Taking into account the fact that what matters in hydroelectric power generation is the potential energy of water, one would be led to the conclusion that we do not need the rest of the reservoir on the picture below, i.e. everything what is to the left of the picture.
Why one has to flood big areas of land if even a reservoir with small surface gives us what we need, i.e. equal pressure at the turbine (given by the height of the water column above it, the hydrostatic pressure is not affected by the area of the dam) and the same flow rate of water, given by river's flow rate?
If one wouldn't be restricted by the need to regulate the flow rate to the turbine, one could in principle build a power plant at a waterfall's edge, e.g. by digging a hole in the rock from the of the waterfall to its foot and putting a turbine there. Much smaller cost, no land flooded. Correct, or are there any other restrictions?