The Central Valley is not a desert and has never been one.
Same can be said for the Sierras.
Average rainfall in Sacramento is 21 in per year and average Sierra snow pack above it, in Apr is about 12-16ft.
Southern California and the Central Valley have always been deserts, or lowland steppe; with a few riparian habitats along permanent or semi permanent river courses.
With the exception of seasonal vernal pools any standing water in the Central Valley was generally created by man.
If you look along the western slope of the Sierra Nevada mountains you primarily see on the lower slopes open oak woodland, grassland and lower elevation chaparral.
As you go up in elevation it transitions to pinyon/juniper/scrub oak/chaparral.
Go up higher and it transitions again to redwood/pine forests.
The westside foothills up into the coastal ranges have always been oak/grassland/chaparral, until they started farming those foothills in the early to mid 20th century.
The valley floor was always high desert/grassland (think Antelope Valley) until irrigation and farming started in the late 19th century.
In essence the central valley floor was always desert/grassland/scattered oak forest, until it was irrigated. Good for grazing livestock but not much else.