Fluctuations in the flow of rivers and streams may have diverse natural or anthropogenic causes. The hydropeaking considered here is caused by the rapid increase or decrease in the release of (operating) water from reservoir hydroelectric power stations when there is a great fluctuation in the demand for power. This method of production changes the flow regime in the rivers downstream of the plant according to the season, in that considerable quantities of water are shifted from summer to winter. But the intermittent turbining also causes short-term, artificial fluctuations in flow, in a weekly and daily rhythm. This frequent and regular occurrence also fundamentally differentiates the change between the high (surge) and low flow produced by the power plant from natural flooding events.
Of the medium-size to large rivers in Switzerland, about one in four is influenced by the return of water from reservoir hydropower stations; the rivers of the Alps and Prealps are particularly affected. In addition to removal or diversion of water (residual flow stretches), and the flushing of hydropower facilities (reservoirs, intakes, equalising reservoirs), hydropeaking is therefore a widespread and severe consequence of the hydroelectric exploitation of rivers.