The water gains kinetic energy as it falls down and loses its potential energy. Assuming no other losses, part of the kinetic energy gain of water is converted into heat , raising the temperature of water.
Why Is The Temperature At The Bottom Of The Waterfall Different Than The Temperature At The Top?
The water loses potential energy as it drops (you=mgh), which increases the kinetic energy and raises the temp.
As water is mixed with air on it's way down there is an evaporation that occurs. That equates to a cooling affect, like a swamp cooler. There are stories of Egyptians making ice by laying out thin pans of water in the night's arid breeze, super evaporation made ice.