Describe how the potential energy of a river is changed into kinetic energy.

The energy possessed by a river when it is on top of a hill or a water fall is the potential energy. That is, as long as water is at some height above the ground level, it will have potential energy due to the influence of gravity. But once it starts flowing, the motion or movement is responsible for the kinetic energy.

Explanation:

Energy due to the position or state of an object is gravitational potential energy. Once the object starts moving, it has a certain velocity. Velocity possessed by a mass (or an object) will give rise to kinetic energy.

For a given object, It is conservation of energy which basically explains this. The total energy should always remain constant during a process.

For the river waters, the potential energy due to its state is converted to kinetic energy of its motion.