San Elijo Lagoon, Coastal wetland reserve in Cardiff-by-the-Sea, US

San Elijo Lagoon is the terminus of the Escondido Creek watershed, where that creek's fresh water meets the Pacific Ocean behind a barrier beach at Cardiff State Beach. The reserve covers 915 acres (370 ha) in southern Encinitas, bordered by Solana Beach to the south. Six plant communities lie within it: coastal strand, salt marsh, riparian scrub, coastal sage scrub, freshwater marsh and mixed chaparral.

Native peoples hunted and gathered along the estuary at least 8,500 years ago, leaving shell middens; the Kumeyaay later used the area seasonally. In 1769 the Portolá expedition named it San Alejo for Saint Alexius. From 1880 to 1940 dikes and levees supported duck hunting, salt harvesting and sewage ponds, while the Santa Fe Surf Line (1887), Pacific Coast Highway (1891) and Interstate 5 (1965) restricted tidal flow.

The name San Elijo derives from San Alejo, given by the Portolá Expedition in 1769 in honor of Saint Alexius. The Kumeyaay traveled seasonally between coast and inland to use the lagoon's resources. Today the San Elijo Lagoon Nature Center, opened in 2009, displays exhibits on Native American history and the lagoon's plant and animal communities.

The lagoon is accessible daily from sunrise to sunset through multiple entry points with trails of varying difficulty. Bring binoculars and wear layers, as wind off the water can change weather conditions quickly throughout the day.

The lagoon's mouth at Cardiff State Beach is mechanically dredged and reopened each spring after the last winter storm, restoring tidal circulation between lagoon and ocean. Continuous and weekly water testing for dissolved oxygen, temperature, salinity, chlorophyll and pH decides when this reopening happens.