Controls on Freshwater Mud Flocculation in Rivers Using a Global Suspended Sediment Dataset

Flocculation is known to aggregate muddy sediment in a range of environments via physicochemical and biological processes, and can cause many-fold increases in aggregate particle size and settling velocity. In rivers, mud flocculation can be an important mechanism for increasing mud deposition rate, which might shape the geomorphology of river channels and their floodplains. However, studies on the controls of mud flocculation in rivers globally are rare. Lamb et al. (2020) demonstrated a ubiquitous pattern of mud flocculation in eight rivers but did not explain the observed variability in floc size and settling velocity. Following up on this work and in line with previous studies, we hypothesized that sediment concentration, shear rate, clay mineralogy, and concentration of organic material are the most important variables that control floc diameter and settling velocity in rivers. To test this hypothesis, we combined and analyzed a global dataset of suspended sediment concentration profiles in rivers with climate data (e.g. aridity, precipitation, runoff) and organic carbon measurements. We also estimated and validated power-law fits for floc diameter and settling velocity, which we compared to previously proposed floc models and scaling relations. Results show that the strongest dependencies for floc size and settling velocity are on sediment concentration and organic carbon concentration. These findings are physically consistent because sediment concentration largely determines collision frequency of particles, and hence aggregation rate, and organic matter in flocs controls the material strength of flocs and thus their susceptibility to shear-induced break up. Our data also support the concept that the Kolmogorov microscale length limits maximum floc diameter in rivers because turbulent stresses enhance break up of flocs larger than this size. Our results provide new insight and predictive tools into the role of flocculation in setting mud transport properties in rivers.