water Article Decadal-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 Department of Geography, University of Florida, Gainesville, FL 32611, USA; mossa@ufl.edu * Correspondence: wuchiayu@ufl.edu Received: 10 May 2019; Accepted: 3 June 2019; Published: 5 June 2019 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. Keywords: thalweg; riffle–pool sequence; lowermost Mississippi River 1. Introduction 1.1. Study Aims 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]. Fluvial geomorphological responses to human impacts are highly dynamic and complex, and the lowermost Mississippi River (LMR) is a notable example (Figure1). Due to preventing the main channel of Mississippi River from suddenly changing its course (avulsion) into the Atchafalaya River [3], the first phase of the Old River Control Structure (ORCS) was completed in 1963 by the United States Army Corps of Engineers (USACE). 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 [4–16]. In a recent modeling study, Wang and Xu [17] found that, proportionally, more riverbed materials were carried downstream in the Mississippi mainstem Water 2019, 11, 1175; doi:10.3390/w11061175 www.mdpi.com/journal/water Water 2019, 11, 1175 2 of 25 Water 2019, 11, x FOR PEER REVIEW 2 of 26 thanMississippi to the outflow mainstem channel. than However,to the outflow they also channel. cautioned However, that a solid they conclusion also cautioned could notthat be a drawn solid becauseconclusion the comparisoncould not be of drawn the modeling because results the comparis is based onon aof very the limited modeling number results of fieldis based measurements. on a very Therefore,limited number given theof field complex measurements. morphodynamic Therefore, behavior given in the the complex LMR region, morphodynamic understanding behavior long-term in (decadalthe LMR scale) region, variations understanding regarding long-term to river thalweg (decadal morphology scale) variations due to anthropogenicregarding to disturbanceriver thalweg is anmorphology important task.due to With anthropogenic hydrographic disturbance data exceeding is an a century important in timespan task. With recorded hydrographic by the USACE data (fromexceeding the 1870s a century to 2013), in timespan the LMR recorded is an ideal by the site US toACE study (from geomorphological the 1870s to 2013 response), the LMR over is decadalan ideal scales.site to study In addition, geomorphological the riffle–pool response undulation over decad on thalwegal scales. profiles In addition, is also the likely riffle–pool to be indicative undulation of longer-termon thalweg morphologicalprofiles is also adjustment likely to be and indicative enables an of analysis longer-term of downstream morphological trends adjustment in the riverbed and formenables resistance an analysis [18– 20of ].downstream trends in the riverbed form resistance [18–20]. Figure 1. The geographical map of the lowermost Mississippi River (LMR) with different locations. Figure 1. The geographical map of the lowermost Mississippi River (LMR) with different locations. Artificial levees along the channel (red lines) are also marked in this map. Our study reach (river Artificial levees along the channel (red lines) are also marked in this map. Our study reach (river kilometer (RK) 490 to RK 0) is located from downstream of Tarbert Landing (TBL, RK 493) to the Head kilometer (RK) 490 to RK 0) is located from downstream of Tarbert Landing (TBL, RK 493) to the Head of Passes (HOP, RK 0). Some important structures are also labeled on the map, such as Old River of Passes (HOP, RK 0). Some important structures are also labeled on the map, such as Old River Control Structure (ORCS, RK 500), Morganza Spillway (MS, near RK 451), and Bonnet Carré spillway Control Structure (ORCS, RK 500), Morganza Spillway (MS, near RK 451), and Bonnet Carré spillway (BCS, near RK 205), as well as the two cities of Baton Rouge (RK 370) and New Orleans (RK 164). (BCS, near RK 205), as well as the two cities of Baton Rouge (RK 370) and New Orleans (RK 164). The The yellow square indicates the location for Figure4B. yellow square indicates the location for Figure 4B. Alluvial river channels commonly exhibit a dynamic, three-dimensional variation in the planform, cross-sectional,Alluvial river and longitudinalchannels commo aspectsnly [ 21exhibit,22]. At a thedynamic, reach scale, three-dimensional the channel thalweg variation is modified in the intoplanform, a series cross-sectional, of bathymetric and lowslongitudinal and highs. aspects These [21,22]. vertical At the undulationsreach scale, the in channel bed elevation thalweg are is generallymodified into referred a series to as of ribathymetricffle–pool sequences. lows and Rihighffless. These represent vertical the topographicallyundulations in bed shallow elevation section are ofgenerally an undulating referred channel to as riffle–pool bed, while sequences. pools are Riffles the topographically represent the topographically deep areas of the shallow channel section bed (Figureof an undulating2A). The ri fflchannele–pool sequencesbed, while are pools commonly are the placed topographically in both straight deep and areas meandering of the channel rivers withbed coarse-grained(Figure 2A). The channels riffle–pool of <2% sequences slope [21 are,23 –commonly25]. The morphology placed in both of riffl straighte–pool sequencesand meandering is considered rivers aswith a fundamental coarse-grained process channels of channel of <2% adjustments slope [21,23–25]. in both The vertical morphology and planform of riffle–pool dimensions sequences [26–30 is]. Furthermore,considered as ri ffla e–poolfundamental morphology process is alsoof channel important adjustments in terms of in mesoscale both vertical habitats, and as planform different physicaldimensions conditions [26–30]. wouldFurthermore, determine riffle–pool different morphology habitat type is also [31 ,32important]. Human in terms activities, of mesoscale such as habitats, as different physical conditions would determine different habitat type [31,32]. Human activities, such as channelization or dredging, could modify the riffle–pool forms [33]. Therefore, to Water 2019, 11, 1175 3 of 25 Water 2019, 11, x FOR PEER REVIEW 3 of 26 channelizationexamine the variation or dredging, of the could riffle–pool modify morphology the riffle–pool after forms massive [33]. human Therefore, alterations to examine is another the variation major ofgoal the for riffl thise–pool study. morphology after massive human alterations is another major goal for this study. In sum, this study aims to examine the variations of thalweg profiles, profiles, as well as the ririffle–poolffle–pool sequences, fromfrom pre-constructionpre-construction of of the the ORCS ORCS (pre-1963) (pre-1963) to to recent recent time time (2010s) (2010s) in thein the LMR LMR by usingby using the hydrographicalthe hydrographical datasets. datasets. This paperThis paper focuses focuses on the on lowermost the lowermost reach of reach the LMR of the (Figure LMR1), (Figure extending 1), fromextending the ORCS from juncturethe ORCS to thejuncture Head to of the Passes Head (HOP), of Passes which (HOP), extends which from extends river kilometer from river (RK) kilometer 490 to 0. In(RK) general, 490 to this 0. In study general, serves this as study an important serves as extension an important/comparison extension/comparison to both studies of to Joshi both and studies Jun [34 of], asJoshi well and