An estuary is a semi enclosed coastal body of water that has a free connection with the open sea. It is formed where a river meets the sea, and the saltwater from the ocean mixes with the freshwater from the river. Estuaries are typically characterized by their brackish water, which is a mixture of saltwater and freshwater. They often have diverse ecosystems and are important habitats for many species of plants and animals. Estuaries also serve as crucial transition zones, where nutrients and sediments from the river are deposited, resulting in nutrient-rich and productive environments. Examples of well-known estuaries include the Chesapeake Bay in the United States and the Thames Estuary in the United Kingdom.