There's not enough information to do a good calculation, but I can give you an upper limit for the heating. Let's say that the 70 feet was straight down and that all the lost gravitational potential energy went into heating the water. (I doubt it's straight down and the turbine had better take most of the energy and convert it to electrical forms if the device has any value.) So that's about a 20m drop. The gravitational potential energy from that drop is around 200 Joules per kg, enough to heat the water about 0.12°C. That's a pretty small effect. In the real case I'd guess it would be at least 10 times smaller.
Mike W.
(published on 01/30/2017)
