One such relief valve is the Morganza Spillway And Floodway, located just north of the city of Morganza, Louisiana. In this area, a swamp exists between the Mississippi River and the Atchafalaya River. The US Army Corps Of Engineers cut a 4 mile wide channel between the two rivers, largely using this swamp. At the Mississippi River end of the channel, a large earthen dam was constructed. A 3,900 foot long concrete control structure was installed in the middle of the dam.
In the event of a monumental flood on the Mississippi River, the control structure of the Morganza Spillway could be opened, sending a significant fraction of the Mississippi River flow through the Morganza Floodway and into the Atchafalaya River, eventually draining into the Gulf Of Mexico. The dam and control structure are needed to prevent the Mississippi River from permanently changing course after such a flood, something that very nearly happened just upriver in 1973, and previously south of Baton Rouge in the mid-1800's.
The Morganza Floodway has never been used as planned to protect New Orleans. A flood that large has not yet happened since the project was finished in 1954. The structure was opened once, however, in 1973. While that was during a major flood, the reason for opening the structure was to take the pressure off of the Old River Control Structure just upstream. The main dam in the Old River Control project was crumbling, and if it failed completely, the Mississippi River would have changed course right then and there. The Morganza Floodway drained off enough water and the Old River Control Structure held, saving the day.
While the US Army Corps Of Engineers has invested billions of tax dollars into holding the Mississippi River in its current channel, some river pundits say it is just a matter of time before the river wins this battle.
Update—The US Army Corps of Engineers opened the Morganza Spillway on May 14, 2011, during the record flooding that spring. This is the first time that the floodway has been used strictly due to flooding. Water released through the Morganza Spillway caused water levels to rise by as much as 15 feet in the Atchafalaya Basin, flooding out entire communities. This was a choice that the Corps had to make. If the floodway had not been opened, levees along the Mississippi River could have been overtopped, possibly leading to devastating flooding in Baton Rouge or New Orleans. The floodway was eventually opened to 21% of its capacity. Gates began to be closed starting on May 25, 2011, but it was expected to take as long as 90 days for the water levels to return back to normal.