Charlotte Harbor Estuary is a natural estuary spanning the west coast of Florida from Venice to Bonita Springs on the Gulf of Mexico and is one of the most productive wetlands in Florida. The estuary has a large watershed, and includes Charlotte Harbor itself as well as the Peace River, Caloosahatchee River (via Pine Island Sound) and Myakka River basins. It covers 12,653 square kilometers (4,885 sq mi), the second largest open water estuary in the state. It is classified as a bar-built estuary, formed when sandbars build up along the coastline. The sand bars block the waters behind them from the sea.