The trends of degradation and aggradation are measured in this study for the Lower Mississippi River. Historical riverbed elevation and stage data from the past hundred years were used from six gages in order to measure changes in riverbed gradation. It was found that using stage data to measure gradation changes is a superior method to using riverbed elevations, due to stage data’s reliability, length of record and daily measurements. Degradation in the Lower Mississippi River was seen during the

The habitat requirements of shovelnose sturgeon populations are unknown for the lower Mississippi River. Active acoustic telemetry was used to measure temporal variation in habitat use of shovelnose sturgeon and preference for depth, surface current velocity, and riverbed rugosity and slope by the population and within sandbar microhabitats. Shovelnose sturgeon occupied habitats differently throughout the year; and, within habitats, areas of moderate depth and surface current velocity and smooth riverbed were preferred. Within sandbars, environmental conditions did not differ from the surrounding environment, yet frequent aggregations of individuals in the lower portion of sandbars often in close proximity suggest habitat preference at a scale greater than the 5 ha measured. Results of this study provide information on seasonal habitat use patterns and methods that can be applied to a long-term dataset to identify the habitat requirements of shovelnose sturgeon.

The Mississippi River is considered the ultimate single channel meandering river. Five hundred km upstream from its mouth, about 25% of the river’s discharge is diverted into the Atchafalaya River. This diversion is controlled by the Old River Control structure, built by the US Army Corps of Engineers in stages since 1963, to stop the avulsion of the Mississippi River into the Atchafalaya. The study area is a 119 km sandy bedded reach near Old River Control that is highly impacted by engineering measures. Channel dimensions average 1,000 m wide with average thalweg depths of 23 m. The mean annual discharge is 15,000 m3s-1 with a water surface slope of 0.06 m per river mile. In a sandy bedded river, the effective discharge is the discharge which cumulatively transports the most sand. This study evaluates how the Old River Control structure has influenced an adjusting effective discharge between 1978 and 2011. The bed load component of sand transport is included by employing a novel, automated, cross-correlation technique. It was found that the upper limit for discharges that cumulatively transport 85% of the total sand load has decreased from 34,000 m3s-1 to 28,000 m3s-1 between 1978 and 2011. Sand transport from 1982 to 2011 occurred during progressively greater ratios of water discharge to the Atchafalaya River and corresponded to an aggradational trend in the nearby Mississippi River at Red River Landing stream gage. The combination of this sand transport trend and nearby channel aggradation is some indication that the diversion may not be stable and that the avulsion of the lower Mississippi River is ongoing. However, sand was transported at similar discharge ratios in the 1978 to 1982 and 2002 to 2011 periods. Future trends will reveal definitively if these findings indicate that the lower Mississippi River avulsion is continuing. Two aims are pursued by placing the effective discharge approach into the geomorphologic context of the study area. Ten zones are categorized by four distinct geomorphologic classes: meander, no islands; meander, geologic control; meander, islands and divided flow; and straight zones. One, these classes have merit for future research and are shown to be geomorphologically distinct in several ways: natural levee height and channel planform adjustment relationships, sinuosity and width to depth ratios, and bedform depth to height scaling. Two, this approach allows the comparison of the effective discharge to the study area’s geomorphology. Process-form linkages can be made between sediment transporting events and the three levels in a fluvial hierarchy: fluvial bedforms and channels, bars, and levees. Median grain size and channel position of sediment samples from these three levels were plotted on combined LiDAR and bathymetric derived cross- sections from specific geomorphologic zones. This analysis indicates that the fluvial hierarchy coincides with the stages of effective discharge but seems to scale to the elevation of natural levees. This study interjects a geomorphologic approach into the lower Mississippi River discourse and raises a number of interesting questions for further research. / text

Alterations along the Mississippi River, such as dams and levees, have greatly reduced the amount of freshwater and sediment that reaches the Louisiana coastal area. Several freshwater and sediment diversions have been proposed to combat the associated land loss problem. To aid in this restoration effort a 1-D numerical model was calibrated, validated, and used to predict the response of the river to certain stimuli, such as proposed diversions, channel closures, channel modifications, and relative sea level rise. This study utilized HEC-RAS 4.0, a 1-D mobile-bed numerical model, which was calibrated using a discharge hydrograph at Tarbert Landing and a stage hydrograph at the Gulf of Mexico, to calculate the hydrodynamics of the river. The model showed that RSLR will decrease the capacity of the Lower Mississippi River to carry bed material. The stage at Carrollton Gage is not significantly impacted by large scale diversions

This dissertation examines the dynamics of sediment transport and channel morphology in the lower Mississippi River. The area of research includes the portion of the river where reach-averaged downstream flow velocity responds to the boundary condition imposed by the relatively uniform water-surface elevation of the receiving basin. Observational studies provided data that are used to identify channel-bed sediment composition, and measure bed-material sediment flux and the properties of the fluid-flow field over a variety of water-discharge conditions. The analyses demonstrate that a significant portion of the channel bed of the final 165 kilometers of the Mississippi River consists of exposed and eroding underlying relict sedimentary strata that qualify as surrogate bedrock. The exposed bedrock is confined to the channel thalweg, particularly in river-bend segments, and actively mobile bed-material sediments are positioned on subaqueous bars fixed by river planform. The analyses for sediment flux provides insight to the nature of sediment transport: during low- and moderate-water discharge, bed-material movement occurs primarily as minimal bedform flux, and so bed materials are not transferred between alluvial bars. During high-water discharge, bed-material transport increases one-hundred fold, and sands move as a part of both suspended and bedform transport. Physical models are used to show that skin-friction shear stress increases by a factor of ten for the measured water-discharge range. This change is not possible given conditions of uniform water flow, and therefore non-uniform flow in response to the Mississippi River approaching its outlet has a significant impact on the timing and magnitude of sediment flux through the lower river. In order to estimate the dynamics of bed material movement from the uniform to non-uniform segment of the river (lower 800 km), data for channel morphology are used to construct a model that predicts spatial changes in water-flow velocity and bed-material flux over a range of water-discharge conditions. The model demonstrates that non-uniform flow tends to produce a region of net channel-bed aggradation between 200-700 kilometers above the outlet, and a region of channel-bed degradation for the final 200. The implication for these results for the spatial variability of channel morphology and kinematics is explored. / text

Low grade rip rap weirs installed in agricultural surface drainage ditches manage downstream eutrophication by slowing water flow, allowing sediments time to settle out of the water column and phosphorus (P) to sorb to soil. A laboratory experiment was conducted in microcosm chambers to simulate increased hydraulic residence time caused by weirs and two field studies were conducted to compare experimental data with field data and determine sediment deposition rates. One field study monitored weirs monthly after installation and the other measured weirs of varying ages. Weirs retained significantly more water and sediment than controls. Longer inundation times led to abiotic factors known to release P during hydrologic flux, but did not translate to reduced P storage. By converting intermittently inundated sediments into more consistently saturated sediments, weirs function as a viable conservation practice for about a year until temporary P retention mechanisms and sediment retention capacities are reached.

The presence of man-made levees along the Lower Mississippi River (MR) has significantly reduced the River sediment input to the wetlands and much of the River's sediment is now lost to the Gulf of Mexico. The sediment load in the River has also been decreased by dams and river revetments along the Upper MR. Freshwater and sediment diversions are possible options to help combat land loss. Numerical modeling of hydrodynamics and sediment transport of the MR is a useful tool to evaluate restoration projects and to improve our understanding of the resulting River response. The emphasis of this study is on the fate of sand in the river and the distributaries. A 3-D unsteady flow mobile-bed model (ECOMSED; HydroQual 2002) of the Lower MR reach between Belle Chasse (RM 76) and downstream of Main Pass (RM 3) was calibrated using field sediment data from 2008 – 2010 (Nittrouer et al. 2008; Allison, 2010). The model was used to simulate River currents, diversion sand capture efficiency, erosional and depositional patterns with and without diversions over a short period of time (weeks). The introduction of new diversions at different locations, e.g., Myrtle Grove (RM 59) and Belair (RM 65), with different geometries and with different outflows was studied. A 1-D unsteady flow mobile-bed model (CHARIMA; Holly et al. 1990) was used to model the same Lower MR reach. This model was used for longer term simulations (months). The simulated diversions varied from 28 m3/s (1, 000 cfs) to 5, 700 m3/s (200, 000 cfs) for river flows up to 35, 000 m3/s (1.2x106 cfs). The model showed that the smaller diversions had little impact on the downstream sand transport. However, the larger diversions had the following effects: 1) reduction in the slope of the hydraulic grade line downstream of the diversion; 2) reduction in the available energy for transport of sand along distributary channels; 3) reduced sand transport capacity in the main channel downstream of the diversion; 4) increased shoaling downstream of the diversion; and 5) a tendency for erosion and possible head-cutting upstream of the diversion.

This research is part of an ongoing effort to improve predictions for bathymetric and morphological changes in the Lower Mississippi River. The utilized model is a subset of a previously calibrated Delft3D model. This shorter model has reduced computational time, and can be deployed for analysis focused on the area between Belle Chasse and HOP, which is the domain of the model. Simulation runs conducted under this study vary from 12 years to 48 years, utilizing a developed 12-year variable hydrograph. The comparison of variable annual hydrograph and repeated representative annual (uniform) hydrograph input data on bathymetric changes indicated that the absolute bathymetric equilibrium is dependent on year to year variability. The utilization of a uniform hydrograph increases the predicted deposition within the river domain. When evaluating diversion sand capture, utilizing a uniform hydrograph can be considered a conservative approach, while utilizing a variable hydrograph will result in more accurate sand load volumes captured by the diversion.In general, sediment capture showed only minor interdependencies amongst multiple diversions, as long as the total diversion flow is less than 140,000cfs. This study shows that morphological changes are dependent on the number and location of multiple diversions. The largest interdependencies occur for the most downstream diversions, which increase with the total diverted flow. A true equilibrium was not achieved within 48 years, with or without sea level rise. It was observed, that the system with diversions responds to sea level rise by an increase in deposition, which increases with total diverted flow.