Energy of a waterfall

Question

Why cannot a waterfall be considered to be never ending source of energy? A waterfall is converting it's potential into kinetic energy and if we "take out" some of it's kinetic energy while the water is falling down and use it to move a turbine of a generator, we get electricity. Why are we worried about saving energy then?

Let's see, A river has its origins in hilly areas and mountains. The mountains get back water from the clouds. The clouds get water from the sun. So the work in bringing water back to hill is done by solar energy. If the sun is doing our work for us, and sun has a lot of energy and it wouldn't possibly end in a billion years, why do we not see waterfalls as an almost 'infinite' energy source?
I know this is a very basic question.
This question may be similar to the previously asked questions but I couldn't find a satisfactory answer, so I'm asking this.

Answer 1

It is likely that most waterfalls will continue flowing, at least intermittently, for hundreds or thousands of years and are powered by the Sun which is expected to continue radiating energy to drive this system for much much longer.

Each waterfall can therefore supply a very large amount of energy. However only at a very limited rate - i.e. power output is limited by the flow rate of the river that feeds the fall.

The reasons this is not infinite include

• limited power output
• limited duration
  • rivers erode their beds and change their routes
  • the Earth has a limited lifetime
  • the Sun has a limited lifetime

A more conventional way to extract power from the flow of water is of course turbines built into dams on rivers.

Answer 2

We take power from Niagara Falls, and make artificial water falls inside of every hydroelectric dam. We've been doing so for a long time.

Hydro makes up most of the renewable power in North America, but less than 10% of the total power. See https://en.m.wikipedia.org/wiki/Hydroelectric_power_in_the_United_States

Why is hydro limited?

• the best places have long been exploited
• it restricts river navigation
• impoundment floods lands with alternative uses

You can find more comprehensive lists. Hydro is never free, but it is an important resource, and is often combined with flood control projects.