An estuary is an ecosystem formed by the interaction of freshwater from rivers and streams with saltwater from the sea. The San Juan Bay Estuary system is a complex network of bays, lagoons, marshes, and mangroves interconnected by channels. Its watershed encompasses 3,400 acres—extending 16.5 miles along the San Juan metropolitan area's coastline and across eight municipalities. The San Juan Bay Estuary Program is a non-profit organization that undertakes comprehensive, cross-sectoral projects to improve the water quality of this important ecosystem. Its agenda is of national importance as it is part of the United States National Estuary Program, established by the federal Clean Water Act.