Complex sentence: The Old River Control Structure, a dam which regulates the outflow from the Mississippi into the Atchafalaya, was completed in 1963 to prevent the Atchafalaya from capturing the main flow of the Mississippi and stranding the ports of Baton Rouge and New Orleans.
Split into simpler sentences: The Old River Control Structure, a dam which regulates the outflow from the Mississippi into the Atchafalaya, was completed in 1963. It is intended to prevent the Atchafalaya from capturing the main flow of the Mississippi and stranding the ports of Baton Rouge and New Orleans.