http://elpjournal.eu/wp-content/uploads/2016/03/itsj-spec-1-1-5.pdfhttps://www.usbr.gov/power/edu/pamphlet.pdf

The potential energy of water at a high elevation is often used, whether the water is stored behind a dam, or waterwheels are inserted into an undammed river. Waterwheels in an undammed river are less harmful environmentally but typically produce less power. The difference in concentration of salt between river water and sea water has also been considered as an energy source, which is available wherever a river flows into the sea. Evaporation of water has also been used; the temperature reduction due to evaporation allowing a heat engine to operate between ambient temperature and the wet-bulb temperature. In the case of evaporation the water gains energy upon vaporizing, so the energy source for evaporation and for heat-engine operation is the ambient thermal environment. The increase in volume of the water upon evaporation and hence increase in configurational entropy more than offsets the decrease in thermal entropy due to cooling to the wet bulb temperature, so the Second Law of Thermodynamics is satisfied.

http://www.ucsusa.org/clean_energy/our-energy-choices/renewable-energy/how-hydroelectric-energy.html

https://www.hindawi.com/journals/isrn/2012/730631/