As the reservoir scale in which there are high quantities of water and enough pressure to spin the turbines and generators in power plants, water the lower end.
A reservoir to a level that is so high that the water pressure is enough to bring water to spin turbines and generators in the powerhouse downstream at a lower level.
A reservoir for coping in higher water content and pressure until there is enough to bring rotating turbine generator in the power plant and the downstream with the lower level.