Atchafalaya Basin - Basin Geology and Flooding

Basin Geology and Flooding

Geologically, the Atchafalaya River has served periodically as the main channel of the Mississippi River through the process of delta switching, which has built the extensive delta plain of the river. The natural levees created by earlier main channels border and help define the Atchafalaya Basin, with the Atchafalaya River's natural levee running southward along the western edge of the basin. The central basin is further bordered by man-made levees designed to contain and funnel floodwaters released from the Mississippi at Morganza south toward Morgan City and eventually to the Gulf of Mexico.

Since the early 20th century, because of manmade alterations in the channel, the Mississippi has sought to change its main channel to the Atchafalaya River. By law, a regulated proportion of the water (30%) from the Mississippi is diverted into the Atchafalaya at the Old River Control Structure. In times of extreme flooding, the US Army Corps of Engineers may open the Morganza Spillway to relieve pressure on levees and control structures along the Mississippi. On May 13, 2011, in the face of a rising Mississippi River that threatened to flood New Orleans and other heavily populated parts of Louisiana, the USACE ordered the Morganza Spillway opened for the first time since 1973. This water floods the Atchafalaya Basin between the levees along the western and eastern limits of the Morganza and Atchafalaya basin floodways.