To protect against this occurrence, the US Army Corps of Engineers installed a relief valve in the river system. A low spot was built in the levee at Birds Point, which is just downriver from Cairo. In the event of a large flood, the waters would overtop this levee, causing it to fail. When it fails, up to one-fifth of the river flow would be diverted through the New Madrid Floodway. A 1,500 foot wide gap in the levee system just north of New Madrid would allow the water in the floodway to rejoin the Mississippi River. The result is that a large fraction of Mississippi and Ohio River flood water would bypass the narrow section of the river, likely sparing much of the adjacent farming regions from flooding.
There turns out to be two glitches in this plan. First, when the floodway was first used, the levee at Birds Point refused to fail, so the US Army was forced to blow the levee with explosives. Second, the outlet near New Madrid allows water from smaller floods to flow backwards into the floodway, needlessly flooding the farmland in the floodway area.
The US Army Corps of Engineers has a plan to fix these issues. First, the levee at Birds Point would be converted into a fuse plug. That is, pipes would be installed in the levee to allow explosives to be pumped in and detonated when needed. This allows the US Army Corps of Engineers to pick the time when the floodway is opened rather than leaving it to chance. Second, the gap in the levees near New Madrid would be closed off. This would prevent the backflow floods. It would also prevent the water from exiting the floodway naturally. The plan is to install two large pumping stations to pump out the floodway.
This plan is not without its critics. They argue that nature should be allowed to do its job without human intervention. They also see the backflow flooding from the outlet gap as beneficial to the wetlands rather than the annual destruction of large areas of farmland.
Update—The Birds Point–New Madrid Floodway was opened for a second time in its history on May 2, 2011, during a record high flood on the Mississippi River. Heavy snows and unusually heavy spring rains caused both the Mississippi and Ohio Rivers to flood, and that water arrived in the Cairo area about the same time. Cairo was already evacuated, but the levee system around Cairo was in danger of failing. The US Army Corps of Engineers blew the upper end of the floodway on May 2. The water levels at Cairo dropped by 6 inches in the first hour that the floodway was open, saving the city. The lower end of the floodway was blown in two places on May 3 and May 5. The second blast was delayed when the Corps ran short on explosives.
The 2011 operation of the floodway was very controversial. In fact, the state of Missouri and local residents went to court to try to stop the opening of the floodway. As it stands, a large area of prime farmland was flooded to save Cairo, and that land might remain flooded for a month or more as the water slowly recedes. The Corps counters that this was a record flood for the Mississippi River, and action had to be taken.
