THE CESAPEAKE BAY BRIDGE
Geology
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 30 feet (9 m), although this soon diminishes to an average of 10 feet (3 m) from the city of Havre de Grace for about 35 miles (56 km), to just north of Annapolis. On average, the depth of the bay is 21 feet (7 meters), including tributaries;[4] over 24% of the bay is less than 6 ft (2 m) deep.[citation needed]
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.[5]
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.)
