In my previous post, I stated that the blowing up of levees by USACE served to open an area for storage of flood waters. I was partly wrong. The area behind the levees is actually a floodway, designed to alleviate flood levels at high discharge levels in the Mississippi river. The floodway was implemented as a flood management measure after the 1927 Mississippi floods and has been used twice since: once in 1937 and now this time in 2011.
Floodways serve to create an extra flow path in rivers that are affected by backwater effects, i.e. rivers where a bottleneck hampers flow. This is similar to opening an extra lane when there is a traffic jam. This will increase flow past the bottleneck and upstream, water levels will drop / the traffic jam will disappear.