I think that though thousands of Acres of the San Joaquin and Sacramento Valley might be underwater within the next few months that:

 The aquifers will tend to be replenished a lot from this years snow storm. So, in the long run this might be good for California even though thousands of Acres of the vast Valley in the middle of California is underwater, even if many roads go underwater too. The place I would be the most concerned about is the city of Sacramento actually because in 1862 in the ark Floods Sacramento was underwater for 6 to 12 months while the water there was 6 to 12 feet deep until it cleared then. Also, the state government had to be moved by boat to San Francisco because there were not passable roads then just the flooded Sacramento River which covered completely Sacramento then in 1862.

Could this happen again with the Sacramento River 20 miles wide like in 1862 from Redding to Stockton?

I don't know. We will have to see.