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

Abstract

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.

Hydrodynamically‐Driven Deposition of Mud in River Systems

“…In the regions of the study area without deposition of mud on the bed, our observed median floc sizes were approximately 100 μm. This is consistent with the range of back‐calculated and observed floc sizes in previous studies of flocs in a variety of other freshwater environments and laboratory conditions (Abolfazli & Strom, 2023; Lamb et al., 2020; Lawrence et al., 2022; Nghiem et al., 2022; Osborn et al., 2023; Soulsby et al., 2013; Zeichner et al., 2021). We find that, despite the range of floc sizes observed in the field between stations, the D 50 of the floc constituent particles remained constant at approximately 14 μm.…”

“…Flocs observed in natural environments typically range in diameter from 50–5,000 μm, while flocs processed in laboratory settings typically range between 30 and 800 μm. In both cases, the flocculation process has been empirically and theoretically demonstrated to increase the rate at which aggregates are able to settle out from suspension in the water column, and thus induce the deposition of fine‐grained, cohesive sediment (Abolfazli & Strom, 2022, 2023; Adachi & Tanaka, 1997; Alldredge & Gotschalk, 1988; Bungartz & Wanner, 2004; Curran et al., 2007; Droppo et al., 2000; Dyer, 1989; Dyer et al., 1996; Fennessy et al., 1994; Fox et al., 2004; Gibbs, 1985; Graham & Manning, 2007; Gratiot et al., 2005; Gratiot & Manning, 2004; Huang, 1994; Krone, 1986; Kumar et al., 2010; Lamb et al., 2020; Lick et al., 1992, 1993; A. J. Manning & Bass, 2006; A. Manning & Dyer, 1999; Mikkelsen et al., 2007; Nghiem et al., 2022; Osborn et al., 2021, 2023; Soulsby et al., 2013; Sternberg et al., 1996; Strom & Keyvani, 2011; Syvitski et al., 1995; Van der Lee, 2000; Winterwerp et al., 2006; Zeichner et al., 2021).…”

From Particles to Flocs: Revealing Where Flocculation Occurs in the Nearfield of a Negatively‐Buoyant River Plume in a Large Lake (Lake Geneva)

“…However, the trapping of silt particles within flocs can have an important impact on floc density, and thus on the settling velocity which may increase by more than 50% (Tran & Strom, 2017; Xu et al., 2022). These results were confirmed by river studies (Osborn et al., 2023) and agree with Droppo and Ongley (1994), indicating that salinity is not a needed for sediments to flocculate. Therefore, it can be assumed that the flocs generated in the Rhône River interflow shear layer are a mixture of clays and silts.…”

“…Flocculation is well documented in fluvial (e.g., de Boer et al., 2000; Droppo & Ongley, 1992; Osborn et al., 2023; Slattery & Burt, 1997), estuarine (e.g., Gratiot et al., 2017; Verney et al., 2009; Winterwerp, 2002) and marine environments (e.g., Franck, 1973; Li et al., 2004; Many et al., 2016) mainly under steady flow conditions. Flocculation, however, has rarely been investigated in lakes.…”

Impact of Salinity on the Erosion Threshold, Yield Stress, and Gelatinous State of a Cohesive Clay

“…(2022) and Osborn et al. (2023) found that turbulence and ionic strength of the ambient water impact the flocculation of mud. Krahl et al.…”