Hyperpycnal (denser) river inflows into lakes bring sediments, nutrients, oxygen and contaminants which are crucial for the water quality. Due to the higher densities of hyperpycnal inflows, they abruptly descend (plunge) upon inflow and generate gravity driven underflows. This plunging is accompanied by mixing and entrainment of ambient lake waters which causes dilution of their initial density excess. The mixing is parameterized by the plunging mixing coefficient Ep and is of critical importance as it decides the fate and final destination of the matter carried by the river inflows. This PhD project officially commenced on November 13, 2023 and builds on a preparatory research phase that suggests that Ep is affected by the geometry of the river mouth. This research presents the first laboratory attempt to affect the pathway of riverine inputs from hyperpycnal inflows into a sloping solid bottom of the ambient water by modifying the geometry, in particular the aspect ratio (AR = B0/H0) and the lateral confinement at the river mouth. It also presents the first laboratory investigation including flow-sediment-bathymetry interactions at the near field region (region between the river mouth, the plunge location and just downstream of the plunge). This study is realised by means of laboratory experiments in the unique Coriolis platform at LEGI (Laboratoire des Ecoulements G´eophysiques et Industriels) which allows to mimic the turbulent flow and mixing processes at the Rhˆone inflow into Lake Geneva. This study is part of a bilateral project funded by Austrian Science Fund (FWF) and Agence Nationale de la Recherche (ANR) that also includes complementary field and numerical investigations.