Nawlins was built where it is because it was a very convenient place for a seaport. The Mississippi, however, continued to carry dirt and eventually the mouth was so clogged up that the Corps of Engineers had to cut a channel to keep NO a seaport.

About the same time, bogs were drained to reduce the problem of Yellow Fever (and other diseases). As the bogs dried, the land subsided.

But it's the river that causes the most problems. You see, the Mississippi doesn't want to flow by the Big Easy any more -- it wants to roil down the Atchafalia (and I probably spelled that wrong). So the NO city father, congressfolk, and people in general pressured the Corps of Engineers into finding some way to keep the city a seaport and the Mighty Miss flowing by the Cafe du Monde.

The Corps did. They built the Atchafalaya (I checked the spelling) Dam and the Old River Structure. This forces the river to continue down its original channel, right by the French Quarter, instead of flowing down the middle of Louisiana like it wants to.

I suggest you check out the Wikipedia articles.