Repeated bathymetrical surveys reveal transient storage of sediment in the near-field region of the plunging Rhône River inflow into Lake Geneva

Negatively buoyant (hyperpycnal) river inflows plunge upon entering lakes or reservoirs, feeding gravity-driven underflows that propagate along the bed [1, 2]. When these underflows encounter a layer of equal density within the water column, they detach from the bed and transition into an interflow. In this light, the hydro-sedimentary processes related to plunging set the upstream boundary conditions for under- and interflows. The interaction of a plunging flow with the local lake bed is understudied as of yet. Using ADCP velocity and backscatter data, Thorez et al. (2024) [3] hypothesized that the plunging Rhône River inflow into Lake Geneva locally eroded the bed near the river mouth during high discharge and high suspended sediment conditions on June 26, 2019, yet they did not prove this using repeated bathymetrical survey data. The aim of the present contribution is to test and demonstrate this hypothesis. Boat-towed ADCP measurements were used to eluci-date the near-field bathymetry of the plunging Rhône River plume in Lake Geneva along longitudinal (away-from-mouth) transects. These measurements were performed on June 23, June 26 and July 11, 2019. Figure 1 reveals that between June 23 and June 26, 2019, the lake bed within the plunging region aggrad-ed by an amount in the order of 1 m and up to a max-imum of 2 m, indicating that the hydro-sedimentary processes related to plunging were nett depositional in nature. No significant changes in the bed height were measured further downstream. Between June 26 and July 11, 2019, the lake bed eroded by up to 10 m in the plunging region and in the order of 5 m further downstream. This is in accordance with the findings of Thorez et al. (2024) [3] that state that the conditions on June 26, 2019, most likely led to ero-sion. The consecutive periods of deposition and ero-sion comply with their hypothesis of transient stor-age of sediment within the plunging region.