Estuaries are bodies of water where freshwater from rivers and streams mixes with saltwater from the ocean. They are semi-enclosed coastal bodies of water that are connected to the open sea. Estuaries are formed by a tidal bore, and water continually circulates into and out of them. Tides create the largest flow of saltwater, while river mouths create the largest flow of freshwater. There are four different kinds of estuaries: coastal plain estuaries, tectonic estuaries, bar-built estuaries, and fjord estuaries. Estuaries are home to unique plant and animal communities that have adapted to brackish water, a mixture of fresh water draining from the land and salty seawater. They are delicate ecosystems that provide essential habitat for wildlife, offer educational opportunities for students, and serve as living laboratories for scientists. Estuaries are lined with marshes and sea grasses that filter water flowing to the ocean and act as a buffer protecting us from coastal storms.