Tijuana River drains a 1.1‑million‑acre (450,000 ha) watershed that originates in the Sierra de Juárez in northern Baja California and flows west‑northwest for 120 miles to the Pacific at the southern edge of Imperial Beach, about 11 miles south of San Diego and 5 miles northwest of Tijuana, Mexico. Imperial Beach, a San Diego residential community, stands on the site of the historic Kumeyaay village Alyshuwiiin. In contrast, the river’s name derives from Tecuan, a Kumeyaay settlement near present‑day Tijuana; Tecuan means ‘by the sea.’ The river runs only during rains. In Mexico, it is impounded southeast of Tijuana for drinking water and irrigation. The residual flow crosses the border about 5 miles (8 km) inland, then runs west along the international boundary before entering an estuary roughly 2 miles (3 km) upstream from its mouth. In Mexico, concrete barriers protect the riverbank from flooding. The United States, expected to build a similar system, instead chose a dissipator flood‑control plan that preserved the estuary. The estuary is underlain by over 3,280 feet (>1,000 m) of sedimentary rocks from the Late Cretaceous, Paleogene, and Quaternary periods, which overlay basement rock. Late Cretaceous formations comprise marine turbidites and alluvial fan deposits that were uplifted and eroded until the Middle Paleogene, when nine layers of siltstone, sandstone and conglomerate—about 2,300 feet (700 m) thick—were deposited. In the Late Paleocene, further uplift and erosion were followed by deposition of marine sandstone and transitional pebble-and-cobble conglomerates. Continued uplift has created wave-cut platforms and marine terraces overlain by Quaternary nearshore marine, beach, estuarine, lagoonal and dune sediments. Natural erosion releases millions of tons of fine sediment into coastal waters each year, playing a key role in nutrient cycling, habitat formation and the geological cycle.

The archaeological record indicates that humans have inhabited the southern California coast for roughly 12,000 years. The Kumeyaay, originally hunter‑gatherers, occupied what are now southern San Diego and Imperial counties, the Tijuana River watershed, and northern Baja California. Divided into northern Ipai and southern Tipai, they emerged about 1,000 years ago from a proto‑culture established 5,000 years ago. Juan Rodríguez Cabrillo first visited in 1542, meeting the Kumeyaay without settling. In 1602, Sebastián Vizcaíno encountered a Kumeyaay band during the feast of San Diego de Alcala, which lent its name to the region, though he founded no colony. In 1769 the Portolá expedition landed in San Diego Bay and visited the Kumeyaay village of Kosa’aay. Shortly afterward, the Spanish established a presidio and Mission San Diego de Alcalá, incorporating the village into their settlement. Indigenous people near the mission were called Diegueños. Brutal treatment by soldiers and mission staff provoked a rebellion that burned the mission; it was later rebuilt as Spanish control reasserted. Mexico gained control in 1810 after its war of independence, secularizing mission lands and awarding grants to soldiers and settlers—later known as Californios. In 1833 Governor José Figueroa granted Rancho Melijo, a 4,439‑acre parcel named for a local Kumeyaay village, to Santiago E. Argüello. The rancho extended 3 miles north from the San Antonio Hills along the Baja border and 3 miles east from the Tijuana River mouth, including its estuary and adjacent plain. Alta California was ceded to the United States in 1848 after the Mexican-American War. The Treaty of Guadalupe Hidalgo honored Mexican land grants but required landholders to prove title. The California Land Commission rejected the Argüello claim amid confusion over its size, though the family retained some land near the Otay River. The treaty also spurred the 1889 creation of the International Boundary and Water Commission. Today, part of former Rancho Melijo is preserved as open space.

The Tijuana River Estuary is an intertidal wetland covering about 2,500 acres (1,012 ha) at the Tijuana River’s mouth. One of southern California’s few remaining salt marshes, it hosts Border Field State Park, Tijuana Slough National Wildlife Refuge, and both are part of the Tijuana River National Estuarine Research Reserve. Border Field forms the reserve’s southern section, featuring dunes and salt marshes that provide vital bird habitat. Tijuana Slough, in the north and part of the San Diego National Wildlife Refuge Complex, is one of the region’s largest salt marshes free of roads or railroad trestles. The area, shared by San Diego County and Tijuana, Mexico, supports 5.3 million residents. In this international bioregion, the refuge preserves essential habitats for migrating shorebirds and waterfowl along the Pacific Flyway, advancing endangered species recovery through research, restoration and environmental education. Recognized for its ecological significance, the estuary serves as a living laboratory for coastal conservation, providing critical data to guide sustainable management amid urban growth and climate change. The reserve is one of 29 nationwide established to enhance understanding of estuaries. A partnership among federal and state agencies—including the National Oceanic and Atmospheric Administration, California State Parks, and the US Fish & Wildlife Service—it safeguards the estuary’s natural and cultural resources through research, education, recreation management and sustainable resource use. Its varied habitats—beach, dune, mudflat, salt marsh, riparian, coastal sage scrub and upland—lie beneath the expanding cities of Tijuana, Imperial Beach and San Diego. Key challenges include habitat conservation and restoration, endangered species management, control of Mexican wastewater, sediment management and integrating recreation. Adjacent to the estuary, the Tijuana River Mouth State Marine Conservation Area is a marine protected area that covers 1,862 acres (754 ha) in the nearshore ocean, restricting marine life removal. An integrated management approach fosters cross-border collaboration, reinforcing the region’s commitment to preserving its unique natural heritage and ensuring that conservation and sustainable use remain priorities. Together, these areas form the most intact contiguous estuarine-marine complex in southern California. Read more here and here. Explore more of the Tijuana River Estuary and Imperial Beach here: