Are reservoirs usually built upstream or downstream of rivers?

Reservoirs are generally built in the upper reaches of rivers for several reasons: first, the terrain on both sides of the river is high, so it is easy to build dams to collect water and the cost is low; Second, the difference between the upper and lower water levels of the dam is large, and there is enough potential energy to facilitate hydropower generation; Third, there are not many residents in the reservoir area, and the submerged area is small, which reduces the difficulty of immigration.