Of course that is exactly what the two modern floodways do today. They were built right after WWII and the first time they were opened was in 1973. I believe one goes into Lake Ponchetrain and the other to the Atchafalaya. I remember crossing the Atchafalaya flood basin at Easter of 1973 and it was completely flooded. The Mississippi in New Orleans was at least eight or ten feet above the street level at Jackson Square - lapping at the top of the levee there. There was some seepage on the west bank and "mattresses" were place at those spots.

The flooding from Katrina was from the Lake side levees not holding. They were not built as "sea wall" type construction and couldn't withstand the wave pounding from the north. We had the same problem with Lake Livingston Dam in Rita and had to lower the lake level to repair the dam. Hopefully the Corps of Engineers has learned their lesson and reinforced those levees to sea wall strength.

I also believe the Red River can be diverted into the Atchafalaya - actually it looks like they are connected.