Here's how a levee failure is expressed in terms river stage. The first levee fails just before midnight on the 8th and the river stage drops approximately 2 feet over the next several hours as the flood waters go out onto the floodplain where they are ''stored'' to be released slowly into the underlying aquifer and eventually back into the river. This storage can be a significant amount of water. There is a second, smaller failure about 1800 hours on the 9th. What appears to be failures on the 11th and 13th have not been confirmed from ground observations, there may be other explanations for the declines, it's a little hard to say just from the graph. The net effect of these failures are lower downstream river levels, i.e. at Jefferson City and St. Louis, which is one reason why the flood crest is going to be less than originally predicted and why this flood isn't going to be as bad as feared.
The orginal predictions posted by the National Weather Service did not account for levee failures and rapidly changing conditions (more rain, levee breaks) indicates the need to be continously monitoring river levels during extreme events. You wouldn't really want to have to evacuate a large metropolitan area unless completely necessary, and if you're a Republican, then you might never see the need. I think the Republican motto is don't send in the National Guard until you see the white shirts on the rooftops.
