You can actually produce a high voltage generator by harvesting the power of falling water.
Back in 1867, Lord Kelvin devised a contraption that would later be known as Kelvin's Thunderstorm or Kelvin's Water Dropper. This simple device uses falling water to charge by electrostatic induction. When the charge separation produced by the water is large enough between the two ends of the water dropper, a spark can be seen. This is the same process that produces lightning and that creates electricity in hydroelectric dams.
For a more detailed explanation of Lord Kelvin's Thunderstorm, Veritasium takes a look at the device in more detail.