The blue lines represent the drainage canals in New Orleans; notice they exist only close to Lake Pontchartrain.
The Mississippi River deposits more silt closer to its banks during the spring floods. For that reason, the land is higher closer to the river. As one moves away from the river's edge, the land sinks to about 5 to 10 feet below sea level, maybe higher in some places. Because the river deposits more silt closer to its banks, the riverbed itself has been built up so much that the river's surface is actually higher than the surrounding land. Only the natural levee along its banks is higher than the river's surface, and the river is naturally prone to overflow this levee and inundate the low land around it. Because of this, the U.S. Army Corps of Engineers has extended the height of this natural levee in an attempt to keep the river within its natural course.All of the high land along the river's bank had long since been claimed and developed, but more was needed. In order to reclaim the swamp, a series of canals and pumping stations were built. The pumping stations were able to drain hundreds of square miles of swampland, pumping the excess water into Lake Pontchartrain to the north of the city and into the swamps and bayous to the south, thus leaving behind a dry, yet still spongy prairie upon which the city could sprawl. As can be seen in the photo-map above, the system of canals drains the land farthest away from the River. These are the lowest areas within the city, lying below sea level and naturally covered by shallow marshes and swamps. No canals are needed to drain the land close to the river's banks since the river has built up this land to an elevation a few feet or more above sea level. As water must flow down, the canals drain into the Lake and bayous surrounding the city instead of into the River itself, as one might expect.
The few [only 2 or three seen here] canals that *do* empty into the River are connected to it in order to allow barge traffic to pass through them.