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

Chia-Yu Wu,Joann Mossa

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.

Highlights

This paper focuses on the lowermost reach of the lowermost Mississippi River (LMR) (Figure 1), from the Old River Control Structure (ORCS)
Wang and Xu [14] utilized the hydrographic survey data from 1992–2013 and identified both degrading and aggrading sub-reaches; this study indicated that a Wang and Xu [14] utilized the hydrographic survey data from 1992–2013 and identified both degrading and aggrading sub-reaches; this study indicated that a vast amount of sediment was deposited in the LMR channel, especially between river kilometer (RK) 386 and 163
An examination of hydrographic surveys from the mid-1950s to 2013 provided insights into morphological adjustment of the thalweg profiles, as well as the pool–riffle sequences in the LMR

Summary

Introduction

Large alluvial rivers in the world usually have immense socio-economic importance. Different human activities, such as channelization, flood control, or dredging for navigation affected several large drainage systems such as the Nile, Mississippi, Indus, Yangtze, and Euphrates rivers [1,2]. After the completion of these structures, different studies discussed whether the ORCS blocked sediments and resulted in sediment deficiency, or stabilized the channel gradient and caused riverbed aggradation [4,5,6,7,8,9,10,11,12,13,14,15,16]. In a recent modeling study, Wang and Xu [17] found that, proportionally, more riverbed materials were carried downstream in the Mississippi mainstem

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