from west to east and 24 miles (39 km) from south to north. As an estuary, its “Lake” title is a misnomer, but only a slight one, as its passage to the open sea is narrow.

In descending order of area, the estuary is located in parts of six Louisiana parishes: St. Tammany, Orleans, Jefferson, St. John the Baptist, St. Charles, and Tangipahoa. The water boundaries were defined in 1979 (see list of parishes in Louisiana).