Asking on behalf of my friend Gabby Manson as she reposted the reference pic attached on her IG story to ask if anyone knew of anywhere in the Southern California region for a safe, muddy landscape to shoot in. [Photo source: nowdoyoucare on IG - personally unsure of if this is their content, but I wanted to credit anyway just in case]. I suppose I'd like to know for myself as well ( and I'm sure my friend Froggie would like to know too since I know she's absolutely dying for the opportunity to play in mud like this ) lol. TIA !
The difficulty is the safe and public aspect of what you're asking for which greatly restricts the list of possibilities and excludes all tidal areas.
You might want to try the playa lake just north of Tecopa Hot Springs, California on the east side of the highway but only after there has been a decent rain. Beware that the biting flies make being there miserable most of the time.