why does the best place for a reservoir have to be a river valley in an upland area with steep sided slopes to act as natural walls. Why cant is be flat?

Answer 1

A flat area requires too much area for a given volume. There will also be less pressure head, and as a result, less extractable power for a given volume. As the reservoir is drawn down, there will be large unsightly areas with no vegetation and a "bath tub ring".

Answer 2

so are you saying that a steep area would collect more water in a smaller area as there is more pressure?