Run of the river
Run of the river hydroelectricity differs in that no dam or reservoir is involved, eliminating many ecologically destructive drawbacks cause by other techniques.
Some water is diverted from upstream in a river, it flows downhill through pipes and turbine generators and it then returned to the river.
In small rivers with fluctuating flow rates, run of the river generation may become intermittent.
Large-scale systems are not entirely green – their construction an maintenance requires roads, bridges and transmission lines as well at the pipes and turbines themselves. However, this impact is far smaller than if a dam or reservoir were involved.