Abstract The research was conducted as a part of the phase III of the KEEP(Kuroshio Edge Exchange Processes)program. Two strings of sediment traps were deployed simultaneously: one near the outlet of North Mien-Hua Canyon (T15), and the other at the south rim of the South Okinawa Trough (T16). The purposes are to collect settling particulates at various depths for the studies of temporal and spatial variations of the particulate flux, 210Pb activity, 210Pb flux and size distribution. The particulate fluxes showed synchronous variations among the traps deployed at different depths of the same site. In general, the particulate flux in the marginal sea areas increases with depth but the maximum values may be at the mid-depth rather than at the deepest trap and remain so for the entire mooring period as observed at T15 and T16. Unusually high particulate fluxes could occur within the same period, probably reflecting an episodic event, such as typhoon or rain storm. 210Pb activity measured from different depths at T15 and T16 increases generally with depth along with the increase of the particulate flux toward the bottom. The temporal variation of 210Pb activity is generally smaller than that of the particulate flux in terms of relative amplitude. At the outlet of the canyon, T15, the temporal mean particulate flux of each trap ranged from 0.16 to 3.3 g/m2/d(a factor of ~ 20), while the mean 210Pb activity of each trap varied from about 98 to 168 dpm/g. At T16 located at the south rim of the South Okinawa Trough, the temporal mean particulate flux ranged from 0.06 to 5.7 g/m2/d(a factor of ~ 100), while the mean 210Pb activity varied from 82 to 192 dpm/g. The particulate 210Pb flux was smaller at T15 than at T16 because T15 has smaller particulate flux. The trapped particulates in the canyon(T1-T3) are mostly silt and sand, while the particulates collected from the Okinawa Trough(T15, T16) are mainly silt and clay. The 210Pb activity of the former is much lower than that of the latter, indicating the enrichment of 210Pb on the fine particulates. As the fine-grained particulates away from landmass have longer residence time in the ambient water, they can scavenge 210Pb more effectively.

La connaissance des flux de contaminants particulaires véhiculés par les cours d'eau est cruciale pour la gestion des pollutions environnementales, mais elle se heurte à d'importantes difficultés technologiques, méthodologiques et scientifiques. Grâce à un effort d'observation intensive rendue possible par l'Observatoire des sédiments du Rhône, ce travail de recherche s'est concentré sur la dynamique spatio-temporelle des flux de contaminants particulaires – mercure et PCB- au sein du Rhône et de ses principaux affluents contributeurs en matières en suspension (MES) : l'Arve, la Saône, l'Isère et la Durance. La stratégie d'observation choisie et validée privilégie un échantillonnage quasi-continu des débits, concentrations en MES (turbidimétrie) et teneurs en contaminants (par piège intégratif), associé à des prélèvements de référence (manuels, ou par centrifugation) et à une caractérisation détaillée des particules en termes de granulométrie et de taux de carbone organique. A partir des données ainsi mesurées et existantes, l'étude de la variabilité spatio-temporelle des différents paramètres et de leurs facteurs d'influence a permis de proposer et valider des méthodes adaptées pour la quantification des flux particulaires à l'échelle événementielle, annuelle et inter-annuelle. Ces résultats fournissent une nouvelle vision des flux particulaires dans le Rhône à l'échelle du réseau hydrographique, et soulignent l'importance des évènements hydro-sédimentaires majeurs et des interactions avec les aménagements hydro-électriques. Ces processus hydro-sédimentaires complexes motivent une contribution à l'amélioration de la simulation numérique du Rhône / The knowledge of particulate contaminant fluxes transported by rivers is crucial for the management of environmental pollution, but it is compromised by significant technological, methodological and scientific difficulties. Through an intensive observational effort made possible by the Rhône Sediment Observatory, this research focused on the spatio-temporal dynamics of particulate contaminant -mercury and PCBs- fluxes in the Rhône and its major tributaries as regards suspended particulate matters (SPM) contribution : the Arve, Saône, Isère and Durance rivers. The chosen and validated observation strategy is based on the quasi-continuous monitoring of discharge, SPM concentrations (by turbidimetry) and contaminant contents (using sediment integrative traps), associated with samples of reference (manual sampling or centrifugation) and a detailed characterization of particles in terms of grain-size and organic carbon contents. From measured and existing data, the study of spatio-temporal variability of parameters and of their factors of influence allowed us to propose and validate suitable methods for the quantification of particulate fluxes at the event, annual and inter-annual scales. These results provide a new insight into the particulate fluxes throughout the Rhône river system and highlight the importance of major hydro-sedimentary events and of the interactions with hydro-power schemes. Such complex hydro-sedimentary processes motivate the improvement of the numerical simulation of the Rhône river