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Malva neglecta
stream.
Hemma, I've hiked on Mt. Diablo, but I'm from San Francisco/South San Francisco. My friend moved from SF to Walnut Creek, but I don't know what high school he went to.
The ID suggestion is based on a couple of assumptions about the size of the flower and leaves. If it's not M. neglecta, is something else in the mallow family (Malvaceae). Calflora says it has been spotted in Contra Costa county (as a kid, I'd visit my best friend in Walnut Creek every summer).
If you come across this again, can you get a picture of the leaves, including whether they have opposite or alternate arrangement?