I thought they deliberately dynamited the levee south of New Orleans during Betsy to save Uptown. That's what I heard, at least. Was that just a rumor?
Yes, but like many falsehoods, loosely based on truth. In 1920 or thereabouts, when the Mississippi was at a seasonally VERY high level, they feared the levees would break and flood the city. The decision was made to dynamite the levee below New Orleans, thus flooding the area where St. Bernard and Plaquemines Parishes meet. This was done, and I believe it had the desired effect. Desired, at least, on the part of New Orleans residents, much less so to those living in the now-flooded area. I went to the St. Bernard Library years ago and looked up the actual microfilmed articles myself. Pretty interesting - they went way overboard, blowing up about 1/2 mile of levee and actually using up all available dynamite in the city!
To this day there is a long body of water known to locals as "the crevasse" which according to their stories is the spot where the first blast was made. The crevasse was supposedly carved out by those initial rushing waters, undermining a nearby railroad track and sending several boxcars to the bottom, where they still lie. (This could simply be local legend, but tracks indeed run right by the end of the water.)
See this map. "The crevasse" is the body of water just south of Saro Lane.
FYI, the Mississippi indeed gets seasonally very high at times. To alleviate stress off the levees when it does, the Army Corps of Engineers opens the Bonnet Carre Spillway, a long channel above New Orleans dammed by floodgates which diverts some of the flow directly into Lake pontchartrain.