Morganza Spillway The Morganza Spillway or Morganza Control Structure is a flood - control structure in the U . S . state of Louisiana along the western bank of the Mississippi River at river mile 280 , near Morganza in Pointe Coupee Parish . The spillway stands between the Mississippi and the Morganza Floodway , which leads to the Atchafalaya Basin and the Atchafalaya River in south - central Louisiana . Its purpose is to divert water from the Mississippi River during major flood events by flooding the Atchafalaya Basin , including the Atchafalaya River and the Atchafalaya Swamp . The spillway and adjacent levee s also help prevent the Mississippi from changing its present course through the major port cities of Baton Rouge and New Orleans to a new course down the Atchafalaya River to the Gulf of Mexico . The Morganza Spillway , operated by the U . S . Army Corps of Engineers , was opened during the 1973 and 2011 Mississippi River floods . History . The Great Mississippi Flood of 1927 was the most destructive river flood in the history of the United States , in large part due to the Mississippi River swelling to wide in spots . To prevent a repeat of the Great Flood of 1927 , and better control river flooding in general , Congress passed the Flood Control Act of 1928 to authorize the United States Army Corps of Engineers to build the Bonnet Carre Spillway ( located above New Orleans and completed in 1931 ) , the Birds Point floodway in Missouri , and the Morganza Floodway as part of the 1928 Mississippi River and Tributaries Project . Construction of the levees that make up part of the Morganza Spillway began in the late 1930s . The Morganza Control Structure portion of the project was completed in 1954 and subsequently became incorporated into the Mississippi River Commission ' s 1956 project design flood , which added the Old River Control Structure ( ORCS ) in 1963 to the protections used to prevent Mississippi River flood . The Flood Control Act of 1965 provided further regulation over the Morganza Spillway ' s role in Mississippi River flood prevention . A concrete pit called a stilling basin was added at the Morganza Spillway in 1977 " to provide erosion protection after the velocity of water pouring through the open bays during a 1973 flood caused severe scour ing of the land behind the bays . " Design and construction . The Morganza Spillway , a " controlled " spillway using a set of flood gates to control the volume of water entering the Morganza Floodway from the Mississippi River , consists of a concrete weir , two sluice gates , seventeen scour indicators , and 125 gated openings which can allow up to of water to be diverted from the Mississippi River into the Atchafalaya Basin during major floods . The project was completed in 1954 . A highway ( Louisiana Route 1 ) and the Kansas City Southern Railway cross the structure . The Corps of Engineers maintains the structures and , in times of flood , monitors their piers for scouring and stability . To lift and lower the flood gates , the Corps of Engineers maneuvers a movable crane along the spillway to the particular opening . Operation during floods . The land on both sides of the Morganza Control Structure is above normal river water levels , and usually dry . In order for water to reach the spillway , the Mississippi must first rise well above its flood stage , overtopping its banks . The Corps of Engineers considers opening the Morganza Spillway when the flow of the Mississippi at Red River Landing , Louisiana , is greater than and rising . Water from the Mississippi is normally diverted into the Atchafalaya Basin at only one place , the Old River Control Structure ( ORCS ) , in use since 1963 , where floodgates are routinely used to redirect the Mississippi ' s flow into the Atchafalaya River such that the volume of the two rivers is split 70 % / 30 % , respectively , as measured at the latitude of Red River Landing . During the 1973 Mississippi flood , the ORCS was being damaged due to high flow rates , leading to the opening of the Morganza Spillway to help relieve this pressure . Subsequently , the nearby Old River Control Auxiliary Structure ( ORCAS ) was constructed , adding additional floodgates for use during major floods . The Morganza Spillway , about downriver from ORCS and ORCAS , is designed for emergency use to divert additional water from the Mississippi River into the Morganza floodway , which merges downstream with the Atchafalaya floodway before entering the Gulf . Diversion of water from the Mississippi ' s main channel has the effect of lowering water level in the Mississippi downstream of the spillway , helping to relieve stress on levees and other flood control structures both upstream and down . Besides controlling flooding in a given event , the system is also designed to prevent the Mississippi River from permanently altering course down the Atchafalaya River , bypassing Baton Rouge and New Orleans . Water that passes the Morganza Spillway first enters the Morganza Floodway , which extends from the spillway at the Mississippi River south to the East Atchafalaya River levee . The floodway , long and wide , includes a stilling basin , an approach channel , an outlet channel , and two guide levee s . From there , diverted water enters the Atchafalaya River Basin Floodway near Krotz Springs , Louisiana , and continues to the Gulf of Mexico . In an extreme flood event , a major release of water from the Morganza Spillway into the Morganza Floodway and Atchafalaya Basin inundates not only the floodways themselves ( between their levees ) , but extensive additional areas of southern Louisiana throughout the Atchafalaya Basin . In such an event , the water level of the Mississippi , high enough to overtop the Spillway , would already be flooding some areas in the Basin downstream of the spillway , due to increased flow through the ORCS and the ORCAS , as well as possible overtopping of levees near the spillway . This flooding , plus any additional water from a Morganza Spillway release , together determine the total extent of flooding throughout the Atchafalaya Basin during a major Mississippi River flood . At risk in the Atchafalaya Basin are Morgan City ( population 13 , 500 ) , various smaller populated places , many farms , thousands of oil and gas wells , and considerable swampland . Inhabitants know that the region is a natural floodplain , and the Corps of Engineers issues written notices annually to all interests reminding them of the possibility that it might open the spillway and flood the area . Any decision to open the spillway must be carefully planned to give ample warning and protect life and property . Part of that planning process includes the Corps ' preparation of maps known as " inundation scenarios " so that interested parties can discuss how much water , if any , should be allowed through the spillway . During both moderate and severe floods of the Mississippi , the Bonnet Carré Spillway can also be opened to help protect New Orleans , many parts of which are below sea level . The Bonnet Carré Spillway , built after the Great Mississippi Flood of 1927 , is located well downstream of Morganza , where the Mississippi River approaches Lake Pontchartrain near New Orleans . The Corps uses Bonnet Carré to divert floodwaters into the lake in order to protect the levees near New Orleans . Integrity of the Morganza Spillway , the Old River Control Structure , and nearby levees is essential to prevent the Mississippi from diverting its main channel into the Atchafalaya Basin ; see " Risk of major course change in the Lower Mississippi River " for further discussion . Flood events and the Morganza Spillway . Since its completion 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 . First opening of gates ( 1973 ) . 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 stilling basin , and substantial flooding occurred in the Atchafalaya Basin . After the 1973 flood , the structure was restored to its original condition . Gates left closed in floods of 1983 , 1993 , 1997 , and 2008 . The issue of opening the gates to keep the Mississippi in its current place was next raised in May 1983 . Although the gates were not opened , Louisiana Highway 1 running across the spillway was closed , and the Corps of Engineers built a ring dike and put a blanket of earth across a section of the spillway to contain water pouring underneath it . During the Great Mississippi and Missouri Rivers Flood in 1993 , officials decided to leave the gates of the Morganza Floodway closed . In March 1997 , the river water flow at Baton Rouge reached ( ) per second . However , the flow rate never reached the threshold of ( ) per second for opening Morganza , and residents of Sicily Island , Louisiana , became angry that the spillway was not opened to prevent flooding in their homes . In March – April 2008 , officials considered but declined to open the spillway gates . While the spillway gates remained closed then , flooding still occurred in part of the Morganza floodway when the Mississippi River crested higher than levees near the spillway . Local residents and National Guard troops attempted to add height to the permanent levee by building a supplemental sandbag levee on top of it . However , additional rainfall raised the flood crest higher than even the supplemental sandbags , sending floodwaters over these levees into of cropland located within the floodway . Second opening of gates ( 2011 ) . The 2011 Mississippi River floods began to become serious in April . In response , the Corp first analyzed the flooding , and then opened the spillway in a controlled manner . The Corps studied four flooding scenarios , all of which assumed the Bonnet Carré Spillway 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 Governor of Louisiana , Bobby Jindal , had meanwhile sent a letter on May 4 , 2011 , to Robert Gates , the United States Secretary of Defense , requesting that the National Guard be deployed under Title 32 of the United States Code status to respond to record water levels in Louisiana , where there was " a significant probability that the U . S . Army Corps of Engineers [ would ] open the Morganza Spillway for the first time since 1973 . " The second opening of the Morganza Spillway began with the lifting of a single floodgate on May 14 , 2011 . Diversion of of water from the Mississippi River to the Atchafalaya Basin was planned during this event , with the structure operating at about 21 % of its capacity . This diversion was deemed necessary to protect levees and prevent major flooding in Baton Rouge and New Orleans , with the tradeoff of creating possibly severe flooding in the Atchafalaya Basin . By May 18 , 2011 , a total of 17 gates ( the largest number for the 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 still open . All bays were closed on July 7 , 2011 . The Corps had estimated that it would take opening one - fourth of the spillway ’ s 125 bays — or 31 bays — to control the flow of the river through Baton Rouge in response to a forecast crest of anticipated on Tuesday , May 17 , 2011 , which must remain below of water per second through Baton Rouge to ensure the integrity of the levee system . Since Morganza never operated above , the flooding in the Atchafalaya Basin was considerably lower than had been anticipated during the initial estimates of . By May 29 , the Corps had also opened 330 of the 350 bays of the Bonnet Carré Spillway located near New Orleans . For other images of the Morganza Control Structure in operation during the 2011 flood , see the U . S . Army Corps of Engineers Flickr page .