Final answer:
Gravity is the force that causes a river to flow from higher to lower elevations, transforming potential energy into kinetic energy as the water moves downstream.
Step-by-step explanation:
The force behind a river's flow from high to low elevations is primarily due to gravity. As water is pulled down by gravitational forces, it moves from places of higher elevation, like a mountaintop or a reservoir behind a dam, to lower elevations. The water at higher elevation holds potential energy, which is converted into kinetic energy as the river flows downhill. This kinetic energy is what powers the river's flow. Furthermore, as the river progresses, changes in the channel's width, depth, and gradient, along with sedimentation, can affect the speed and volume of its flow.
For example, at the top of Victoria Falls, water possesses a high level of potential energy due to its elevation. When it cascades down the falls, this potential energy is transformed into kinetic energy. Similarly, when water behind a dam is released, such as at the Hoover Dam, it flows rapidly downstream, and this kinetic energy can then be harnessed to generate electrical energy through turbines.
It's important to consider that although water behind a dam exerts a force, this force is relatively small compared to the weight of the water. The structures, like dams, are strategically designed to withstand these forces and manage the energy transitions of the water they hold.