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.
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.
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.