#### Answer

54% of the energy

#### Work Step by Step

The river descends 15m and has an initial speed of 3.2 m/s. It leaves with 13 m/s. Considering 10kg of water (as in the hint), we can find the initial potential energy using U = mgd = (10kg)(9.8)(15m) = 1470 Joules.
Using conservation of mechanical energy, we know that the change in kinetic energy must come due to a decrease in kinetic energy. We can find the change in kinetic energy by $\Delta KE = (1/2)m(v_{f}^2-v_{i}^2) $ = 793.8 Joules.
The percentage of the original is therefore (793.8J/1470J) = 0.54 or 54%