Why is hydropower bad?
Hydropower is not necessarily bad, and is actually one of the cleaner power sources people have come up with. The problem with it is caused by damming rivers, a necessary step in creating hydropower. These dams sometimes cause problems for fish and for the natural flow of the water.
Hydropower is a renewable energy source which can be environmentally neutral. Conventional hydropower (dams, diversion, pumped storage), regarded as clean energy sources, may be superseded by emerging hydropower technologies (current, tidal, wave, thermal inversion) which would have minimal impact on the environment because they won't flood or divert the natural water flow.
Hydropower transform potential energy in water (height) and create electricity using dynamo. The dynamo is powered by hydropower. Simple mechanical design to use hydropower such as Rice mill using water flow and weight to make mechanical motion or Ram pump using the velocity of the stream make pumping of water to higher head.