The flow induced by a compositional (e.g., temperature or salinity driven) gravity current propagating over a fixed non-erodible triangular bottom-mounted obstacle is investigated based on 3-D large eddy simulations. The paper discusses how the flow physics (e.g., type and characteristics of the reflected bore, dynamics of flow instabilities affecting mixing, and turbulence structure) and main flow variables (e.g., the proportion of the flow advected over the obstacle, the height of the reflected flow and speed of the reflected bore, the height of the lower layer and front speed of the current downstream of the obstacle, and the drag force) change as a function of the incoming gravity current type (lock-exchange vs constant-flux), relative obstacle height, and Reynolds number. A particular focus is on the flow structure during the two possible quasi-steady regimes that can occur in such flows. The predictive capabilities of shallow flow theory models to estimate the main flow parameters during these two regimes are investigated. An analytical model is proposed to estimate the mean streamwise drag force on the obstacle during the two regimes. Finally, the bed friction velocity distributions are used to identify regions where significant erosion will occur in the case of a loose surface. (C) 2015 AIP Publishing LLC.