Think you would a better leg to stand on if California’s reservoirs actually filled up during the year.

As it stands California’s reservoirs never seem to fill completely anymore. Adding more reservoirs won’t help store extra water if it simply isn’t falling from the sky.

The Oroville dam was so full it was at risk of overtopping just a few years ago.

https://en.m.wikipedia.org/wiki/Oroville_Dam

If you distributed the total available water to more reservoirs then the loss rate from evaporation would actually go up due to an increased ratio of surface area to volume. Simple math, really.

More reservoirs doesn't make more water in the first place. This is about how much that falls from the sky in the first place.

You should not stop all the water from reaching the ocean in the first place, bad things happen when you do that. We are reaching the point where we just have enough water to keep the rivers flowing so we don't kill all the wildlife that exists in them, we're not adding significant amounts of water to storage.