The Flocculation State of Mud in the Lowermost Freshwater Reaches of the Mississippi River: Spatial Distribution of Sizes, Seasonal Changes, and Their Impact on Vertical Concentration Profiles

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. We also examine whether measured floc sizes can explain observed vertical distributions of mud concentration through a Rouse profile analysis. Data were collected at the same locations during summer and winter at similar discharges and suspended sediment concentrations. Measurements showed that the mud in both seasons was flocculated and that the floc size could reasonably be represented by a cross-sectional averaged value as sizes varied little over the flow depth or laterally across the river at a given station. Depth-averaged floc sizes ranged from 75 to 200 microns and increased slightly moving downriver as turbulence levels dropped. On average, flocs were 40 microns larger during summer than in winter, likely due to enhanced microbial activity associated with warmer water. Floc size appeared to explain vertical variations in mud concentration profiles when the bed was predominately composed of sand. Average mud settling velocities for these cases ranged from 0.1 to 0.5 mm/s. However, Rouse-estimated settling velocities ranged from 1 to 3 mm/s at two stations during winter where the bed was composed of homogeneous mud. These values exceeded the size-based estimates of settling velocity. Plain Language Summary Large rivers, such as the Mississippi, carry a substantial amount of fine (muddy) sediment. Where this mud deposits, be it within the river channel itself, the adjacent floodplain, or the coastal zone, depends in part on how fast the mud settles within the water. Mud can exist as a collection of individual particles ranging in size from 1 to 63 microns and/or as aggregates of these particles, known as flocs, whose size, density, and settling speed change with physical, chemical, and biological conditions within the water. Whether mud exists as flocs and how big the flocs are if they do exist in different conditions within a river is difficult to know. The challenges come from the dynamic nature of the aggregate sizes and the difficulty in measuring these flocs within the river itself. In this study, we present data, for the first time, on the flocculation state of mud in the lower freshwater sections of the Mississippi River. Such data aids in understanding where mud may travel to and deposit within the lower Mississippi River Delta and whether or not engineering solutions to land loss, such as diversion structures, can help to promote the emergence of new land.