That’s not actually true, in the case of the Birds Point levee that was blown. If you look at the maps provided by the Corps of Engineers, an enormous area, much bigger than the New Madrid Floodway, including a big chunk of SEMO farmland, was at risk, if that floodway was not operated. Many other towns / parts of towns in Western KY and Southern IL were also at risk. The MEDIA, to make it a jucier story, turned it into a bogus “Cairo vs. the farmers” issue.
Now, that said, operating the floodway in MO is a pipsqueak (except for the actual levee blasts!), compared to opening up the Morganza. I am not sure how much farmland floods if the Morganza Floodway is not operated — probably quite a bit — but intentionally flooding an area with (I read somewhere) 25,000 people in it... Morgan City alone is 12,000 residents. Wow, what a decision to have to make...
Keep in mind though that again, this is not just a “Floodway area” vs. one city issue. Much more than New Orleans is at risk, and that risk includes the “Old River Control Structure” that presently prevents the Mississippi River from changing course, as others here have discussed.
Wikipedia has some fairly good info.:
http://en.wikipedia.org/wiki/Morganza_Floodway
http://en.wikipedia.org/wiki/Old_River_Control_Structure
(Note: Both the above links also have a link to an article on the current flood, which article contains a significant amount of erronious and contradictory information.)
THIS article says 2500 residents (not 25,000) and 18,000 acres of crops would be affected by opening up the Morganza.
http://www.nola.com/weather/index.ssf/2011/05/mississippi_river_floodway_ope.html
Dang news media — hard to get accurate info., as usual...