Boston Harbor is an Estuary.

What is an Estuary?

Fresh and salt water mix when a river drains to the ocean, and together they produce one of the most biologically rich and productive environments in the world, called an estuary. Estuaries create ideal wildlife habitats and provide abundant food sources for thousands of plant and animal species. Their lush marshy edges welcome wading birds and waterfowl. Seventy percent of finfish, and many species of shellfish, depend on estuaries for spawning, feeding, and a safe place to live. Boston Harbor is an important estuarine habitat. It boasts one of the state's largest herring runs; more than 200,000 fish travel annually through the harbor and up the Back River to spawn. Each of the Harbor's major tributaries, the Charles, Mystic and Neponset Rivers, also support various anadromous (i.e. migrating up rivers from the sea to breed in freshwater) fish runs. On an average winter, bird watchers can expect to see up to 39 different species wintering in the Harbor. Marine mammals, harbor seals and harbor porpoises call Boston Harbor their home every Spring and Fall.