A hydroelectric dam converts mechanical energy (gravity acting against water) into electrical energy.
What form would this energy take if the dam did not exist? Would it be thermal energy (from friction as the water flows)? Would it be purely mechanical (the dam reducing the water's momentum)?
A solar panel, for example, converts radiant energy (from the sun) into electrical energy — diverting it from the ground that it occludes (likely becoming thermal energy). But I'm having difficulty figuring out the equivalent for a hydroelectric dam.