This polygon shapefile depicts estuaries on the California coast that form at the mouths of rivers and streams where freshwater and saltwater meet, and where the salinity varies seasonally and over longer timeframes when the river mouths get closed by sand spits or other barriers. Lagoons are coastal water bodies that are cut off from the sea and generally have low freshwater inputs. Estuaries also differ in their geomorphic origin (coastal plain estuaries, river mouth estuaries, canyon mouth estuaries and tectonic estuaries). The California Marine Life Protection Act (MLPA) defines estuaries as being seaward of the mean high tide line or the mouth of a coastal river, including any area of intertidal or subtidal terrain, together with its overlying water and associated flora and fauna. These estuaries were defined for the northern study region (Point Arena to Oregon) and for the southern study region (Point Conception to Mexico). Areas were outlined by committees comprised of experts from the MLPA Initiative, the MLPA Science Advisory Team and the California Department of Fish and Wildlife biologists. All other areas are from The Nature Conservancy Ecoregional Assessment data. Most of that content is derived from the National Wetlands Inventory (NWI) published by U.S. Fish and Wildlife.