This is Glen Canyon Dam in Page, Arizona. It is a hydroelectric dam, converting the potential energy of the lake water into electrical energy. Energy does not just appear out of nowhere, so the potential energy of the lake water had to come from some other energy source. What is that source?
The energy stored in the lake water is positional energy. Gravity pulls the water downwards, changing that positional energy into the energy of motion. To trace that energy to its source, we need to follow the water back to its highest position. Where was that water at its highest elevation? That is when it had the most positional energy. The point where the water was highest was when it was in the clouds as rain or other precipitation. OK, now how did the water get up in the clouds? Evaporation changed liquid water into water vapor, and then convection lifted that water vapor up to form the clouds. (see Cloud Formation, part 2) And where did the energy come from to provide the heat for that convection? In most cases, that energy came from the Sun, so solar energy is the source of the energy stored in the lake water.