Chesapeake Bay Facts
Width and Depth
The 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.
A watershed is an area of land that drains into a particular river, lake, bay, or other body of water. The Chesapeake Bay watershed stretches from Cooperstown, New York, to Norfolk, Virginia, and includes parts of six states—Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia—and the entire District of Columbia. The 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 Chesapeake. Each year, the Susquehanna River transports more than a million tons of sediment to the Bay. See a map of the Chesapeake Bay watershed.
The 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. Major rivers that empty into the Bay include the James, York, Rappahannock, Potomac, Patuxent, Patapsco, and Susquehanna from the west and the Pocomoke, Wicomico, Nanticoke, Choptank, and Chester from the east. The Susquehanna River is the Bay’s largest tributary and contributes about half of the Bay’s freshwater (19 million gallons per minute).
The Bay receives about half its water volume from the Atlantic Ocean in the form of saltwater. The other half is freshwater that drains into the Bay from its enormous watershed.Salinity 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.
The 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.
Formation of the Bay
How did the Chesapeake Bay begin?
Geographic history—the story of the structure of the Earth—has several components that need to come together in order to tell the whole tale. The geologic history of the Chesapeake Bay includes physiographic provinces, tectonics, ice ages, and ocean processes. Each of these storylines helps us understand this complex environment.
The Chesapeake Bay watershed includes six different geologic areas, called physiographic provinces: Continental Shelf, Coastal Plain, Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateaus. The appearance of each of these geographically distinct regions is a direct result of the rocks underneath. Young, flat-lying sedimentary rocks create the Coastal Plain. An ancient ocean makes up the Piedmont province. Tectonic forces lifted 1.2-billion-year-old crystalline basement rock upward to build the Blue Ridge and Appalachian Mountains. Wrinkles in sedimentary rock layers comprise the Valley and Ridge province. These same layers form the Appalachian Plateaus.
The modern Chesapeake Bay watershed is framed by mountains on the north and west and the lower elevations on the east and south that formed from the slow collision of the African and North American tectonic plates.
Ocean and Coastal Plain
The Atlantic Ocean started to form about 200 million years ago. A rift or gap opened in the supercontinent of Gondwanaland, separating South America and Africa. Sediments eroded from higher elevations into the low-lying areas created as the Atlantic Ocean opened. The sediments draped over the edge of the continent, extending seaward for hundreds of miles to form the Coastal Plain. Glaciation frequently changed the sea level, contributing to the formation of the coastal plain. When the sea level rose, water covered the Coastal Plain, depositing marine sediments to form a type of sand that geologists call the Chesapeake Group.
Thirty-five million years ago a warm, humid tropical rain forest covered the Appalachian Mountains. In an instant, this landscape changed forever when a bolide, a large meteor, 1.8–3.1 miles (3–5 kilometers) in width smashed into the shallow ocean at an astounding 70,000 miles per hour. The bolide hurled millions of tons of debris and rocks into the atmosphere, leaving a gaping, smoldering crater in the continental shelf. A tsunami, or tidal wave, resulting from the impact devastated the eastern margin of the North American continent, wiping out much life on land and surrounding marine areas.
Ice sheets that covered the poles and Canada some 2 million years ago periodically pushed their way into the present-day United States. The ice was a major cause of changing sea levels: sea level rose as ice melted and fell as the water froze and more ice formed. Water from the melting ice sheets fed the ancestral Susquehanna River, giving it the power to move massive amounts of sediment as it cut its way to the coast. As glaciers and sea level adjusted, so did the rivers. During glacial times (lower sea level), rivers had to work their way across the exposed continental shelf to dump their sediment loads into the ocean.
Five substantial rivers feed the western half of the Bay: the Patuxent, Potomac, Rappahannock, York, and James. The Potomac is the longest and has the largest watershed. The enormous watersheds of these rivers produce huge flows of freshwater, which cause the western half of the Bay to be less salty, more silty, cluttered with woody and other vegetative debris, and generally more active than rivers on the eastern side. See a larger image of the rivers to the Bay.
The eastern rivers of the Chesapeake Bay are quite different from those in the west. The largest of these rivers draining into the Bay are the Pocomoke, Wicomico, Nanticoke, Choptank, and Chester. These eastern rivers are not particularly long, do not collect freshwater from large drainage areas, and are tidal for most of their length. Because they contribute only a fraction of the water produced by the western rivers, the eastern half of the Bay is saltier, slower moving, more given to marshes, and more productive of saltwater plants such as submerged aquatic vegetation.
Enclosing the Bay
During periods of high sea levels, the Delmarva Peninsula, the Chesapeake's "Eastern Shore," lengthened into a major barrier spit. Progressive phases of high sea levels/renewal of Delmarva Peninsula development and low sea levels/cessation of landform development continued throughout the last 2 million years.
Re-shaping the Bay
Erosion, transportation, and deposition of sediments are constantly changing the Bay's shorelines. Currents and tides erode and smooth peninsulas and headlands and deposit materials in other parts of the Bay. Rivers transport sediments and deposit them at the mouths of tributaries and along margins of the Bay, forming broad, flat deposits of mud and silt. These natural causes of sedimentation are often accelerated by human activity. By the mid-1700s, as farmers cleared more and more land for agriculture, sedimentation filled some of the navigable rivers used for travel. For example, Joppatowne, Maryland, once a seaport, is now more than 2 miles (3 km) from water. The clearing of forests and other impacts of population growth continue to alter the Bay's landforms and increase sedimentation. Sea-level rise has also changed the landscape. Many of the islands that existed in the Bay during colonial times are now submerged. For example, in the early 1600s, Poplar Island in Talbot County, Maryland, encompassed several hundred acres. Today, a chain of small islands is all that remains of the original Poplar Island.
Last updated: December 14, 2018