The public in North America has a low tolerance for reservoirs for any purpose, so many new hydro stations are designed as run-of-river facilities. This means that they only have a small head pond to divert flow from a river or stream through an intake and water conveyance leading to a hydro station.
The diversion structure (normally a low dam or overflow weir) is designed to pass all flows that are too large for the hydro station to use, including major floods. The excess flow is not diverted, but continues down the river. The hydro station is designed to operate from the rated station flow down to a flow approaching the minimum historical river flow, so that it can continue to produce at least some power year round.
A run-of-river plant is never allowed to divert all the available river flow. It is required to maintain a minimum flow (called a riperian flow, fisheries flow, or environmental flow) to sustain life in the river downstream of the diversion. A riparian outlet is normally provided from the head pond, equipped with a recording flow meter, so that the Owner can demonstrate compliance with the riparian flow requirement.
While run-of-river plants have no reservoir, they still need careful design to avoid or mitigate harmful Environmental impacts from their construction and operation.