Editing Chesapeake Bay (section)

==Physical geography==
===Geology and formation===
[[File:Chesapeake Crater boundaries map.png|thumb|Boundaries of the [[Chesapeake Bay impact crater]].]]
The Chesapeake Bay is an [[estuary]] to the North Atlantic, lying between the [[Delmarva Peninsula]] to the east and the North American mainland to the west. It is the ''[[ria]]'', or drowned valley, of the [[Susquehanna River]], meaning that it was the alluvial plain where the river flowed when the sea level was lower. It is not a ''[[fjord]]'', because the [[Laurentide Ice Sheet]] never reached as far south as the northernmost point on the bay. North of [[Baltimore]], the western shore borders the hilly [[Piedmont (United States)|Piedmont]] region of Maryland; south of the city the bay lies within the state's low-lying [[Atlantic coastal plain|coastal plain]], with sedimentary cliffs to the west, and flat islands, winding creeks and marshes to the east. The large rivers entering the bay from the west have broad mouths and are extensions of the main ria for miles up the course of each river.

The bay's geology, its present form, and its very location were created by a [[bolide]] [[impact event]] at the end of the [[Eocene]] (about 35.5 million years ago), forming the [[Chesapeake Bay impact crater]] and much later the Susquehanna River valley. 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.<ref name="fs102-98"/> 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 State Park|Calvert Cliffs]], are famous for their [[fossil]]s, 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.<ref>{{cite web|url=http://www.scientistscliffs.org/HTML/FAQs/FAQs.htm |title=FAQ |publisher=Scientists Cliffs community |access-date=2008-05-08 |url-status=dead |archive-url=https://web.archive.org/web/20090107050207/http://www.scientistscliffs.org/HTML/FAQs/FAQs.htm |archive-date=January 7, 2009 }}</ref>

===Hydrology===
[[File:Sunset - Eastern Bay 5.jpg|thumb|View of the eastern bay in Maryland at sunset]]
[[File:Annapolis, Maryland, Usa P1010854.jpg|thumb|The [[Chesapeake Bay Bridge]], near Annapolis, Maryland]]
Much of the bay is shallow. At the point where the Susquehanna River flows into the bay, the average depth is {{convert|30|ft|m|0|}}, although this soon diminishes to an average of {{convert|10|ft|m|0}} southeast of the city of [[Havre de Grace, Maryland]], to about {{convert|35|ft|m|0}} just north of Annapolis. On average, the depth of the bay is {{convert|21|ft|m}}, including tributaries;<ref>{{cite web |url=http://www.cbf.org/site/PageServer?pagename=exp_sub_watershed_geography |title=Geography |publisher=Chesapeake Bay Foundation |access-date=2008-04-21 |url-status=dead |archive-url=https://web.archive.org/web/20071014181514/http://www.cbf.org/site/PageServer?pagename=exp_sub_watershed_geography |archive-date=October 14, 2007 }} Other sources give values of 25 feet (e.g. {{cite web |url=http://www.msa.md.gov/msa/speccol/sc2200/sc2269/html/chartfront.html |title=Charting the Chesapeake 1590–1990 |publisher=Maryland State Archives |access-date=2008-04-21 |archive-date=2021-02-15 |archive-url=https://web.archive.org/web/20210215094045/https://msa.maryland.gov/msa/speccol/sc2200/sc2269/html/chartfront.html |url-status=dead }}) or {{convert|30|ft|m}} deep ({{cite web |url=http://ian.umces.edu/pdfs/iannewsletter1.pdf |title=Healthy Chesapeake Waterways |publisher=University of Maryland Center for Environmental Science |access-date=2008-04-21 |archive-url=https://web.archive.org/web/20080626004705/http://ian.umces.edu/pdfs/iannewsletter1.pdf |archive-date=2008-06-26 }})</ref> over 24 percent of the bay is less than {{convert|6|ft|m|0|abbr=on}} deep.<ref name="Chesapeake Bay Program: A Watershed Partnership: Facts & Figures">{{cite web |url=http://www.chesapeakebay.net/factsandfigures.aspx?menuitem=14582 |title=Chesapeake Bay Program: A Watershed Partnership: Facts & Figures |publisher=Chesapeake Bay Program Office |date=2010-05-04 |access-date=2010-10-29 |url-status=dead |archive-url=https://web.archive.org/web/20101204081804/http://www.chesapeakebay.net/factsandfigures.aspx?menuitem=14582 |archive-date=2010-12-04 }}</ref>

Because the bay is an estuary, it has fresh water, salt water and [[brackish]] water. Brackish water has three [[salinity]] zones: oligohaline, mesohaline, and [[polyhaline]]. The freshwater zone runs from the mouth of the Susquehanna River to north [[Baltimore]]. The oligohaline zone has very little salt. Salinity varies from 0.5&nbsp;[[Concentration#"Parts-per" notation|ppt]] (parts per thousand) to 10&nbsp;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 [[Rappahannock River]]. Salinity there ranges from 1.07% to 1.8%. 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 1.87% to 3.6%. (3.6% is as salty as the ocean.)<ref>{{cite web |url=https://www.researchgate.net/figure/232679222_fig1_Figure-1-Map-of-Chesapeake-Bay-and-salinity-zones-Salinity-zones-based-on-Chesapeake |title=Figure 1. Map of the Chesapeake Bay and salinity zones. Salinity zones... |website=ResearchGate }}</ref>

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 [[mudflat|flats]] often freeze in winter. It is rare for the surface of the bay to freeze in winter, something that happened most recently in the winter of 1976–77.<ref>{{Cite magazine |magazine=Time |title=The Big Freeze |date=1977-01-31 |url=http://www.time.com/time/magazine/article/0,9171,918620-2,00.html |archive-url=https://web.archive.org/web/20080308235254/http://www.time.com/time/magazine/article/0,9171,918620-2,00.html |url-status=dead |archive-date=March 8, 2008 |access-date=2007-03-19 }}</ref>

The Chesapeake Bay is the end point of over 150 rivers and streams.<ref>{{cite web |title=Facts & Figures |url=https://www.chesapeakebay.net/discover/facts |access-date=18 August 2021 |publisher=Chesapeake Bay Program}}</ref> The largest rivers flowing directly into the bay, in order of discharge,<ref>{{cite journal |last1=Zhang |first1=Qian |last2=Brady |first2=Damien C. |last3=Boynton |first3=Walter R. |last4=Ball |first4=William P. |title=Long-term trends of nutrients and sediment from the nontidal Chesapeake watershed: An assessment of progress by river and season |journal=Journal of the American Water Resources Association |date=2015 |volume=51 |issue=6 |pages=1534–1555 |doi=10.1111/1752-1688.12327|bibcode=2015JAWRA..51.1534Z |s2cid=129432081 }}</ref><ref>{{cite journal |last1=Sprague |first1=Lori A. |last2=Langland |first2=Michael J. |last3=Yochum |first3=Steven E. |last4=Edwards |first4=Robert E. |last5=Blomquist |first5=Joel D. |last6=Phillips |first6=Scott W. |last7=Shenk |first7=Gary W. |last8=Preston |first8=Stephen D. |title=Factors Affecting Nutrient Trends in Major Rivers of the Chesapeake Bay Watershed. Water Resources Investigations Report 00-4218 |date=2000 |url=https://va.water.usgs.gov/online_pubs/WRIR/00-4218.htm}}</ref> are the [[Susquehanna River]], [[Potomac River]], [[James River]], [[Rappahannock River]], [[York River (Virginia)|York River]], [[Patuxent River]], and [[Choptank River]].

For more information on Chesapeake Bay rivers, see the [[List of Chesapeake Bay rivers]].

[[File: Chesapeake Bay from airplane.jpg|thumb|The bay viewed from a plane]]