The changing geomorphology of the Atchafalaya River, Louisiana: A historical perspective Joann Mossa Department of Geography, University of Florida, Gainesville, FL 32611, United States abstract article info Article history: Received 13 August 2014 Received in revised form 24 August 2015 Accepted 25 August 2015 Available online 1 September 2015 Keywords: Fluvial geomorphology River regulation Channel geometry Human impacts Atchafalaya River Mississippi River Red River Wax Lake Outlet Louisiana 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 resolu- tion 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 degra- dation (including the Red River and uppermost section of the Atchafalaya) near the Old River diversion. Aggra- dation 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The Atchafalaya River in south-central Louisiana (Fig. 1) is an impor- tant river for numerous reasons. Receiving all the flow and sediment from the Red River and ~30% of the Mississippi flow through Old River, it serves as a major floodway for the lower Mississippi River. The increasing flow through this system has threatened avulsion at Old River near the 31st North latitude (Fisk, 1952), produced the only notable area of accretion on a rapidly eroding coast (Britsch and Kemp, 1990; Barras et al., 2003; Couvillion et al., 2011), and resulted in geomorphic adjustments that are not yet well documented. It is also the largest contiguously forested bottomland in North America (Hupp et al., 2008), has been labelled peculiar because it is largest at its source and deepest in places of excessive width (Ockerson, 1907), and influences adjoining coastal and marine areas including the chenier plain (Draut et al., 2005) and inner continental shelf (Allison et al., 2000). Despite its importance and having a story of popular interest (McPhee, 1987), and recent works overviewing the history (Reuss, 1998) and ecology (Piazza, 2014), prior geomorphic studies and synthesis of the varied data sources that collectively could give insights regarding channel change in the nation's largest distributary have been limited. 2. Background and regional setting As the Mississippi River migrated laterally near the 31st North paral- lel in the sixteenth century, it intersected the Red River (Fisk, 1952) (Fig. 2, Table 1). The Atchafalaya River begins at this intersection, essen- tially the continuation of the Red River at the juncture where input from the Mississippi began historically. In 1831, Shreve's cutoff at the mean- der bend where these rivers were connected reduced the length of the bendway on the Mississippi to improve navigation; this created Upper Geomorphology 252 (2016) 112–127 E-mail address: mossa@ufl.edu. http://dx.doi.org/10.1016/j.geomorph.2015.08.018 0169-555X/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph