First, in order for the hydro plant to produce power at cheap, competitive levels the plant and the river must be very large. In other words, potential sites for dams are limited. The width of the river should be narrow. Ideally the soil should be firm on the bank to support the weight of the dam. A drought year can empty a reservoir and seriously cut the amount of water available for power production.
Silt can build up behind a dam to limit the amount of water stored in the reservoir.
But perhaps the greatest limitation on power generated from dams is that limitation common to all forms of electrical energy. It cannot easily be used as a portable power source, such as the engine for a vehicle or the motor for a lawn-mower. Instead, the energy must first be stored in batteries, which are often heavy and inefficient. In many cases, several dams are built on the same river, thus increasing the overall amount of energy drawn from the same river system. Nearly all large-scale dam projects in 20th century have entirely ignored their environmental and social costs.