You might be thinking of the Bonne Carre Spillway.
That's it, thanks.
http://www.mvn.usace.army.mil/pao/bcarre/bcarre.htm
The Bonnet Carre' Spillway is a vital element of the comprehensive plan for flood control in the Lower Mississippi Valley. This multi-state plan, called the Mississippi River and Tributaries Project (MR&T), provides flood protection for the alluvial valley between Cape Girardeau, Missouri, and the mouth of the river.