That's the point you can pump the water higher using excess renewable energy (in the event we get past 100% peak renewable power) to be used later when there's a deficiency. It's a fairly efficient method of time-shifting renewables without having to mine huge piles of lithium.

Pumped hydro if geography permits is very good storage method. Efficiency is not horrible, it is simple to ramp up and down. Technology is extremely mature. It scales pretty well. And as final bonus in some cases it can contribute to grid stability.

Yeah the biggest downside is the ecological impact of the flooding involved in creating a new hydro source. They could maybe look at retrofitting it onto an existing hydroelectric dam so it's not flooding a new area.

A hydro power reservoir doesn't need a watershed, so can have overwhelmingly less ecological impact.

It still needs volume to flood to hold the water though and unless we're building up artificially the cheap way to do that is to build fairly wide. All that area is habitat and covering it has an impact. In the end it's probably well worth it so long as we're not destroying rare habitats to do it but it is still destruction that has to be considered and weighed.

Pumping desalinated water up to Sierra Nevada reservoirs would take a hell of a lot of pipe. We use canals and rely on gravity to move water, today.

I wonder whether the most efficient way to get water to the Sierra Nevada reservoirs would be to pump ocean water onto the desert, and let evaporation take it from there. Siphoning from the ocean into Death Valley, you wouldn't even need to pump it.

I'm going to assume you mean "up there" as in northern cali -- if we could get some clean hydrogen/electric transport trucking it shouldn't be impossible to move it around either!