Arriving in 1849 or 1850 was Dr Charles van Geldern.

Here he is in the 1850 census:
1850 Sonoma Co, California
Dwelling 98, Family 99
VanGelden Chas. 34 M Physician Hanover
VanGelden Eliza 34 F Brunswick

"Dr van Geldern was perhaps the most interesting of the new citizenry. Because doctors were scarce and therefore more than welcome, no one questioned then, and we do not know now, whether he had a medical degree, but he served his entire life in Sonoma as a medical jack-of-all trades, practicing pharmacy, dentistry and medicine; his office was a veritable curiousity shop, displaying skulls and bones of Indians carefully collected and preserved by the doctor; and though obviously devoted to helping mortals, he spent his leisure hours with the immortal, his hobby being necromancy. His home, doctor's office and hospital were combined in one building, which he purchased from A C McDonald on First Street East, about the middle of the block, facing the plaza." (Robert Parmalee "Pioneer Sonoma")

Doctor van Geldern wrote a description of some of the places in Sonoma facing the plaza which is published in the "History of Sonoma County" 1880, author J P Munro-Fraser, on pages 445-447