MOSSA and WUDecadal-Scale Variations of Thalweg Morphology and Riffle–Pool Sequences in Response to Flow Regulation in the Lowermost Mississippi River

Chia-Yu Wu and Joann Mossa

Article first published online: 5 JUN 2019 Water

DOI: 10.3390/w11061175

ABSTRACT: The lowermost Mississippi River (LMR) is one of the largest deltaic systems in North America and one of the heavily human-manipulated fluvial river systems. Historic hydrographic surveys from the mid-1900s to the early 2010s were used to document the thalweg morphology adjustments, as well as the riffle–pool sequences. Extensive aggradation was observed during 1950s to 1960s, as the Atchafalaya River was enlarging before the completion of the Old River Control Structure (ORCS). Following the completion of the ORCS, reductions in sediment input to the LMR resulted in net degradation of the thalweg profile patterns since the mid-1960s except for the 1992–2004 period. Different flood events that supplied sediment might be the cause of upstream aggradation from 1963–1975 and net aggradation along the entire reach from 1992–2004. Furthermore, the change pattern of thalweg profiles appear to be controlled by backwater effects, as well as the Bonnet Carré spillway opening. Results from riffle–pool sequences reveal that the averaging Ws ratios (length to channel width) are 6–7, similar to numerous previous studies. Temporal variations of the same riffles and pools reveal that aggradation and degradation might be heavily controlled by similar factors to the thalweg variations (i.e., sediment supply, backwater effects). In sum, this study examines decadal-scale geomorphic responses in a low-lying large river system subject to different human interventions, as well as natural flood events. Future management strategies of this and similar river systems should consider recent riverbed changes in dredging, sediment management, and river engineering.

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Mossa – The Changing Geomorphology of the Atchafalaya River, Louisiana:  A Historical Perspective

Joann Mossa

Article first published online: 18 Aug 2015 Geomorphology

DOI: 10.1016/j.geomorph.2015.08.018

ABSTRACT: 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.

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