The other way round: the reservoir catches excess rainwater and feeds it downstream through dams and power plants, often aiming to catch water to the reservoir and keep constant flow, at least not exceeding the generation capacity (trying to avoid bypass discharge). The reservoir is where the "lumps" are caught and level of water can change considerably.

The absence of lumps, i.e. floods downstream, is of course not always good for nature of the water systems, as in many places the river system ecology is based on having some floods. And specific arrangements need to be made for fish to pass dams and power plants.