More knee-jerk stupidity.
The <a href="https://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant" rel="nofollow">Helms Pumped Storage Plant</a> has an upper reservoir with a capacity of 151,718,266 m^3, and a lower reservoir with a capacity of 159,119,157 m^3, Let's assume that about 100,000,000 m^3 is transported daily between these reservoirs. The head is about 500 meters.
That's 10^8 tons * 10^4 newtons/ton * 500 meters = 5*10^14 Joules (Watt-seconds). Divide this by 100,000 seconds for daily balancing with some margin, yields 5*10^9 watts, 5 GWatts. With plenty of extra margin.
From <a href="http://energyalmanac.ca.gov/electricity/total_system_power.html" rel="nofollow">here</a>:<blockquote>In 2014, total system power for California was 293,268 gigawatt-hours (GWh), about 1 percent lower than 2013. California's in-state electricity production remained virtually unchanged from 2013 levels at 198,908 GWh, a difference of less than 1 percent compared to the year before.</blockquote>Dividing 293,268 GWh by 8766 (hours/year) yields an average of ~33.5 (33.45516769336071) GWatts total usage, of which 5 GWatts is ~15% (0.1494537419698024).
Dividing 198,908 GWh (actual production) by 8766 yields ~22.7 (22.69085101528633) GWatts total in-state production, of which 5 GWatts is 22% (0.220353128079313).
And this is for just <b>one</b> (admittedly the largest) existing pumped storage facility. Using (slightly) under 2/3 of the total available storage capacity.
Of course new turbines/motor/generators would have to be installed (which could add significant phase stabilization), and probably bigger tunnels, etc.:<blockquote>Connecting the reservoirs, in order from upper to lower, is first a 10,511 ft (3,204 m) long head-race tunnel which turns into a 2,248 ft (685 m) long steel penstock which drops in elevation and trifurcates into three individual penstocks which feed a separate pump-generator. After the water is used to generate electricity, it is discharged into the lower reservoir via a 3,797 ft (1,157 m) long tail-race tunnel.</blockquote>