The changing geomorphology of the Atchafalaya River, Louisiana: A historical perspective

• Channel geometry of the Atchafalaya River changed before and after placement of the Old River Control Project in 1963.
• Thalweg elevations at an upstream and downstream location have risen since 1880, although other locations may differ.
• Between 1963 and 2004, thalweg elevation differences show net degradation upstream and marked aggradation downstream.
• As discharge increased, widening has been the preferred mode of adjustment owing to the bed being below base level.
• Wax Lake Outlet, designed to divert 20% of the flow, now carries similar flow and sediment as the lower Atchafalaya.

The Atchafalaya River in south-central Louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower Mississippi River, notwithstanding its potential to become an avulsion pathway at Old River. Since its origin in the sixteenth century, it has undergone many transformations and currently receives flow from the Red River and partial flow of the Mississippi River through multiple pathways. Flow increased progressively beginning in the mid-1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the Mississippi and Atchafalaya rivers in 1963. Prior studies, that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal, width, and depth increases in the uppermost 80 km (50 mi) of the Atchafalaya River from 1880 to 1974.This study assesses how channel geometry of the Atchafalaya River has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works. While structures have regulated and limited flow into the Atchafalaya since 1963, the highest values of discharge, cross-sectional area, and width at Simmesport (upstream) follow river regulation. Associated geometry data at Simmesport and Morgan City (downstream) document that mean bed and thalweg elevations have risen. Comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggradation occurs in the lower Atchafalaya, a zone where along-channel lakes are filling, flow and sediment quantities are changing in part caused by the constructed Wax Lake Outlet, and deltas have been growing in Atchafalaya Bay. Findings are relevant to river engineering and flood management, especially navigation, and help in better understanding the evolution of large distributaries and the role of human impacts.