1. Introduction The state of the art of plunging hyperpycnal river plumes is based largely on two-dimensional (laterally confined) laboratory experiments that led to the development of a conceptual model for such geometries, which are typical for river-dammed reservoirs (Fischer et al., 1979). More recent works have focused on increasing the understanding of three-dimensional (laterally unconfined) plunging plumes, which are typical for lakes (Best et al., 2005; Hogg et al., 2013; Kostaschuk et al., 2018). These works led to several hypotheses regarding the flow processes in these geometries, but detailed in-situ measurements to confirm these are lacking. The sedimentary processes related to plunging have, to the authors’ knowledge, never been investigated in detail. The objective of the present work is to uncover the three-dimensional flow structure and the sedimentary processes of a plunging plume and their influence on the local bathymetry through field work, in order to extend the existing conceptual model of plunging plumes for unconfined geometries and flow-sediment-bed interactions. 2. Methods A combination of gridded, boat-towed ADCP measurements and remote time-lapse imagery was used to elucidate the hydro-sedimentary processes of the plunging plume of the Rhône River inflow into Lake Geneva in Switzerland. 3. Results The ADCP velocity data reveals that the Rhône River plume plunges at contact with Lake Geneva. While plunging along the river axis, the inflow undergoes a lateral slumping motion, caused by its density excess. Just outside the plume, this slumping drives two secondary currents transporting river water back to the surface. The remote time-lapse imagery shows that the inflowing, sediment-rich river water forms a triangle-shaped surface pattern leading away from the river mouth towards a sharp tip due to the lateral slumping. It also provides evidence for the transport of sediment-rich water to the surface by the secondary currents. The shape of the bathymetry indicates the presence of zones of mostly erosion and deposition, while an increase in sediment flux in downstream direction indicates net erosion during the measurement campaign. 4. Conclusions Outside of the along-river-axis plunging, all other processes described are additions to the existing conceptual model for plunging, which therefore can be extended for laterally-unconfined plunging and flow-sediment-bed interactions. Acknowledgments We thank CIPEL for the lake data and FOEN for the river data. This project was funded by the Austrian Science Foundation (FWF) under project number I 6180-N. References Best, J. L., Kostaschuk, R. A., Peakall, J., Villard, P. v., & Franklin, M. (2005). Whole flow field dynamics and velocity pulsing within natural sediment-laden underflows. Geology, 33(10), 765–768. doi:10.1130/G21516.1 Fischer, H. B., List, E. J., Koh, R. C. Y., Imberger, J., & Brooks, N. H. (1979). Mixing in Inland and Coastal Waters. Elsevier. doi:10.1016/C2009-0-22051-4 Hogg, C. A. R., Marti, C. L., Huppert, H. E., & Imberger, J. (2013). Mixing of an interflow into the ambient water of Lake Iseo. Limnology and Oceanography, 58(2), 579–592. doi:10.4319/lo.2013.58.2.0579 Kostaschuk, R., Nasr-Azadani, M. M., Meiburg, E., Wei, T., Chen, Z., Negretti, M. E., Best, J., Peakall, J., & Parsons, D. R. (2018). On the Causes of Pulsing in Continuous Turbidity Currents. Journal of Geophysical Research: Earth Surface, 123(11), 2827–2843. doi:10.1029/2018JF004719