Tijuana River drains a watershed of 1.1 million acres (450,000 ha) starting from the Sierra de Juárez of northern Baja California and flowing generally west-northwest for 120 miles (195 km) to the Pacific Ocean at the southern city limits of Imperial Beach, about 11 miles (18 km) south of San Diego and 5 miles (8 km) northwest of Tijuana, Mexico. Imperial Beach is a residential community of San Diego situated at the site of a historical Kumeyaay village called Alyshuwiiin. The city was named after Imperial County in southeastern California. The river name is derived from a historical native Kumeyaay settlement called Tecuan, meaning ‘by the sea’, in the vicinity of present-day Tijuana. The river is intermittent, flowing naturally only during rains. The river is impounded in Mexico southeast of Tijuana for drinking water and irrigation and what remains crosses the border approximately 5 miles (8 km) upstream from the Pacific where it then flows generally west, skirting the international border, and entering an estuary about 2 miles (3 km) upstream from the river mouth. Concrete barriers were constructed along the riverbank in Mexico to prevent flooding, and the United States was supposed to build a similar system of flood control, but instead opted for a dissipator flood control plan that preserved the estuary. The estuary is underlain by a succession of Late Cretaceous, Paleogene, and Quaternary period, on the geological time scale, sedimentary rocks over 3,280 feet (>1,000 m) thick that overlie basement rocks. The Late Cretaceous rocks are composed of marine turbidites and continental alluvial fan deposits that were uplifted and eroded until the Middle Paleogene when a sequence of siltstone, sandstone, and conglomerate in nine layers were deposited about 2300 feet (700 m) thick. During the Late Paleocene time, the coastal margin again underwent uplift and extensive erosion followed by more sediment deposits consisting mostly of marine sandstone and transitional marine and nonmarine pebble and cobble conglomerate. The coastal margin is still being uplifted and a series of wavecut platforms have been eroded and uplifted creating marine terraces covered by Quaternary deposits of nearshore marine, beach, estuarine, lagoonal, and continental dune sediments. Each year millions of tons of fine sediment are released into the coastal waters from natural erosion of the adjacent landscape. The majority of these sediments enter from coastal streams and rivers, such as the Tijuana River, and have an important role in nutrient cycling, habitat formation, and the geological cycle.

The archaeological record suggests that humans have inhabited the southern California coast for about 12,000 years. The Kumeyaay were originally a hunter-gatherer group whose territory comprised the present-day southern San Diego and Imperial counties, the Tijuana River watershed, and northern Baja California. The Kumeyaay are divided into two groups, the northern ‘Ipai’ and southern ‘Tipai’. These people emerged around 1,000 years ago from a proto-Tipai-Ipai culture that became established about 5,000 years ago. The first European to visit the region was Juan Rodríguez Cabrillo in 1542 and met with the Kumeyaay, but did not establish a colonial settlement. In 1602, Sebastián Vizcaíno also met with a band Kumeyaay during the feast of San Diego de Alcala, giving the region of San Diego its name, but this also did not lead to colonial settlement. In 1769, the Portolá expedition landed in San Diego Bay and visited the Kumeyaay village of Kosa’aay. The Spanish then established a presidio and the Mission San Diego de Alcalá, eventually incorporating the village into the pioneer settlement. Under the Spanish mission system, Indigenous Peoples living near Mission San Diego de Alcalá were called Diegueños. After years of brutal treatment by the presidio soldiers and mission staff, the Kumeyaay rebelled and burned down the mission. The mission was rebuilt and the Spanish solidified their control over the area. In 1810, Mexico defeated Spain in the Mexican War of Independence and assumed ownership of the mission lands. The mission lands were secularized and given as land grants to Mexican soldiers and settlers who became known as Californios. In 1833, Rancho Melijo, a Mexican land grant of 4,439 acres (1,800 ha), was given by Governor José Figueroa to Santiago E. Argüello. The rancho was named after a local Kumeyaay village. The rancho extended north for 3 miles (5 km) from the San Antonio Hills along the present-day Baja border, and east for 3 miles (5 km) from the mouth of the Tijuana River including its estuary and the plain along the lower Tijuana River valley. In 1848, Alta California was ceded to the United States following the Mexican-American War and the Treaty of Guadalupe Hidalgo provided that the Mexican land grants would be honored; however, the burden of proof of title was placed on landholders. The land claim by the Argüello family was rejected by the California Land Commission partly over confusion about size. The family retained some of the lands by homesteading in the vicinity of the Otay River and San Diego Bay. The Treaty of Guadalupe Hidalgo also led to the establishment in 1889 of the International Boundary and Water Commission to maintain the border, allocate river waters between the two nations, and provide for flood control and water sanitation. Today a portion of former Rancho Melijo is included in a large open-space preserve.

Tijuana River Estuary is an intertidal coastal wetland that covers about 2,500 acres (1012 ha) at the mouth of the Tijuana River. The estuary is one of the few remaining salt marshes in southern California. It is the location of 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 southern part of the National Estuarine Research Reserve and is an important wildlife habitat, comprised of dunes and salt marshes that provide refuge to birds. Tijuana Slough National Wildlife Refuge forms the northern part of the reserve and is also part of the San Diego National Wildlife Refuge Complex. The slough is one of southern California’s largest remaining salt marshes without a road or railroad trestle running through it. This salt marsh is surrounded by San Diego County and Tijuana, Mexico, with a combined population of 5.3 million people. Within this international bioregion, the refuge maintains essential habitats for many migrating shorebirds and waterfowl along the Pacific Flyway. The refuge’s habitat and wildlife management programs focus on the recovery of endangered species through research, habitat restoration, and environmental education. The Tijuana River National Estuarine Research Reserve is one of 29 reserves created nationwide to enhance scientific and public understanding of estuaries and thereby contribute to improved estuarine management. The reserve is a partnership between federal and state agencies including the National Oceanic and Atmospheric Administration, California State Parks, and the U.S. Fish & Wildlife Service. It protects the natural and cultural resources of the Tijuana River Estuary by focusing on research and education and managing recreation and resource use. The reserve encompasses beach, dune, mudflat, salt marsh, riparian, coastal sage scrub, and upland habitats, all surrounded by the growing cities of Tijuana, Imperial Beach, and San Diego. Critical issues confronted by the reserve include habitat conservation and restoration, endangered species management, management of the wastewater from Mexico, sediment management, and the integration of recreation. Adjacent to the estuary in the nearshore ocean is the Tijuana River Mouth State Marine Conservation Area, a marine protected area that covers 1,862 acres (754 ha) and limits the removal of marine plants and animals. The conservation area is geographically connected with the Tijuana River Estuary creating 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: