Bayou Saint John was originally a distributary channel of the Mississippi River: it carried water across the cypress swamps to Lake Pontchartrain. Because it offered a straightforward route from the lake to the river, the stream played an essential role in New Orleans’ founding in the early 18th century. At the turn of the 19th century the stream’s course was extended by the construction of the Carondelet Canal and its turning basin, but those structures were filled in the early 20th century because they had fallen out of use. Bayou Saint John is still connected to Lake Pontchartrain, but because of subsidence, it no longer carries water out of New Orleans: it lies higher than the land on either side of it.