Freshwater flocculation in the Usumacinta River delta during the high flow season was confirmed and supported by field data and the results from experiments conducted in a rotating annular flume and in a settling column device. Although the stream intensity and a bottom macroform generated and disturbed the 3D turbulence components in the cross section, the flow patterns followed the logarithmic law and the Nezu and Nakagawa models. The Rouse model can be applied to predict suspended sediment flux; however, flocculation led to diffusivity ratios less than 0.63 and effective settling velocities close to $1\mathrm{mm}/\mathrm{s}$ that were consistent with the mass conservation equation. When comparing the suspended load patterns of several lowland rivers, flocculation stages are related to shear rate; furthermore, in contrast to saline environments, the representative aggregate size in freshwater systems is presumably higher than the Kolmogorov microscale.

Particle aggregation modifies sediment dynamics, which is a determining factor for morphodynamic and ecological processes in deltaic plains. Here, we investigated the link between intra-annual hydrodynamics variability and flocculation in the Grijalva-Usumacinta system. Monthly (2016–2017) and seasonal (2021–2022) river data was processed using analytical methods and the simplified sonar equation. Flocs were reformed and characterized in the laboratory, validating the in situ settling velocities (0.5–3.8 mm/s) and the existence of large low-density macro-flocs (>300 μm). We verified that flocculation prevailed, exhibiting seasonal patterns; (1) the highest aggregation rates matched the increase in total suspended solids at rising-flow (>100 mg/L), (2) periods of high-flow showed stable aggregation rates, and (3) an influence of marine conditions occurred at low-flow. Particulate phosphorous and organic fraction showed seasonal patterns linked to flocculation. Due to damming, the shear rates varied slightly (7–11 L/s) in the Grijalva, leading to high flocculation intensities affecting the diffusivity ratio. In the Usumacinta, aggregation was limited by shear rates that normally exceed 15 1/s. We found seasonal Rouse parameters representative of sediment dynamics.