That's a good question without one definite answer!
Generally speaking, an estuary is anywhere that a river meets the ocean, typically seen as a place of mixing between fresh and salt water (though not always!).
The largest example of this in the United States is the Chesapeake Bay, which is 200 miles long and holds more than 18 trillion gallons of water. Along the length of an estuary, the water will shift from being mostly fresh near the continent to being mostly saline near the sea. Since many estuaries open up into the ocean, the tides will usually have an impact on their waters, altering height and salinity. These factors make estuaries extremely dynamic ecosystems able to sustain a rich variety of habitats and species, making them very attractive places to live for both animals and people! In the United States, estuaries provide habitat for more than three-quarters of all commercial and recreational fish catch, and of the 32 largest cities in the world, 22 are located on estuaries.
Do you live near an estuary?
Before you answer, remember: not all estuaries have salt water! In Ohio, Old Woman Creek is an example of a freshwater estuary, where river water meets lake water, as the creek pours out into Lake Erie. Freshwater estuaries can be found across the Great Lakes, with more than 20 just on Wisconsin's Lake Superior shore.
The Chesapeake Bay is 4,480 square miles and its watershed includes six states - that's pretty big! If you want to see more of this majestic estuary without driving through Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia, and the District of Columbia to do so, check out the virtual tour from the Chesapeake Conservancy.
Sources: American Institute of Biological Sciences “The Value of Healthy Estuaries” http://www.actionbioscience.org/environment/christian.html; Chesapeake Bay Program “Facts & Figures” http://www.chesapeakebay.net/discover/bay101/facts; National Estuarine Research Reserve System “An Estuary is…. Freshwater Estuaries” http://estuaries.noaa.gov/About/Default.aspx?ID=216; National Estuarine Research Reserve System “Quick Facts About Estuaries” http://www.nerrs.noaa.gov/ECDefault.aspx?ID=414; Sapper, B. (2008). St. Louis River freshwater estuary. Wisconsin Great Lakes Chronicle. Retrieved from http://lsnerr.uwex.edu/Docs/Nom-Docs/Article-St-Louis-Estuary.pdf; Virginia Institute of Marine Science “How Big is the Bay?” http://www.vims.edu/bayinfo/faqs/estuary_size.php)