Fresh water, salt water and eighteen million neighbors.

Where fresh meets salt

An estuary is where river water mixes with ocean water, and the Chesapeake is the largest one in the United States. Up near Havre de Grace, at the head of the Bay, the water runs close to fresh. Down toward Norfolk, where the Bay opens to the Atlantic, it's much saltier. Everything in between is some mix of the two, and the plants and animals here have sorted themselves along that line.

Wide, long and surprisingly shallow

The Bay runs about 200 miles from top to bottom and anywhere from 3.4 to 35 miles across, yet its average depth is only 21 feet. Count every marsh edge, island and tidal creek, and its shoreline comes to 11,684 miles.

Upstream is part of the Bay

More than 64,000 square miles of land drain into it, across parts of Delaware, Maryland, New York, Pennsylvania, Virginia and West Virginia, and all of Washington, D.C. More than 18.5 million people live on that land. What goes down a storm drain hundreds of miles away can end up in the same water where someone sets a crab pot.

The oyster's day job

A single adult oyster can filter more than 50 gallons of water a day. That's why oyster reefs matter to people who never eat one: they're part of how the water clears.

What's broken

When too much nitrogen and phosphorus wash in, algae bloom. When those blooms die, they decompose, and the decay uses up oxygen faster than the water can replace it. The result is a dead zone, water with little or no oxygen, and it can suffocate blue crabs and fish that can't get clear of it. That isn't only an ecology problem. It's felt by everyone who makes a living from the water.