The Tijuana Estuary is the largest coastal wetland in Southern California and it is located on the international border between the U.S. and Mexico. The estuary is primarily a shallow water habitat, though it is often termed an "intermittent estuary," as it is subjected to extreme changes in streamflow at different times of the year. Extended periods of drought leave parts of the estuary dry during some periods, while flooding inundates the same areas during others. For this reason, Tijuana Estuary is considered to be a very unique part of the National Estuarine Research Reserve System.