Bayou Bienvenue, formerly called St. Frances River, drains all the waters of a swamp-basin, of triangular form and about eighty square miles in surface, bounded on the west by New Orleans, on the northwest by Chef Menteur, and on the east by Lake Borgne, into which it empties. It receives the waters of several other bayous from the surrounding cypress swamps and prairies. It is navigable for vessels of one hundred tons burden as far as the junction with old Piernas Canal, twelve miles from its mouth. It is about one hundred and twenty yards in width, and has from six to nine feet of water at the bar, according to the flow of the tides. Its principal branch is Bayou Mazant, which runs to the southwest and receives the waters of the canals of the old plantations of Villere, Lacoste, and Laronde, on and near which the British army encamped, about eight miles below New Orleans. The banks of these bayous, which drain the swamp lands on either side of the Mississippi, are usually about twelve feet below the banks of the river, which have been elevated by the deposit of sediment from overflows for centuries. These slopes, from the banks back to the swamps, usually ten to eighteen hundred yards, drain off the waters and form the tillable lands of the sugar and cotton planters. They are protected from overflows by levees thrown up on the banks of the river. These plantation lands formed the only ground in this country for the encampment of a large army, or available for a march on New Orleans. On nearly all the large sugar plantations canals were cut from the bank of the river running back to the swamp, to furnish at high tides water-power for mills which did the grinding or sawing for the plantations.