Spillways are used to control the flow of water downstream. Spillways do not relieve pressure from upstream areas, but rather divert the flow of water to take pressure off of the downstream systems.
The Morganza Floodway diverts the excess floodwater from the Mississippi into the Atchafalaya Basin. The control structure and Floodway is designed to pass 600,000 cfs of excess water to the Gulf, which takes pressure away from mainline levees downstream from the Mississippi. Operation of the Floodway occurs when the flow of the Red River Landing is predicted to reach above 1.5 million cfs.
The Bonnet Carre Spillway was designed to divert floodwaters away from the New Orleans metropolitan area as well as other downstream communities. The floodwaters are redirected to flow into Lake Pontchartrain to relieve stress on downstream levees.