Width and depthThe Chesapeake Bay is about 200 miles (300 km) long. At its narrowest point, the Bay is 2.8 miles (4.5 km) wide. At its widest point, just south of the mouth of the Potomac River, it is 30 miles (50 km) wide. The Bay and its tributaries contain an astounding 11,684 miles (18,804 km) of shoreline.
Much of the Bay is quite shallow; more than 24 percent of the Bay is less than 6 feet (2 m) deep. The average depth is 21 feet (7 m). The deepest channel in the Bay is 175 feet (53 m).
WatershedThe Chesapeake Bay watershed is about seven times larger than the state of New Hampshire, encompassing approximately 64,000 square miles (166,000 sq km). It takes six days for water to flow from the farthest corner of the watershed—the head or source waters of the Susquehanna River in New York—to Havre de Grace, Maryland, where it empties into the Bay. Each year, the Susquehanna River transports more than a million tons of sediment to the Bay. See a larger map of the Chesapeake Bay watershed.
TributariesThe Chesapeake Bay has the largest land-to-water ratio (14:1) of any coastal water body in the world. More than 100,000 streams and rivers thread through the watershed and eventually flow into the Bay. Everyone within the Chesapeake Bay watershed is just minutes from one of the streams or rivers. These tributaries are direct conduits carrying runoff and pollution into the Bay.
SalinitySalinity is the primary physical and ecological variable that changes through the length of the Bay. Brackish water, a combination of saltwater and freshwater, fills most of the Bay. The mingling of the freshwater with ocean water creates an estuary. The Chesapeake Bay is the largest estuary in North America, and one of the largest in the world.
Water circulationThe Bay contains two distinct layers of water. Warmer, less dense freshwater flows near the surface of the Bay from the north to the south and into the Atlantic Ocean. Denser, colder, and saltier ocean water flows near the bottom of the channel from the ocean up into the Bay. A zone of intensive mixing called the pycnocline separates these two layers.
This stratification, or layering, of water varies depending on the season and weather conditions. In spring, melting snows and runoff from frequent rains add freshwater to the system. In autumn, the freshwater cools faster than the deep saltwater. The freshwater layer sinks as it cools, usually resulting in rapid mixing. As the freshwater sinks and the saltwater rises, nutrients from the Bay's bottom are pulled nearer to the surface where phytoplankton and other organisms can utilize them. The sinking freshwater carries much-needed dissolved oxygen to deep waters. Where mixing occurs, nutrients and sediment are suspended. The available nutrients and minerals in the water create ideal breeding and nursery areas for fish and other Bay dwellers.