Louisiana lies wholly within the Gulf Coastal Plain. Alluvial lands, chiefly of the Red and Mississippi rivers, occupy the north-central third of the state. East and west of this alluvial plain are the upland districts, characterized by rolling hills sloping gently toward the coast. The coastal-delta section, in the southernmost portion of the state, consists of the Mississippi Delta and the coastal lowlands. The highest elevation in the state is Driskill Mountain at 535 ft (163 m), in Bienville Parish; the lowest, 8 ft (2 m) below sea level, in New Orleans.

Louisiana has the most wetlands of all the states, about 11,000 sq mi (28,000 sq km) of floodplains and 7,800 sq mi (20,200 sq km) of coastal swamps, marshes, and estuarine waters. The largest lake, actually a coastal lagoon, is Lake Pontchartrain, with an area of more than 620 sq mi (1,600 sq km). Toledo Bend Reservoir, an artificial lake along the Louisiana-Texas border, has an area of 284 sq mi (736 sq km). The most important rivers are the Mississippi, Red, Pearl, Atchafalaya, and Sabine. Most drainage takes place through swamps between the bayous, which serve as outlets for overflowing rivers and streams. Louisiana has nearly 2,500 coastal islands covering some 2,000 sq mi (5,000 sq km).