Very nice finds Reed!

If you were at the spot I think you were, that is out of the Diablo Range Garter Snake range. The snake would be an aquatic intergrage, Thamnophis atratus intergrade.
check the page; Thamnophis atratus intergrades

The other "unknown grey snake" was probly a C. c. mormon, yellow bellied racer. I've seen them out there before on several times. Too fast to get ahold of usually
Coluber constrictor mormon (=Coluber mormon) - Western Yellow-bellied Racer

Sincerely, Jerry