[8bitsrule]

Here in the PNW,

"The Columbia River Gorge is a canyon ... up to 4,000 feet (1,200 m) deep [and which] stretches for over eighty miles (130 km) as the river winds westward [toward Portland]." - WPedia

Hmmm. Now I'm wondering how many reservoirs already (or could) exist above and along 80 miles of river. (No need to dig tunnels to the generators.)

[usrusr]

Exactly one reservoir (or a number of tiny ones not worth mentioning), because even all the way from Richland to Portland there is hardly any elevation drop.

[usrusr]

PS: oh, you meant reservoirs in contributory valleys left and right of the main river. Sorry about the misread, "above and along" should have been clear enough.

[s1artibartfast]

this works for water storage, and some for hydro generation, but for hydro stroage, you really want two large basins with as big of a elevation change as possible.

This is because dams are very expensive, have height limitations, and some inherent risk. Hover dam is only ~200 meters high. The big hydro storage projects have height changes ~400 meters.