The Nacimiento River watershed comprises , with about 54 percent in San Luis Obispo County and 46 percent in Monterey County. The crest of the Santa Lucia Range, part of the Coast Ranges, forms the southwestern boundary of the Nacimiento River watershed. The San Antonio River watershed divide forms its northeastern boundary. The San Antonio River, which feeds Lake San Antonio and is also a Salinas River tributary, mirrors the Nacimiento's path several miles to the northeast.