The average elevation of the city, which is below the highwater levels of both the Mississippi River and Lake Pontchartrain, is but one foot above mean Gulf level. The highest natural formations in the city, about fifteen feet above mean Gulf level, are the strips of land adjacent to the river, the natural levees which confine the water to the channel during ordinary and all but the highest stages of the river.
The greater portion of the city would suffer from floods every year were it not for the surrounding artificial levee system. Levees constructed along the river and the Pontchartrain lake-front, across the swamps and along the waterways are all interconnected, thus enclosing completely the builtup section of the city, which is drained by means of canals and pumping stations. The levees along the river average about 23 feet and those along the lake-front and across the swamps and marshes about nine feet above mean Gulf level. Approximately thirty-nine per cent of the total land area of New Orleans is enclosed within levees. The unprotected sixty-one per cent is the peninsula and lands which lie along Lakes Pontchartrain and Borgne and extend northeastward from Micheaud to the Rigolets Pass. This area, for the most part subject to overflow by high tides from the Gulf, consists of delta fingers, coastal islands and ridges of low elevation, and intervening coastal marshes.
There are several navigable waterways within the municipal limits of the city, all connecting with Lake Pontchartrain. The New Orleans Navigation Canal begins at South Rampart Street at the edge of the business district and runs northward, entering the lake near the northwestern corner of the city. Farther east, the Inner Harbor Navigation Canal, commonly known as the 'Industrial Canal,' provides a channel five and one half miles long, with a depth of thirty feet and a width of three hundred feet, connecting the river and the lake. Bayou St. John, formerly a navigable stream, begins at Lafitte Avenue and Jefferson Davis Parkway and runs northward to the lake. Other navigable waters include Chef Menteur Pass, Lake St. Catherine, and a number of small passes and canals in the marsh area northeast of the built-up section of the city; the Mississippi River, Lakes Pontchartrain and Borgne, Rigolets Pass, and Bayou Bienvenue, all navigable, form part of the boundaries.
Lake Pontchartrain on the north, one of the largest lakes in the United States, is approximately forty-one miles long and twenty-five miles wide and comprises an area of 635 square miles. Of this area 146 square miles are included within the boundary of New Orleans.