It was done to prevent flooding in New Orleans and to protect the levees, said Jim Addison, New Orleans District Army Corps of Engineers spokesman. The spillway diverts about 14 percent of the river into Lake Pontchartrain at a rate of 240,000 cubic feet a second. Lake Pontchartrain flows into Lake Borgne, which empties into the Gulf of Mexico. Through this path, the fresh waters of the flooding Mississippi River are diluting the brackish waters of the Mississippi and Louisiana Sound. Given proper conditions and enough time, part of the Gulf of Mexico from the mouth of the Mississippi River eastward will become freshwater.
Oysters can survive in freshwater for about a week before they start dying.
While this would create a short-term disaster, the diluted saltwater should mean good news later. With the spillway closed, freshwater cannot flow into the marshes east of the Mississippi River as it did before levees were built in the 1930s. Instead, brackish areas where oysters and young fish grow become too salty. The added freshwater should restore the natural balance. The spillway has been opened just eight times in its 66 years.