The sun evaporates water. Water vapor in the form of clouds travels around the world. When these clouds encounter mountains the air is cooler and the clouds can't hold as much water so the water is released as rain or snow to fill mountain lakes and streams. Without this free ride on the sun's energy, we could never get net energy out of falling water, because it would take more energy to pump the water uphill than we could get out of it's run downhill. No significant greenhouse gases are created.
Large dams prevent the natural migration of fish.
Large dams usually displace people and animals. Large dams stop the natural flushing of rivers and streams thus reducing the deposit of silt downstream. Large dams disrupt the natural flows of the rivers and streams often creating niches for pests which couldn't prosper before. All dams will eventually fill with silt. The power of waves can be used to generate electricity.