I haven't seen that many stream salamanders in California, but I think this is a Coastal Giant Salamander (Dicamptodon tenebrosus). This species is one of the largest salamanders in North America — adults can be 33 cm (13") long! It ranges from Mendocino County (Point Arena) north to British Columbia.
The Coastal Giant Salamander is now separated from the California Giant Salamander (Dicamptodon ensatus), which is found in the San Francisco Bay area, including Sonoma County. The most recent local sighting I've heard about was in Coleman Valley.
Coastal Giant Salamander larvae have broad heads, a mottled color pattern, and prominent tail fins (see below).
Below are close-ups of the head — the first out of water and the second under water (look for the gills at the back of the head).
Giant salamander larvae live in streams for 1.5–3 years before metamorphosing into adults. Terrestrial adults live underground or on damp forest floors, often within 50 meters of a stream.
For photos of the adults (and a couple of videos of the larvae), check the California Herps website.
P.S. If anyone disagrees with this identification, please let me know!