Ummmmm ... while Enviros and Dems pretty much shut down new dam projects in the 1970s, there are multiple dams on many California rivers and creeks. So no, the rain water and snow melt doesn't just run off straight into the ocean or SF Bay. Ever heard of Lake Shasta, on the Sacramento River? Dam. Remember the Oroville Dam on the Feather River that was endangered in 2017? Yeah, dam. Folsom Dam and Nimbus Dam on the American River near Folsom and Sacramento ... dams.
Ever heard of the Yolo Bypass? It's a large flat area into which water from the Sacramento River can be released, the major weirs for releasing water having been built in 1916 and 1929. Upstream from the Yolo Bypass is the Sutter bypass.
Moving on to a few creeks, Lake Berryessa is formed by a dam on Putah Creek. Clear Lake is formed by a dam on Cache Creek. Lexington Reservoir and Lake Vasona on Los Gatos Creek are behind dams. San Pablo Reservoir is on San Pablo Creek behind the San Pablo Dam.
Obviously I could go on and on and ... if I resorted to a search engine. So, no, California rain and snowpack does not "all just run off into the ocean". !