Grand Canyon Pumped Storage
Pumped hydro energy storage requires two nearby reservoirs with a large elevation change between them. Finding a suitable location with two very voluminous canyons and a large vertical drop between them is difficult - the highest energy (peak capacity times hours) storage pumped storage site in the US is the Raccoon Mountain Pumped-Storage Plant near Chattanooga, Tennessee. Power companies aren't stupid; they build the best sites first, so to get more pumped storage we will need a much larger number of lower capacity sites, eventually destroying the all the high canyons in the US if we look for a lot of pumped storage.
As long as "green energy" is driving us to destroy nature, let's evaluate the highest capacity site in the US - the Grand Canyon. We will call this awful idea LMOC: Lake Mead On Crack.
The volume of the Grand Canyon is 4 trillion cubic meters; but we need at least two reservoirs, and they must have a large elevation change between them. Let's assume we can move water equivalent to 1/4 of the volume - 1 trillion cubic meters. The Grand Canyon drops about 600 meters from one end to the other - again, lets divide that by 2, for the average upper and lower depth, resulting in 300 meters. The gravitational energy of a cubic meter of water lifted one meter is about 10,000 joules. And as the upper reservoir empties and the lower reservoir drops, the distance shrinks to zero - divide the maximum drop by two. So, the total energy storage in LMOC is 10,000 times 1 trillion times 300 over 2 , or 1.5e18 joules. Chances are, this is way too optimistic.
Total US electric power use averages about 3.3e12 watts, so at 100% efficiency (Hah!) we could store 5.25 days of electrical power in LMOC. If we moved to an all-electric economy. replacing transportation fuel with electricity, we might reduce the current 16TW total energy consumption to a mere 10TW. Assuming a more realistic 50% efficiency for generation, pumps, and continent-spanning transmission lines, LMOC would store 0.9 days of power. That might be enough to do day/night averaging for the country, but not average over large weekly variations in wind or solar power.
And, of course, LMOC would be a monstrous crime against nature. If we want to use un-schedulable energy sources like wind that mostly add noise to the power grid, atrocities like LMOC are in our future.