The Gulf Coastal Plain's southern boundary is the Gulf of Mexico in the U.S. and the Sierra Madre de Chiapas in Mexico. On the north, it extends to the Ouachita Highlands of the Interior Low Plateaus and the southern Appalachian Mountains. Its northernmost extent is along the Mississippi embayment (Mississippi Alluvial Valley) as far north as the southern tip of Illinois. To the east the Gulf coastal plain meets the South Atlantic Coastal Plain in southern Georgia along the basin divide between the rivers flowing into the Gulf and those flowing to the Atlantic and south along the Apalachicola River through the Florida panhandle. The flat to rolling topography is broken by many streams, river riparian areas, and marsh wetlands.