Darrieus horizontal axis hydrokinetic turbines (DHAHT) have shown their ability to provide hydro-power in shallow rivers. Investigation of DHAHT performance and interactions with the free surface are of great importance and are investigated numerically in the present work using a multiphase CFD solver. The free surface is modeled using the volume of fluid (VOF) method associated with an unsteady k - omega SST turbulence closure. Fully developed boundary layer conditions are imposed at the inlet. Two submergence configurations are considered by varying the water level: partially submerged (configuration 1) and fully submerged (configuration 2). The flow solver has been carefully validated against experimental data available in the literature for a three-blade vertical axis Darrieus wind turbine and a dam break experiment. Total immersion of the DHAHT leads to an improvement in the power coefficient C-p by 36.8% compared to configuration 1. For high tip-speed ratios, the Froude number downstream the DHAHT is observed to increase. The momentum extracted by the DHAHT from the flow is also quantified. The submerged DHAHT extracts the most momentum at high tip-speed ratios. The novelty of this study is to show how DHAHT performance is influenced (diminished) when the dynamics of the free surface is included in the numerical model. (C) 2019 Elsevier Ltd. All rights reserved.