The Los Osos Valley Groundwater Basin, DWR Bulletin 118 Basin No. 3-08, is bounded on the north by Park Ridge, on the south by Irish Hills, on the west by Morro Bay, and on the east by a drainage divide separating Los Osos Valley from San Luis Valley. The Los Osos Basin, as defined by DWR, is located in San Luis Obispo County and covers 6,990 acres (10.9 square miles).
On February 11, 2019, DWR released the 2018 Final Basin Boundary Modifications for Los Osos Valley Groundwater Basin, including the following basin modifications:
- DWR approved the creation of two jurisdictional subbasins:
- Los Osos Area Subbasin (3-08.01)
- Warden Creek Subbasin (3-08.02)
- DWR approved the removal of the southern fringe area
- DWR denied the removal of the minor northern fringe area