Floods in the United States: 2001–present floodgates of the Morganza Spillway were opened, marking the first use of the gates since the 1973 flood. 330 of the Bonnet Carré Spillway's gates were opened to save the levees protecting New Orleans. There was concern that if the Old River Control Structure, the Morganza Spillway, or the Bonnet Carré Spillway failed, the Mississippi River could change its course, flowing either into the Atchafalaya Basin or Lake Pontchartrain. In early September, the remnants of Tropical Storm Lee had stalled due to Hurricane Katia. As a result, the main cyclone stalled over the Midwest, and a cold front associated with
2011 Mississippi River floods to open more gates on the spillway, the Corps studied four flooding scenarios, all of which assumed the Bonnet Carré Spillway near New Orleans would be concurrently operating at full capacity (100%). Following this analysis, which showed that extensive flooding was expected in the Atchafalaya Basin regardless of the choice made regarding the Morganza Spillway, the Corps decided to start the 2011 diversion by opening the spillway a bit less than described in scenario 1a (21%, not 25%) The Corps of Engineers subsequently released a map showing the estimated times it would take the flood waters to reach the various