River inflows are an important input of oxygen, sediment, pollutants, nutrients, momentum and heat for lakes and reservoirs. Therefore, they have a large influence on such water bodies in terms of the water quality, reservoir storage capacity and hazards. When such river inflows are negatively buoyant with respect to the upper water layers of the receiving lake or reservoir, they will plunge and form a gravity-driven density current near the bed. When a high sediment load causes the density excess, such currents are called turbidity currents. Density currents can supply the aforementioned constituents to any depth layer of lakes and reservoirs depending on their relative buoyancy. The plunging process at the river-lake interface provides upstream boundary conditions for density currents and controls the pathway and fate of all constituents they carry. Understanding the turbulent flow and mixing processes involving entrainment of ambient water into the plunging flow and quantifying them as a function of the relevant inflow parameters is therefore important. The amount of mixing induced by the plunging process is commonly defined by the plunging mixing coefficient Ep = Qd/Q0 - 1, where Q_0 signifies the discharge of the river inflow before plunging and Q_d the discharge of the density current immediately after plunging. Previous studies were mainly concerned with laboratory investigations of laterally confined configurations, which are relevant for dammed river reservoirs, and hypothesized that the inflow densimetric Froude number Frd is the main control parameter for the plunging process. These studies indicated a plunging mixing coefficient Ep in the range 0.1-0.3. In the present study, we report a field investigation of a laterally unconfined plunging river inflow into a lake. Six field measurement campaigns of the plunging inflow of the negatively buoyant Rhône River into Lake Geneva (Switzerland/France) were carried out. Vessel-mounted ADCP velocity measurements were performed along a raster of longitudinal and transverse transects, whose locations were informed using simultaneous remote-sensing time-lapse camera imagery. The ADCP data provide, for the first time, the full three-dimensional flow field of the plunging process. Moreover, they provide a direct quantification of the plunging mixing coefficient Ep from the discharges measured in transverse transects at the river mouth and just downstream of the plunging. All ADCP data confirm that the inflowing river water plunges and forms a density current. For all field campaigns, the transverse transects show a general increase in discharge as the inflowing water moves away from the river mouth, indicating that ambient water is being entrained into the plunging flow. The data reveal that the plunging mixing coefficient Ep is up to an order of magnitude larger than in laterally confined configurations and decreases with increasing inflow densimetric Froude number, which contrasts with observations made in laterally confined configurations.