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The Mississippi River Delta has been continuously losing land since the 1930s due to several factors, chief of which is the reduced sediment supply. A few recent studies have estimated individual components of short-term sediment transport, i.e., bedload and suspended load, separately for some locations along the Lowermost Mississippi River (LmMR, commonly considered as the last 500-km reach of the Mississippi River before entering the Gulf of Mexico). However, the combined effects of both components on the long-term sediment supply along the river reach are still unclear. One of the major obstacles here hindering our understanding is that it is difficult and impractical to accurately measure bedloads in large alluvial rivers, such as the Mississippi. In this study, we estimated bedloads of three medium grain sizes (D50 = 0.125, 0.25 and 0.5 mm) for three locations along the uppermost 140-km reach of the LmMR: Tarbert Landing (TBL) at river kilometer (rk) 493, St Francisville (St F) at rk 419, and Baton Rouge (BTR) at rk 367.5 during 2004-2015. We also estimated suspended loads at St F during 1978-2015 and at BTR during 2004-2015 in order to discern the interactive relationship between bedloads and suspended loads. We found gradually increasing bedloads for all the three medium grain sizes from TBL (83, 41.5 and 20.75 million tons (MT), respectively) to BTR (96, 48 and 24 MT). We also found that suspended loads at TBL (reported previously) were significantly higher than those at St F and BTR during corresponding overlapping periods. Bedloads increased almost linearly with suspended loads, river discharge and river stage at the upstream locations (TBL and St F); however, such a relationship was not evident downstream at BTR. The total sediment load (bedload + suspended sediment load) was substantially higher at TBL (931 MT), while lower and nearly equal at other two downstream locations (550 MT at St F and 544 MT at BTR) during 2004-2010 (the matching period of availability of both loads). These findings indicate that the uppermost 20-25 km LmMR reach (covering TBL) has potentially entrapped substantial suspended load over the last three to four decades, while bedload transport prevails in the lower reach (covering St F and BTR).We suggest that future sediment management in the river should seek engineering solutions for moving trapped coarse sediments downstream towards the coast for the Mississippi River Delta restoration. [ABSTRACT FROM AUTHOR]

The Mississippi River is the ultimate single-thread meandering river. Five hundred kilometers upstream from its mouth, about 25% of the river's discharge and sediment load is diverted into the Atchafalaya River. This diversion is controlled by the Old River Control Structure (ORCS), built by the U.S. Army Corps of Engineers (USACE) in stages since 1961, to stop the avulsion of the Mississippi River into the Atchafalaya. The effects of ORCS on sediment and water discharge and geomorphic change to the Lower Mississippi River (LMR) channel are not yet completely understood and require placing the river into a geomorphic context, first classifying the channel into similar categories before evaluating change. The objectives of this study are to estimate the LMR bedload, develop and apply a geomorphic classification of the LMR near the ORCS, and explore geomorphic change within the classified areas. We studied a 115-km-long stretch between ORCS and Baton Rouge that is highly impacted by engineering. We used six sets of bathymetric multibeam echosounder surveys conducted by the USACE, multitemporal cartographies, and a field survey supported by multibeam echosounder bathymetry, acoustic Doppler current profiler (ADCP) measurements, sediment samples collection, and geomorphic observations. A three-dimensional method for estimating bedload from time-elapsed bathymetric surveys was developed and applied on seven sets of time-elapsed surveys downstream from ORCS from 2010, 2011, and 2012. We estimate that the fraction of bedload as a percentage of total sand load between 2003 and 2011 was 13.2%. A bedform classification scheme, based on bedform height, was developed. The bed was almost completely mantled by sandy bedforms above the − 24-m isoline. The studied reach was divided into ten zones according to four geomorphic types based on channel planform, geologic controls, presence of islands, and other morphometric parameters. These zones were shown to be physically distinct in terms of bedform location, depth to bedform size, channel sinuosity, channel width-to-depth ratios, and adjustments in the thalweg and channel width from 1948 to 2012. The 1948 to 1975 period displayed a relatively large amount of thalweg aggradation, with a spatial average of 1.8 m and an average channel narrowing of 80 m. The 1975 to 2012 period had much lower spatial aggradation of 0.5 m and a lower average channel narrowing of 35 m. Thalweg sinuosity did not adjust very much, with an average reduction of 0.02 from 1948 to 2012 and an average of zero adjustment between 1975 and 2012. The shape and spatial extent of a very large, anomalous bar near ORCS varied at seasonal and decadal timescales and may be influenced by the ORCS. Bed morphology was specific to geomorphic zones, which were shown to be physically significant, and should be employed in future studies of large rivers. [ABSTRACT FROM AUTHOR]

A series of community-led, large-scale laboratory experiments, termed 'StreamLab', were performed by the National Center for Earth-surface Dynamics (NCED) with the purpose of advancing multidisciplinary research, education, and knowledge transfer at the interface of physical/chemical/biological processes in streams, science-based stream restoration practice, and environmental sensing technologies. Two series of experiments, StreamLab06 and StreamLab08, were conducted in the Main Channel of the St. Anthony Falls Laboratory at the University of Minnesota, a flume 84 m long and 2.75 m wide with water fed by the Mississippi River at a rate of up to 8.5 m3/s. The purpose of this paper is to share with the broader community the data collected with the hope of stimulating further analysis and future experimental campaigns toward advancing our predictive understanding of the physical, chemical, and biological processes in streams. Toward this end, a brief summary of the results to date is included and some ideas for further research are provided. [ABSTRACT FROM AUTHOR]