This is not true. Rivers are often dammed to create an artificial body of water that can be used for power generation. You are storing water from the river whe

This is not true. Rivers are often dammed to create an artificial body of water that can be used for power generation. You are storing water from the river when it has a high flow rate, in order to be able to generate power when needed. There is nothing that requires the area downstream from the dam to have been previously underwater.