Pondage is the water that can back-up behind larger run-of-the-river systems. This pondage is essentially a small amount of water storage wherein water is built up during off-peak periods and used during peak periods. In periods of low electricity demand, or when the plant is inactive the water that would otherwise fall through the spillways and be "wasted" is stored and used when it is needed. Water that is built up in this pondage can result in the elevation of the water varying between 1-1.5 meters during operation. A significant amount of pondage allows the plant to accomodate hourly fluctuations.
Generally speaking, since small amounts of water storage in the form of pondage allow run-of-the-river systems to run fully even during off-peak periods, plants with pondage tend to be more efficient than those without.