Very few dams have the capacity to pump water back uphill. And they don't need it, because power-generating dams are generally only built in places that provid

Very few dams have the capacity to pump water back uphill. And they don't need it, because power-generating dams are generally only built in places that provide lots of downstream flow naturally, like the U.S. Pacific Northwest, or in places where a reservoir is already desired for other reasons (typically drinking water, navigation, and/or flood control). Dams have the capacity to route downstream flow around some turbines to lower their generating capacity. It's not really accurate to think of that routed water as "wasted capacity," because that water would flow naturally even if the dam wasn't there.