Morganza Spillway in 1954, the Morganza Spillway has been opened twice (in 1973 and 2011), and considered for opening during four other major floods of the Lower Mississippi River. On April 17, 1973, in order to lower the water level of the Mississippi River and relieve pressure on the Old River Control Structure, the Corps of Engineers opened 42 of the 125 steel gates of the Morganza Spillway for the first time since its completion, allowing about half of its maximum designed flowrate to pass from the Mississippi into the Atchafalaya Basin. The spillway received minor scouring and slight damage to the
Morganza Spillway 2011 event) had been opened by the Corps of Engineers. The Corps estimated the flow rate at . However, on May 25, new estimates from the U.S. Geologic Survey (USGS) described a much higher rate of , resulting in the closure of 3 bays by May 26, and additional closures by May 29, bringing the total to 11 bays with an estimated diversion rate of . The Corps continued to evaluate the flow and close additional bays as appropriate. By June 6, the number of open gates had been reduced to seven, and by June 8, only two gates were