As the Mississippi River flows south, its natural inclination is to take the shortest possible route to the Gulf of Mexico. Given its close proximately wth the Atchafalaya River and possible short cut through Old River, a change in the distribution of flow was changing with the rivers. If left unchecked, eventually the Mississippi River would change course, overtaking the Atchafalaya River route through Morgan City. To stop this action, the Corps constructed the Old River Control Structures to maintain the Mississippi-Atchafalya rivers distribution at 70-30. Following the flood of 1973, the Old River Auxillary Control Structure was added for additional strength in maintaining this ratio.