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

Abstract

AbstractThe dynamics of sediments entering lakes in river plumes is virtually unknown. This field study provides unprecedented evidence of the initiation and evolution of suspended sediment flocculation in the nearfield of the negatively‐buoyant Rhône River plume, flowing as interflow in the thermocline of stratified Lake Geneva. Sediment floc property changes (formation, size, composition, shape) with depth and distance from the mouth, were determined by combining digital holographic camera LISST‐HOLO data with full‐depth in situ profiles of particle size (LISST‐100X), density, turbidity, currents and water samples taken along the plume path. The total suspended matter volume of inflowing Rhône River waters (∼155 mg l−1) mostly consisted of clays (<4 μm), very fine silts (4–8 μm) and small contributions of microflocs (20–100 μm). This composition was also found in the interflow plume core. Above the plume, in the epilimnion, fine silts, microflocs and numerous phytoplanktonic organisms (∼200 μm) were observed, representative of the lake background. High levels of shear (15–27 s−1) and turbulence occurred in the shear layer that formed between the interflow bottom and the hypolimnion below. It was found that macroflocs only formed in this shear layer. In the hypolimnion, sediment load was the lowest and macroflocs (up to ∼300 μm) composed of inorganic particles were dominant. The size of the largest flocs was limited by the size of the smallest turbulent eddies determined by the Kolmogorov microscale. Floc 3D fractal dimensions of ∼2.1–2.5 suggest an intermediate shape complexity between marine snow and sludge flocs.