The Chesapeake Bay is about 200 miles (300 km) long, from the Susquehanna River in the north to the Atlantic Ocean in the south. At its narrowest point between Kent County's Plum Point (near Newtown) and the Harford County shore near Romney Creek, 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. Total shoreline for the Bay and its tributaries is 11,684 miles (18,804 km), and the surface area of the bay and its major tributaries is .
The bay is spanned in two places. The Chesapeake Bay Bridge crosses the bay in Maryland from Sandy Point (near Annapolis) to Kent Island; the Chesapeake Bay Bridge-Tunnel in Virginia connects Virginia Beach to Cape Charles.
The word Chesepiooc is an Algonquian word commonly believed to mean "Great Shellfish Bay"; however, a reconstruction of Virginian Algonquian implies that the word may mean something like "Great Water" or it might just be the name of a village at the mouth of the bay.
The Chesapeake Bay is the ria, or drowned valley, of the Susquehanna, meaning that was where the river flowed when sea level was lower. It is not a fjord, as the Laurentide Ice Sheet never reached as far south as the northernmost point on the bay. The Bay was formed starting about 10,000 years ago when rising sea levels at the end of the last ice age flooded the Susquehanna river valley.
The Bay's geology, its present form and its very location have also been affected by a bolide impact event at the end of the Eocene (about 35.5 million years ago), forming the Chesapeake Bay impact crater. Parts of the bay, especially the Calvert County, Maryland coastline, are lined by cliffs composed of deposits from receding waters millions of years ago. These cliffs, generally known as Calvert Cliffs, are famous for their fossils, especially fossilized shark teeth, which are commonly found washed up on the beaches next to the cliffs. Scientists' Cliffs is a beach community in Calvert County named for the desire to create a retreat for scientists when the community was founded in 1935.
Much of the bay is quite shallow. At the point where the Susquehanna River flows into the bay, the average depth is , although this soon diminishes to an average of from the city of Havre de Grace for about , to just north of Annapolis. On average, the depth of the bay is 21 feet (7 meters), including tributaries; over 24% of the bay is less than deep.
The climate of the area surrounding the bay is primarily humid subtropical, with hot, very humid summers and cold to mild winters. Only the area around the mouth of the Susquehanna River is continental in nature, and the mouth of the Susquehanna River and the Susquehanna flats often freeze in winter. It is exceedingly rare for the surface of the bay to freeze in winter, as happened most recently in the winter of 1976-1977.
Since the bay is an estuary, it has fresh water and brackish water. Brackish water has three salinity zones — oligohaline, mesohaline, and polyhaline. The fresh water zone runs from the mouth of the Susquehanna River to north Baltimore. The oligohaline zone has very little salt. Salinity varies from 0.5 ppt to 10 ppt and freshwater species can survive there. The north end of the oligohaline zone is north Baltimore and the south end is the Chesapeake Bay Bridge. The mesohaline zone has a medium amount of salt and runs from the Bay Bridge to the mouth of the Rapahannock River. the salinity ranges from 10.7 ppt to 18 ppt. The polyhaline zone is the saltiest zone and some of the water can be as salty as sea water. It runs from the mouth of the Rappahannock River to the mouth of the bay. The salinity ranges from 18.7 ppt to 36 ppt. (36 ppt is as salty as the ocean.)
Captain John Smith of England explored and mapped the bay between 1607 and 1609. The "Captain John Smith Chesapeake National Historic Trail", the United States' first-ever all-water National Historic Trail, was created in July 2006. The bill passed by voice vote in the House of Representatives and by unanimous consent in the Senate.
Today, the Calvert Cliffs Nuclear Power Plant uses water from the bay to cool its reactor.
The largest rivers flowing into the bay, from north to south, are:
The bay was once known for its great seafood production, especially blue crabs, clams and oysters. The plentiful oyster harvests led to the development of the skipjack, the state boat of Maryland, which is the only remaining working boat type in the United States still under sail power. Other characteristic bay area workboats include:
Today, the body of water is less productive than it used to be, because of runoff from urban areas (mostly on the Western Shore) and farms (especially on the Eastern Shore), overharvesting, and invasion of foreign species. The bay though, still yields more fish and shellfish (about 45,000 short tons or 40,000 tonnes yearly) than any other estuary in the United States.
The bay is famous for its rockfish, also known as striped bass. Once on the verge of extinction, rockfish have made a significant comeback due to legislative action that put a moratorium on rockfishing, which allowed the species to repopulate. Rockfish are now able to be fished in strictly controlled and limited quantities.
Oyster farming is a growing industry for the bay to help maintain the bay's productivity as well as a natural effort for filtering impurities in the bay in an effort to reduce the disastrous effects of man-made pollution.
In 2005, local governments began debate on the introduction to certain parts of the bay of a species of Asian oyster, to revive the lagging shellfish industry.
In the 1970s, the Chesapeake Bay contained one of the planet's first identified marine dead zones, where hypoxic waters were so depleted in oxygen they were unable to support life, resulting in massive fish kills. Large algae blooms, nourished by the runoff of farm and industrial waste throughout the watershed, prevent sunlight from reaching the bottom of the bay. The resulting loss of aquatic vegetation has depleted the habitat for many of the bay's animal creatures. One particularly harmful algae is Pfiesteria piscicida, which can affect both fish and humans. The depletion of oysters due to overharvesting and damaged habitat has had a particularly harmful effect on the quality of the bay. The bay's oyster industry has also suffered from two diseases: MSX and Dermo. Oysters serve as natural water filters, and their decline has further reduced the water quality of the bay. Water that was once clear for metres is now so turbid that a wader may lose sight of his feet before his knees are wet.
Efforts of federal, state and local governments, working in partnership through the Chesapeake Bay Program, and the Chesapeake Bay Foundation and other nonprofit environmental groups, to restore or at least maintain the current water quality have had mixed results. One particular obstacle to cleaning up the bay is that much of the polluting substances arise far upstream in tributaries lying within states far removed from the bay itself.
It has been estimated that in pre-colonial times, oysters could filter all the water in the Bay in about 3.3 days, but in 1988 it was calculated that depletion of oyster beds had increased this time to 325 days. Efforts at oyster farming is one method Marylanders are using to clean up the pollution.