Ray I usually spec DC chargers that can tolerate a wide voltage input. Then I discovered the manufacture had changed the spec so the same charger could handle Dc input as wellas widely fluctuating frequency.
Hmmm? Then it dawned on me this was to accomodate the new breed of solar panels intended for grid tie as well as other types of generator sources. For European grid tie you need 230V @50Hz but their equipment is able to select either 50Hz or 60Hz still at 230V. To accomodate 120V equipment I find it easier to design in a stepdown transformer than deal with that idiocy of a center tapped neutral with the attendant unbalanced neutral current. In one job we ordered a 4kW transformer and it arrived de;overed docvkside in Floridqa for around $100
If you look at the lowtechmagazine you will see how the floating boat mills accomnodated river floods havi0ng water levels chaging by 10 - 20 feet
It is far safer to adjust wheel blade immersion where you are not fighting the force of the water.
Spillways are used to adjust water flow to modern turbines but then you get into expensive civil works construction. A floating undershot wheel is not as efficient but it is easier to regulate especially with electronic devices these days.