In the context of power systems research, the analysis of the water-energy nexus is crucial. The high amount of water required to meet the needs of irrigation, human consumption and other uses may affect to the scheduling and dispatch of the thermal power plants, since they need freshwater for cooling. Power system models worldwide tend to neglect this water-energy interaction in order to reduce mathematical and computational complexity of the models. However, recent generation adequacy-related episodes (in Poland in 2015 and 2016 or France, Germany, and Spain in 2006) show the importance of these interactions for the operation of the power system. Most analyses expect these incidents to occur with increasing frequency due to climate change. This first report of the WATERFLEX Exploratory Research Project proposes a medium-term hydrothermal coordination problem where the hydro-specific features of the power system are well represented by means of (i) the water balance in each hydropower plant; (ii) the bounds on water release, spillage, and reservoir levels; as well as (iii) the hydraulic network with water time delays for representing cascade hydropower plants. Also, dispatch constraints on thermal generators are also included in the model. The problem is thus formulated as a linear programming problem. The proposed model is linked to the dispatch and unit commitment Dispa-SET model in which the thermal generators are precisely represented. Dispa-SET provides short-term operational decisions on aggregated hydropower and disaggregated thermal power plants. These two models are linked to the hydrological LISFLOOD model in order to accurately capture the water-power interactions. LISFLOOD could provide not only the water inflows of the hydropower plants but also the water needs of thermal power plants for a given plan of reservoir levels.