Border Field State Park is part of the Tijuana River National Estuarine Research Reserve, adjacent to the urban neighborhood of Playas de Tijuana, Mexico, located about 12 miles (19 km) south of San Diego, it lies within the city limits of Imperial Beach, California. Playas de Tijuana is the westernmost borough of Tijuana, Baja California. The Tijuana River originates at an elevation of 614 feet (187 m) in the Sierra de Juárez and flows generally westward for 120 miles (193 km), draining an arid watershed of 1,119,999 acres (453,247 ha). The river flows intermittently during rainfall and is prone to flooding. Its floodplain consists mostly of unconsolidated alluvium, including silt, sand, and cobble-sized particles. The river is impounded in Mexico for drinking water and irrigation. Its lower channel is constrained by concrete barriers to prevent flooding. The United States adopted a dissipator flood control plan that preserved the river estuary, which was subsequently designated as an estuarine research reserve and wildlife sanctuary. The lower river provides the last undeveloped coastal wetlands in southern California. In recent decades, it has been the subject of significant controversy over pollution, flood control, and U.S. border protection.

The Tijuana River National Estuarine Research Reserve protects vital saltwater and freshwater marsh habitats for migrating waterfowl and resident wading birds, including the black-necked stilt, American avocet, green-winged teal, American wigeon, and pelicans. Border Field State Park forms the southernmost part of the reserve and includes the International Friendship Park, a 0.5-acre (0.2 ha) binational park located just inland from where the border meets the ocean. The park was originally intended to be a place where people from each side of the border could meet under the supervision of the U.S. Border Patrol. The park historically was part of the Monument Mesa picnic area which is the location of a marble monument that was erected at the western end of the border near the Pacific shoreline in 1894. Today, the monument stands on the Mexican side of the border wall and is part of the Plaza Monumental de Tijuana. This plaza includes the Tijuana Lighthouse and a bullring, popularly known in English as the Bullring by the Sea. The bullring, located just 200 feet (60 m) from the Mexico-United States border, first opened in 1960. It is primarily used for bullfighting but also hosts boxing matches, concerts, and various cultural and sporting events.

In 1848, the Treaty of Guadalupe Hidalgo officially ended the Mexican-American War. It stipulated that a joint boundary survey establish the new international border, starting at Border Field. According to the treaty, the initial border point is set 3 nautical miles (5.6 km) south of the southernmost point of San Diego Bay. From there, the border proceeds in a straight line toward the confluence of the Colorado and Gila Rivers, about 140 miles (225 km) east, near Yuma, Arizona. Until 1994, a simple barbed wire fence marked the border. However, amid widespread fear of illegal immigration, 14 miles (23 km) of steel mesh fence were constructed. Despite the new barrier, people on opposite sides of the border could still touch and pass objects through it. In 2009, the U.S. Department of Homeland Security closed Friendship Park and built a second parallel fence extending into the ocean. In late 2011, a surf fence extended the barrier another 300 feet (91 m) into the ocean, and the park was reopened for limited hours on weekends. In 2022, a new border wall was approved, expected to permanently close the U.S. side of Friendship Park. Read more here and here. Explore more of Border Field State Park and Playas de Tijuana here: