The Stemple Creek watershed drains 50 square miles of Sonoma and Marin counties. The Sonoma-Marin County line cuts the Stemple Creek watershed almost exactly in half.
It begins west of Petaluma and empties into the Pacific Ocean through the Estero de San Antonio.
Estero de San Antonio is a small coastal lagoon that's part of the Gulf of the Farallones National Marine Sanctuary and the Central California Coast Biosphere Reserve.
