Bay Basics

The Chesapeake Bay is an estuary – What is an estuary? 

Estuaries are bodies of water in which freshwater flowing in from streams and rivers mixes with 
salt water coming from the ocean. Estuaries host some of the most productive ecosystems in the 
world. The water at the head of Chesapeake Bay, (the end of the bay furthest from the ocean) is 
fresh – that is it contains no measurable salinity.  The water at the mouth of the bay (where it 
empties into the Atlantic Ocean) is just like seawater – that is, it contains high concentrations of 
sea salts. The further we travel from the mouth of the bay towards the head (the upper bay where 
we are) lower the salt concentrations are, and the fresher it is. Bay water that is somewhere 
between saltwater and freshwater is called brackish water. 
 
Saltwater is denser (we’ll say heavier) than fresh water because of its high concentrations of salt. 
Since it is heavier, it flows up the bay from the ocean along the bottom, while freshwater flows 
over the saltwater. So if we checked the salinity somewhere around the Bay Bridge (near 
Annapolis), we would find that the water on the bottom of the bay is saltier than the water at the 
surface. Both samples would be considered brackish. 

Healthy estuaries around the world host extremely productive ecosystems because they provide 
nutrients and habitats for a wide variety of plants and animals from the ocean to the rivers – they 
maintain a splendid biodiversity. The Chesapeake Bay is one of the largest and most productive 
estuaries in the world. Its present size and shape has existed for over 10,000 years – since the end 
of the last ice age. Our bay formed as continental glaciers melted, sea level rose, and the Atlantic 
Ocean flooded the Susquehanna River valley, which extended well onto the continental shelf of 
the Atlantic. 

The entire Chesapeake Bay, from its head at Havre de Grace to its mouth at Cape Henry, is tidal. 
This means that the water in the bay rises and falls twice each day. The gravity of the moon and 
sun pull on the earth causing the water of the earth’s oceans and estuaries to bulge. This bulge 
creates the twice daily rising and lowering of the bay surface. 
Upper Bay Ecosystems 

Ecosystems are groups of plants and animals that live in a particular area because of what the 
environment in that area provides, such as food, and places to live and hide from predators. Two 
of the main kinds of ecosystems on the upper Bay are fresh tidal marshes, and bay grass beds.  
Fresh tidal marshes usually form near the shores of the Bay. They grow in fresh water that is 
tidal. The plants in the fresh tidal marsh, like cattail, pickerel weed, arrowhead, and wild rice 
provide lots of food and cover (hiding places) for fish, ducks, birds, turtles, frogs, and mammals 
like the muskrat. 
   
The average depth of the Chesapeake Bay is only about 20 feet. But the Susquehanna Flats are 
even shallower with an average depth of about 5 feet. The shallowness of the Bay allows great 
expanses of underwater or bay grasses to grow. Underwater plants need the same things to grow 
as do land plants: sunlight and nutrients (food). Since the Bay and Susquehanna Flats are shallow, 
sunlight can reach the bottom in most places there allowing grass beds to grow.