The site plan says it will be about 10 miles west of Maxwell, CA.
The plan is to store excess "storm water" flows from the Sacramento river. Ok, sounds reasonable...
The Sacramento river is 10 miles east of Maxwell.
Are they really going to pump large volumes of water uphill? How is that "green" and eco-friendly? It will also be wildly inefficient. To capture excess storm water flows they'll need really large capacity pumping systems, that only operate periodically. This is the opposite of efficiency. But I'm sure some engineering company charged them tens of millions of dollars for a "study" that said sure, this is a wonderful idea, you should do it immediately.
IF most of the pumping uphill is done during periods of low demand, the idea might make sense. The potential energy (well, most of it) can be extracted on the way back down.
Basically, you’ve got a giant battery.
Ameren, in Missouri, does this, albeit on a smaller scale. It works well enough that after a spectacular catastrophic failure of their upper reservoir at Taum Sauk Mountain, they rebuilt the upper reservoir.
https://www.ameren.com/missouri/company/environment-and-sustainability/hydroelectric/taum-sauk
In the CA case, they can also SELL the water itself after it comes down, also in periods of high demand. If the pumps in reverse can run as reasonably efficient generators(?), and if the river stays high enough to pump from, oh, maybe 60 days a year, on average, the whole thing might not be as cockeyed as it 1st seems.