The upper part of Turnbull's Bend filled in and separated from the Mississippi River while the lower channel grew into Old River, the point where the Red, Old, and Atchafalaya Rivers meet. Long ago the Red River began flowing toward a large meander of the Mississippi later called Turnbull's Bend, which intercepted the Red River and turned it into a tributary, and the Atchafalaya River also eventually connected with the meander as a distributary. In the mid-1800s, a cut through the narrow neck of Turnbull's Bend made navigation on the Mississippi more efficient, and as the upper channel gradually silted in, the lower channel became an important connection between the three rivers. After extensive logjams in the Red and Atchafalaya rivers were removed in the 1840s, the Mississippi began sending increasing amounts of water through Old River to the Atchafalaya, a shorter route to the Gulf of Mexico, showing signs that it might eventually change course. By the early 1950s, it was clear that without further human intervention the Mississippi would eventually shift to the Atchafalaya, so multiple structures and floodways were built over several decades to prevent that change and address flooding. The Old River Control Complex includes, from north to south, the Sidney A. Murray Jr. Hydroelectric Station completed in 1990, the Overbank Structure and Low Sill Structure completed in 1963, the Auxiliary Control Structure completed in 1986, and the now closed Lower Old River channel. The Old River Lock, the southernmost part of the complex, lies 11 miles downstream from the main control structures in a canal near the old channel where the Mississippi and Atchafalaya rivers once connected; built in 1963 for navigation, it provides stable passage for barges and small vessels between the Mississippi and the Red and Atchafalaya rivers. The lock is 75 feet wide, 1,185 feet long, and can handle vessel draft depths of 11 feet, while a dam north of the lock across Old River prevents the Mississippi River from changing course. The lock is one of four within the Atchafalaya Basin, along with those at Bayou Sorrel, Bayou Boeuf, and Berwick, and combined cargo through the four locks reached 45 million metric tons per year in 2001. Besides flood control, the complex supplies fresh water to the Atchafalaya Basin, America's largest river swamp, supporting extensive plant and animal life and providing a critical corridor linking the Tensas and Atchafalaya subpopulations of the Louisiana black bear, though sediment from upstream farmland and changes in water flow caused by floodway construction have stressed swamp life and required continued human efforts to maintain remaining habitats.
