Lake Borgne is a lagoon of the Gulf of Mexico in southeastern Louisiana. Although early maps show it as a lake surrounded by land, coastal erosion has made it an arm of the Gulf of Mexico. In southern Louisiana, three large lakes—Maurepas, Pontchartrain, and Borgne—cover 55% of the Pontchartrain Basin. A brackish marsh land bridge and Lake St. Cath…Lake Borgne is a lagoon of the Gulf of Mexico in southeastern Louisiana. Although early maps show it as a lake surrounded by land, coastal erosion has made it an arm of the Gulf of Mexico. In southern Louisiana, three large lakes—Maurepas, Pontchartrain, and Borgne—cover 55% of the Pontchartrain Basin. A brackish marsh land bridge and Lake St. Catherine separate Lake Pontchartrain from Lake Borgne. The Rigolets and Chef Menteur Pass are the two open water connections between Pontchartrain and Borgne.