The Tijuana River Estuary is an intertidal coastal wetland at the mouth of the Tijuana River in San Diego County, California, in the United States bordering Tijuana, Baja California, Mexico. It is the location of the Tijuana River National Estuarine Research Reserve, Tijuana Slough National Wildlife Refuge, and Border Field State Park. The estuary is a shallow water habitat. Often termed an intermittent estuary since its volume is subject to the discharge controlled by the seasons of the year, the volume of the estuary fluctuates and at times there is dry land, or flooded areas. It was designated a National Natural Landmark in 1973.