Evolution of particle size and concentration in the Rhone river mixing zone: influence of salt flocculation

The salt induced flocculation phenomenon is often proposed as a major mechanism influencing the deposition of the suspended matter near the mouth of major rivers. However, depending on the particular mixing conditions between fresh water and marine water and on suspended solids (SS) reactivity, salt induced aggregation may be a minor factor controlling fast deposition of SS. This work combines field studies and laboratory experiments in order to assess the importance of salt induced flocculation in the case of the saltwedge estuary of the Rhone river. Sampling of the mixing zone has been performed in contrasted hydrodynamic conditions from a low water discharge period (500 m(3) s(-1)) to a small flood event (2400 m(3) s(-1)) for particle counting and salinity measurements. Through laboratory experiments, it is shown that the Rhone river particulate matter has a poor average reactivity regarding salt induced flocculation. Considering the hydrodynamics of the estuary, we show that the evolution of the concentration of larger particles (> 5 mum) can be explained by settling and dilution. In contrast, the smallest measured fraction (2-5 mum) shows a more complex behavior and settling processes alone cannot explain the observed particle concentrations during the field studies. Four hypotheses are discussed for explaining the 2-5 um particle concentration evolutions. (C) 2001 Elsevier Science Ltd. All rights reserved.