Cerrito Creek is one of the principal watercourses running out of the Berkeley Hills into San Francisco Bay in northern California. It is significant for its use as a boundary demarcation historically, as well as presently. In the early 19th century, it separated the vast Rancho San Antonio to the south from the Castro family's Rancho San Pablo to the north. Today, it marks part of the boundary between Alameda County and Contra Costa County. The main stem, running through a deep canyon that separates Berkeley from Kensington, is joined below San Pablo Avenue by a fan of tributaries, their lower reaches mostly in culverts. The largest of these is Middle or Blackberry Creek, a southern branch.