The potential energy of the water going over the falls each second is given by

PE = mgh = (1.6 x 10^6 kg)(9.8 m/sec^2)(55 m) = 8.6 x 10^8 Joules

Power is the rate of conversion of potential energy to work and is expressed with the unit Watt, which is a Joule per second. 

To get power you divide work (energy expended) by time. The time it takes for this amount of water to go over the falls is one second, so the power produced is 8.6 x 10^8 Joules per second or Watts.

When a force acts on an object and the object moves, work is done on the object equal to force x distance. In this example the work is being done on water by the force of gravity, moving it to a lower height.