We use in situ measurements of suspended mud to assess the flocculation state of the lowermost freshwater reaches of the Mississippi River. The goal of the study was to assess the flocculation state of the mud in the absence of seawater, the spatial distribution of floc sizes within the river, and to look for seasonal differences between summer and winter. The data was also used to examine whether measured floc sizes could explain observed vertical distributions of suspended sediment concentration through a Rouse profile analysis. The surveys were conducted at the same location during summer and winter at similar discharges and suspended sediment concentrations, and in situ measures of the size distribution of the mud over the longitudinal, transverse, and vertical directions within the river were obtained using a specially developed underwater imaging system. These novel observations show that mud in the Mississippi is flocculated with median floc sizes ranging from 50 to 200 microns depending on location and season. On average flocs were found to be 40 microns larger during summer than in winter and to slightly increase in size moving downriver from the Bonnet Carré Spillway to Venice, LA. Floc size statistics varied little over the depth or laterally across the river at a given station. Bulk settling velocities calculated from size measurements matched values obtained from a Rouse profile analysis at stations with sandy beds, but underestimated settling velocities using the same equation parameters for measurements made during winter over muddy beds.

The joint probability distribution of streamwise particle hop distance, lateral particle hop distance, and travel time constrains the relationships between topographic change and sediment transport at the granular scale. Previous studies have investigated the ensemble characteristics of particle motions over plane-bed topography, however it is unclear whether reported distributions remain valid when bedforms are present. Here, we present measurements of particle motion over bedform topography obtained in a laboratory flume and compare these to particle motions over plane-bed topography with otherwise similar conditions. We find substantial differences in particle motion in the presence of bedforms that are relevant to macroscopic models of sediment transport. Most notably, bedforms increase the standard deviation of streamwise and lateral hop distances relative to the mean streamwise hop distance. This implies that bedforms increase the streamwise and lateral diffusion lengths and, equivalently, increase diffusive-like fluxes.

Sedimentary bed configurations that are stable under weak fluid-driven transport conditions can be divided into two groups: (1) meso-scale features that influence flow and sediment transport through roughness and drag partitioning effects (“mesoforms”), and (2) grain-scale features that can effectively be ignored at the macroscopic scale (“microforms”). These groups produce distinct sedimentary structures and are thought to be separated by a transition in process regime characterized by the onset of nonlinear coarsening associated with flow separation and scour. However, the physical mechanisms responsible for this transition are poorly understood. Previous studies suggest that interactions between moving particles lead to stabilized bed disturbances that initiate nonlinear morphodynamic feedbacks. This study presents a quantitative interpretation of this hypothesis that is tested using experimental observations of particle motion over stable and unstable quasi-planar topography. We find that the microform/mesoform transition corresponds to the transition from rarefied to congested transport quantified by the dimensionless ratio of particle collision frequency to particle entrainment frequency. Combined with empirical relations for bedload flux and particle travel time, theory presented herein enables prediction of bed configuration under weak bedload transport conditions.

A physically rational model for river width is critical to predict macroscopic landscape evolution driven by fluvial sediment transport. Growing evidence suggests that rivers widen until the stress exerted by the fluid on the bed surface is close to the critical entrainment stress of the bank material. In this study, we test the limits of this model as a closure assumption in dynamically evolving river systems. We consider a simple laboratory channel with a fixed water discharge, monodisperse bed material, no sediment supply, and an initial relief that was sufficiently large to guarantee a finite transport capacity. Over time, the transport capacity approaches zero through changes in channel morphology. Concurrent measurements of width and sediment load highlight departures from theory that mirror empirical trends in bankfull alluvial rivers. We suggest that lateral instability limits channel width at high sediment loads.

Bedload flux is notoriously challenging to measure and model. The dynamics of bedload therefore remains largely unknown in most fluvial systems worldwide. We present a global scale bedload flux model as part of the WBMsed modeling framework. Our results show that the model can very well predict the distribution of water discharge and suspended sediment and well predict bedload. We analyze the model’s bedload predictions sensitivity to river slope, particle size, discharge, river width and suspended sediment. We found that the model is most responsive to spatial dynamics in river discharge and slope. We analyze the relationship between bedload and total sediment flux globally and in representative longitudinal river profiles (Amazon, Mississippi, and Lena Rivers). We show that while, as expected, the proportion of bedload is decreasing from headwater to the coasts, there is considerable variability between basins and along river corridors. The latter is largely responsive to changes in suspended sediment and river slope due to dams and reservoirs. We provide a new estimate of water and sediment fluxes to global oceans from 2,067 largest river outlets (draining 67% of the global continental mass). Estimated water discharge (30,579 km3/y) corresponds well to past estimates however sediment flux is considerably higher. Of the total 22 Gt/y estimated average sediment flux to global oceans, 19 Gt/y is transported as washload, 1 Gt/y as bedload, and 2 Gt/y as suspended bed material. The largest 25 rivers are predicted to transport over 55% of total sediment flux to global oceans.

Estimates of fluvial sediment discharge from in situ instruments are an important component of large-scale sediment budgets that track long-term geomorphic change. Suspended sediment load can be reliably estimated using acoustic or physical sampling techniques; however, bedload is difficult to measure directly and can consequently be one of the largest sources of uncertainty in estimates of total load. We propose a physically-informed predictive empirical model for bedload sand flux as a function of variables that are measured using existing acoustic or physical sampling techniques. This model depends on the assumption that concentration and grain size in suspension are in equilibrium with reach-averaged boundary conditions. Bayesian inference is used to fit model parameters to data from eight sand-bed rivers and to simulate bedload flux over the available gage record at one site on the Colorado River in Grand Canyon National Park. We find that the cumulative bedload flux during the nine year period from 2008 to 2016 was 5\% of the cumulative suspended sand load; however, instantaneous bedload flux ranged from as little as 1\% of instantaneous suspended sand load to as much as 75\% of instantaneous suspended sand load due to fluctuations in flow strength and sediment supply. Changes in bedload flux at a constant discharge are indicative of short-term sediment supply enrichment and depletion. Long-term average bedload flux cannot be expected to remain constant in the future as the river adjusts to changes in sediment runoff and the dam-regulated discharge regime.