Re: Fag packet calculation time...
"but at a national scale, rivers aren't a significant energy resource"
True, unless nature has been very good to you, and there are not really that many sites what fit that description, and even fewer where you want the power.
If you are really lucky, you live near the Niagara River, which has a head in the 50m range, and a natural reservoir about as big as the UK.
Elsewhere, the river can be most helpful as a source of water for topping up the reactors' cooling towers.