Hydroelectric dams use the energy from the water flow of rivers.
Pressure builds at the dam, causing water to rush and turn turbines. The turbines turn generators as with other electricity production.
A large hydroelectric dam can have an efficiency of up to 95%. A solar panel has efficiency of only 20-40%
This graph shows that hydroelectric dams cost less per Kilowatt than burning coal, gas, and nuclear energy.
Hydroelectric energy production makes no harmful waste products. Also, it doesn't take constant maintainance, like fossil fuels do.
Hydroelectric dams are only used to create 1% of the energy in the UK, but this is still 1.7 Gigawatts. There is potential for only another 2 Gigawatts in the UK, so it could never produce more than 2% for us.
Surprising when you see how many are in the UK. However, the Peak and Lake Districts don't contain hydroelectric dams, as they are protected areas.
However, hydropower produces more than 40% of the world's renewable energy (this does include tidal as well but countries don't distinguish between them in the data so I don't know how much is tidal and how much is dams.)
This is due to its reliability as a source, and the huge amount of area in which dams can be installed.