The Ashepoo Combahee Edisto Basin National Estuarine Research Reserve (often shortened to ACE Basin NERR) is a 140,000-acre (570 km2) reserve area located in the ACE Basin, one of the largest undeveloped estuaries on the Atlantic coast of the United States. It is named for the Ashepoo, Combahee and Edisto Rivers, which flow past cypress swamps, historic plantation houses, old rice fields and tidal marshes to meet at South Carolina's biologically rich St. Helena Sound.