On top of everything GSY has said, what do you do with the water during heavy stormy conditions, when the river is in spate and wind turbines are going flat out, but nobody needs that much power?
These are all great ideas, but there is a scale here. If you want to store even one day of power for the USA, you're going to have to dam every river in the country and flood hundreds of millions of square miles into lakes.
It's not feasible. There will be some places where it can be done and locally it will make sense. But as a total solution? Not a chance.
More chance to put 2Mw/h of battery storage in every home in the country and use smart technology to do G2H and H2G depending on the renewable need at the time. Of course you would have to renew that storage every couple of decades but, still, it is more feasible than trying to do pumped hydro.
For instance there are approximately 26m residential homes in the UK. If we were to put 2Mw/h capacity in each home that is 52 Tw/h of capacity. Or put another way, 1/28th of the total power generated in the UK in a year.
Great we say, let's go for it, no way we'd ever run out of capacity or power or need to burn any carbon ever again, we could charge our EV's wherever and whenever we wanted (assume commercial buildings have to do the same on the same scale) and electricity will be cheap as chips.
Erm, just one little problem. at $100 per kw/h that's 10 quadrillion and 400 trillion dollars to deploy. Even at $1 per Kw/h that it $100Trillion
Now let's do that figure for the US.....
Just to inject a little reality into the scale of things...
Nuclear decommissioning looking so expensive now??