Yes, I think upgrade of old infrastructure is the real story here. Most of what is out there is very old technology. In Minneapolis, some of the turbines are a century old. Estimates are that replacing those with the latest technology would yield more new net energy than would damning up more of the Mississippi. I suspect that this is true in many areas around the country.

Hydro is not renewable in exactly the same sense as wind and solar. Damns, unlike wind or sunlight, silt up. And damage is not only to ecosystems but often to human lives and cultures.

A possible use of existing hydro that the author didn't mention, as far as I could see, was as storage for other renewables that are intermittent. Filling the reservoir when it is windy and sunny and draining it when it is neither is an essentially "free" way to even out the intermittency that is so famously put forward as a drawback to these true renewables.

What seems to be a permanent drying of the west will make hydro less and less viable in much of that part of the country. How low is Lake Powell now? Over 100 feet?