When solar energy falls on the water surface then evaporation of water from water surfaces like oceans, rivers and other water bodies takes place to from clouds. The clouds are then taken to distant places by air currents, and ultimately water comes back to the surface in the form of rain and snow. During evaporation, a part of solar energy gets converted into potential energy of water molecules. The potential energy of water molecules gets converted into kinetic energy during rain and snowfall.
Thus, energy of water flowing in a river is considered to be an indirect from of solar energy.
