Poly campus. The third largest sub-basin, Prefumo Creek, drains Laguna Lake, one of the only natural lakes in San Luis Obispo County, as well as much of the Irish Hills Natural Reserve and the eastern end of Los Osos Valley. Ecology Various barriers to fish migration have been created on San Luis Obispo Creek and its tributaries, since the city of San Luis Obispo developed. Stage Coach Dam on the upper reaches of the creek was removed in 2002. It had been built in the early 1900s to create a water supply reservoir, but was filled in with sediment. Other barriers have been removed as well, often by creating notches in the middle to concentrate low flows or adding rock weirs to back up the water over an obstacle or provide a more gradual change in elevation. North American beaver (Castor canadensis) have been thought to be non-native to San Luis Obispo Creek but Bolton recorded in "Anza's California Expeditions" that in April 1774, Father Cavaller of Mission San Luis Obispo de Tolosa gave Juan Bautista de Anza "thirty-odd beaver skins" along with other local gifts including fine Indian baskets and "the skins of eight bears, the animals for which the region was renowned". See also List of rivers of California References City of San Luis Obispo - Natural Resources Department The Land Conservancy of San Luis Obispo County Rivers of San Luis Obispo County, California San Luis Obispo, California Santa Lucia Range Rivers of Southern California