What is an estuary?

An estuary forms at a river's mouth, where the tide pushes seawater in and the river pushes fresh water out, creating brackish water. This mixing, together with nutrients carried by rivers, makes estuaries among the most productive ecosystems on Earth. They provide nursery grounds for many fish and shellfish species and feeding grounds for migrating birds. Chesapeake Bay is the largest estuary in the United States, fed by more than 150 rivers and streams. Many great cities, including London on the Thames and New York on the Hudson, grew up beside estuaries.