"Cities rise and fall depending on what made them go in the first place,"
Well, New Orleans still has the Mighty Mississippi - so it will rise again.
But maybe not forever. Maybe just one big flood will take it away. Check out this. Here's an excerpt:
In each decade since about 1860, the Atchafalaya River had drawn off more water from the Mississippi than it had in the decade before. By the late nineteen-forties, when Rabalais was in his teens, the volume approached one-third. As the Atchafalaya widened and deepened, eroding headward, offering the Mississippi an increasingly attractive alternative, it was preparing for nothing less than an absolute capture: before long, it would take all of the Mississippi, and itself become the master stream. Rabalais said, “They used to teach us in high school that one day there was going to be structures up here to control the flow of that water, but I never dreamed I was going to be on one. Somebody way back yonder—which is dead and gone now—visualized it. We had some pretty sharp teachers.”
The Mississippi River, with its sand and silt, has created most of Louisiana, and it could not have done so by remaining in one channel. If it had, southern Louisiana would be a long narrow peninsula reaching into the Gulf of Mexico. Southern Louisiana exists in its present form because the Mississippi River has jumped here and there within an arc about two hundred miles wide, like a pianist playing with one hand—frequently and radically changing course, surging over the left or the right bank to go off in utterly new directions. Always it is the river’s purpose to get to the Gulf by the shortest and steepest gradient. As the mouth advances southward and the river lengthens, the gradient declines, the current slows, and sediment builds up the bed. Eventually, it builds up so much that the river spills to one side. Major shifts of that nature have tended to occur roughly once a millennium. The Mississippi’s main channel of three thousand years ago is now the quiet water of Bayou Teche, which mimics the shape of the Mississippi. Along Bayou Teche, on the high ground of ancient natural levees, are Jeanerette, Breaux Bridge, Broussard, Olivier—arcuate strings of Cajun towns. Eight hundred years before the birth of Christ, the channel was captured from the east. It shifted abruptly and flowed in that direction for about a thousand years. In the second century a.d., it was captured again, and taken south, by the now unprepossessing Bayou Lafourche, which, by the year 1000, was losing its hegemony to the river’s present course, through the region that would be known as Plaquemines. By the nineteen-fifties, the Mississippi River had advanced so far past New Orleans and out into the Gulf that it was about to shift again, and its offspring Atchafalaya was ready to receive it. By the route of the Atchafalaya, the distance across the delta plain was a hundred and forty-five miles—well under half the length of the route of the master stream."