Introduction When a river entering a lake or reservoir is denser than the receiving body of water, it will plunge and form a gravity-driven current (underflow) near the bed. When a layer of equal density is encountered, these underflows will detach from the bed and intrude horizontally into the lake waters to form an interflow. Alternatively, they may continue to the bottom of the lake. As underflows carry a number of constituents fed to them by the river or eroded from the bed, such as sediment, contaminants, nutrients and oxygen, their pathway and final destination has an impact on lake water quality. Turbulent mixing processes dilute the excess density of the river plume (as compared to the ambient lake water) and thereby exert a primary control on the final intrusion depth. Quantifying these turbulent mixing as a function of the inflow conditions is therefore key. The focus of this contribution is on the mixing processes in the plunging region, which are known to be of dominant importance (Rueda et al., 2007). In the past, most estimations of the amount of mixing in the plunging region were performed in laterally confined lab experiments and/or through passive tracer methods. In this study, an effort was made (a) to quantify the mixing in the plunging region directly from flow velocities in a laterally unconfined inflow in the field and (b) to investigate the influence of the inflow conditions on this mixing. Methods Boat-towed ADCP measurements were used to elucidate the flow field of the laterally unconfined, plunging plume of the Rhône River in Lake Geneva along gridded longitudinal (awayfrom-mouth) and transverse transects. These measurements were performed for 6 different inflow conditions characterized by the densimetric Froude number at the inflow: Frd = 𝑈0/√𝑔′𝐻 (1) where U0 is the average velocity of the inflowing river water, g’ = g∙Δρ/ρa is the reduced gravity defined as the gravitational acceleration g times the density difference between the river and the receiving ambient lake epilimnion Δρ, normalized by the lake epilimnion density ρa, and H the river depth at the mouth. The mixing in the plunging region was quantified using the plunging mixing coefficient (Akiyama & Stefan, 1984; Ford & Johnson, 1983): 𝐸p = 𝑄u/𝑄0 − 1 (2) where Q0 signifies the discharge of the river inflow at the mouth and Qu the discharge of the underflow immediately after plunging. The longitudinal velocity transects were used to define the downstream end of the plunge region for each measurement campaign. The underflow discharge in the transversal transect immediately downstream of this point was then estimated by integrating the longitudinal velocity us over the plume extent. Results For larger values of Frd (> 4) the resulting estimates of Ep are of the same order of magnitude as those found in laterally confined lab experiments (O(0.1)). For smaller values of Frd (≈ 2) the estimated Ep is of the same order of magnitude as those found in field tracer measurements of laterally unconfined inflows (O(1)). Ep was found to decrease for increasing Frd, in contrast to existing literature in which numerical simulations of a plunging inflow were performed (Shi et al., 2022). Conclusions In conclusion, the field measurements of the plunging plume of the Rhône River in Lake Geneva presented here provide valuable insights into the turbulent mixing rates associated with the plunging of hyperpycnal river inflows. The observed trend in the plunging mixing coefficient (Ep) reveals an inverse relationship with the densimetric Froude number at the inflow (Frd), challenging existing literature based on numerical simulations. This research delivers useful input for future work aimed at modeling plunging river inflows and their eventual intrusion depth. In a wider scope, it contributes to a more comprehensive understanding of the intricate dynamics governing plunging river inflows and their implications for lake water quality, emphasizing the importance of considering realistic field conditions in quantifying turbulent mixing processes.