As a moving object slows down,its potential energy incrases.As an object speeds up,its potential energy decreases.
One of the natural "driving forces" behind change is a decrease in potential energy. You can probably think of many changes that occur of their own accord because they result in a decrease in potential energy. Water always runs downhill, spontaneously. Of course, we can move water back uphill, but to do so we must use an external source of energy to run a pump. By itself, water will not run uphill because to do so would require it to increase in energy.
As water flows downhill it undergoes a decrease in potential energy. This energy is not disappearing, but is being converted into kinetic energy of motion of the water. However, from the point of view of potential energy, there has been a decrease in energy. At the top of the falls the water has high potential energy. At the bottom it has lower potential energy. Therefore the net energy change is negative. In the notation used to represent H for a chemical reaction,