Decadal‐Scale Riverbed Deformation and Sand Budget of the Last 500 km of the Mississippi River: Insights Into Natural and River Engineering Effects on a Large Alluvial River

Abstract

Alluvial rivers are shaped by interactions of flow and sediment transport. Their lower reaches to the world's oceans are highly dynamic, often presenting engineering and management challenges. Here we analyzed over 6,000 single-beam cross-sectional measurements surveyed in 1992, 2004, and 2013 in the last 500-km reach of the highly engineered Mississippi River, also known as the lowermost Mississippi River or LmMR, starting from the river's Gulf outlet to its avulsion into the Atchafalaya River. We applied inverse distance weighted interpolation to downscale the survey records into 10 × 10 m digital elevation models. We assessed riverbed deformation from bank to bank and quantified changes in riverbed sediment volume. The goal of our study is to test the hypothesis that the lower reach of a large alluvial river can function as a conduit for sediment transport under the current engineering focus of navigation safety and flood control. Our analysis shows that in the past two decades, nearly 70% of the riverine sand was trapped within the LmMR, and continuous riverbed aggradation occurred below the Mississippi-Atchafalaya diversion, presenting favorable backwater conditions for avulsion. Backwater effects have mainly controlled riverbed deformation in the LmMR, while flow reduction may have also contributed to the channel aggradation in the uppermost and lowermost reaches. The study reveals the considerable complexity of geomorphic responses of a large alluvial river to human interventions, strongly suggesting that future river engineering and management of the world's alluvial rivers should focus on strategies and solutions that will improve sediment transport.Over the past century, the natural fluvial processes of many alluvial rivers have been altered by human interventions, including levee construction and other engineering practices, that is, channel cutoffs, dredging,

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

“…They concluded that, since the LMR generally received less flow than it did beforehand due to ORCS regulation, there were lower water velocities, which could further benefit in sediment deposition along the reach. Similar observations with sediment deposition in the LMR were also observed by Biedenharn et al [50], as well as Wang and Xu [14]. Biedenharn et al [50] analyzed the stage and discharge data from gauging stations from 1880-1992 and summarized that the river in the post-cutoff period (1943-1992) had a much greater stream power than in the pre-cutoff (1880s-1930s).…”

“…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][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 than to the outflow channel.…”

“…The average values of maximum and minimum discharge in Tarbert Landing (TBL) at RK 493 vary from 3143 to 45,845 m 3 /s, respectively [14,15]. The drainage area above TBL is estimated to be 2,913,478 km 2 .…”

“…The ORCS allows 30% of the combined discharge to pass into the Atchafalaya River, while the other 70% of the total discharge is carried by the LMR [4,48,49]. Despite several other engineering activities after the completion of ORCS [14], it is widely recognized that the LMR is a completely engineered river after the completion of the ORCS in 1963 [49]. However, several studies [4][5][6][7][8][9][10][11] found channel degradation and sediment deficits in the Mississippi deltaic area resulting from dams and other structures (especially the ORCS) trapping vast amounts of sediments upstream within its upper stream.…”

Estimating bed material fluxes upstream and downstream of a controlled large bifurcation ‐ the Mississippi‐Atchafalaya River diversion

“…Although Wang and Xu pointed out that the diverted sediment may be lower than the transport capacity of the diversion Outflow Channel, it is largely unknown how much sediment, especially the bed material load (i.e., sediment comprising the bed that travels either as bedload or suspended load), is actually diverted via the Outflow Channel to the Atchafalaya River. The information is critical because studies have found considerable channel aggradation immediately downstream of the ORCS on the Mississippi River mainstem (Mossa, 2013; Wang & Xu, 2018a), which could cause backwater during floods threatening the diversion structure.…”