Is there a reason why you wouldn't combine pumped hydro with a diverted fraction of a natural river? You get the ability to store electricity along with some pure generation capacity while still allowing the river to mostly flow unimpeded. You're already building the generation capacity, and this allows you to generate some electricity 24/7 if the need arises. It seems feasible to divert a fraction of a river's flow, provided you work with natural, seasonally fluctuating flow rates, and not severely disturb wildlife. i.e. distribute the diversion plumbing over a mile of the river course.