Boulder Creek is a census-designated place in Santa Cruz County, California, with a population of 4,923 as of the 2010 census.Boulder Creek served as the upper terminus of the San Lorenzo Valley Logging Flume terminating in Felton, which began construction in 1874 and when formally opened in October 1875 was augmented by a new rail line to transport logs to the wharf in Santa Cruz. In the 1880s, this lumber town which was called Lorenzo took the name of the Boulder Creek post office that had been established in the 1870s.