Pontchartrain, Lake (pŏnˈchərtrān) [key], shallow lake, c.630 sq mi (1,630 sq km), 41 mi (66 km) long and 25 mi (40 km) wide, SE La., N of New Orleans. It is linked with Lake Maurepas at its western end and with the Gulf of Mexico at its eastern end through Lake Borgne. Tidal and brackish, the lake forms a popular resort and recreational region. The Bonnet Carre Spillway diverts part of the floodwaters of the Mississippi River into Pontchartrain. Two causeways, each c.24 mi (40 km) long, connect New Orleans with Covington, La.; the long, multispan Pontchartrain Causeway II opened in 1969.