The Chesapeake Bay fills the drowned valleys of the Susquehanna and James rivers, plus tributaries that intersected the Susquehanna. The current shape of the bay is less that 10,000 years old - so Native Americans arrived in in Virginia in time to witness the conversion of multiple freshwater river channels into the brackish estuary of the modern Chesapeake Bay.
The bay is a feature on the Coastal Plain. The Coastal Plain which was created over the last 150-200 million years by erosion from the Appalachian Mountains and deposition closer to the Atlantic Ocean. Various ridges, valleys, lakes, and rivers have formed and disappeared over that time; topography and hydrography have not been static.
For at least several million years, the rivers we currently call the Susquehanna and Delaware have flowed from the uplands of New York and Pennsylvania to the Atlantic Ocean, while the Potomac River has eroded the remains of the Appalachian Mountains further south. The specific paths of those rivers have varied. They carved new channels across the Piedmont and Coastal Plain through soft sediments and hard bedrock, carrying dissolved and suspended particles downstream to be deposited on the ocean floor.
The rivers flowed down to the low spots on the edge of the continent formed by the Salisbury Embayment and Baltimore Canyon Trough. The mouths of each river have migrated in discrete "jumps," shifting to new channels after sea levels rose/fell during glaciation periods in the last 2 million years. When sea levels rose, sand and gravel was deposited at the mouth of the rivers, blocking the old channel. Deltas, with barrier islands and spits, formed at river mouths during the periods of high sea level ("highstands").
When sea levels dropped, an ancestor of today's river carved a new channel nearby, seeking the easiest path to the ocean - but bypassing the old river mouth, now blocked by accumulated sediments. The paleochannels, various scarps and terraces on the Coastal Plain, and canyons carved into the Continental Slope/Continental Rise document the different stages of dropping sea levels.1
The shape of the modern Chesapeake Bay was formed through a combination of both erosion by the Susquehanna River and deposition by ocean currents. The trigger for shifting sands and river channels was the rise and fall of the Atlantic Ocean during various ice ages; water levels were as much as 300-400 feet higher or lower than today.
Over the last 500,000 years, the Delmarva peninsula has grown towards the south, fed by sediments eroded off the Appalachian Mountains in Pennsylvania/New York and carried by ocean currents. The channel of the Susquehanna River has been pushed south, past the former deltas of the Potomac/Rappahannock/York rivers and down to the channel of the James River.
Sediments were deposited during periods of high sea level and extended the ancient Delmarva "spit," while erosion of new Susquehanna River channels occurred during periods of low sea level. Old channnels of the Susquehanna River can be identified underneath the current Eastern Shore, and on the edge of the continental shelf where "fossil" canyons such as the Washington Canyon show the former route of the river.
When the Susquehanna River carved new channels during low sea levels, they migrated to the west of the old riverbed and the river mouth moved further south as the Delmarva Peninsula grew.
The Exmore Channel, the oldest paleochannel dating back to 300,000-500,000 years ago, was the path of the combined Susquehanna and Potomac rivers east across the Continental Shelf.
After sea level rose and covered the Continental Shelf, an estuary formed at the ancient mouth of the Susquehanna River. In the low-energy estuary, without strong river currents to flush away the sediments, the Exmore Channel filled up. Rising sea levels also triggered longshore currents that brought sediments from the north. The Accomack Spit grew southward, forming the northern portion of today's Delmarva Peninsula.
When sea level dropped again roughly 150,000 years ago, in one of the Ice Age stages, the Exmore Channel was filled with sediment and the old path was blocked by the Accomack Spit. The Susquehanna River did not return to its old path or carve a gap through the Accomack Spit. Instead, the river cut a new channel in softer sediments on the west side of the estuary, creating the Belle Haven channel. To get around the barrier of the Accomack Spit, the mouth of the Belle Haven channel reached the Atlantic Ocean south of the old Exmore Channel, roughly at the latitude of today's Rappahannock River.
When sea levels rose again as ice sheets melted on the continents, an estuary developed at the mouth of the Belle Haven channel. At the same time, ocean currents deposited more sediments at the tip of the evolving Delmarva Peninsula, further blocking the Belle Haven channel. When sea level dropped again, the Susquehanna River abandoned its old path and once again carved a new channel further to the west. Repeating the earlier pattern, the new Eastville Channel formed to the west of the old (Belle Haven) channel, and with an outlet to the Atlantic Ocean further south.
Again, sea levels rose, drowning the Continental Shelf and a portion of the Coastal Plain, creating yet another estuary at the mouth of the river. The Eastville Channel filled with sediments. Longshore currents carried more sediments south from New York, New Jersey and Pennsylvania, and the Nassawadox Spit grew to the south of the Accomack Spit, extending the Delmarva peninsula.
As summarized very well by a Chesapeake Quarterly article in 2011:2
The Accomack spit, which blocked the Exmore channel, was deposited when sea levels were 12-15m above present level. The later Nassawadox spit, which blocked the Belle Haven and Eastville channels, was deposited when sea levels were 7.5-9m above present level. Ames Ridge on the Eastern Shore marks the boundary between the two.3
former channels of Susquehanna River were obliterated, as growth of Eastern Shore extended length of Chesapeake Bay (green dots show current Susquehanna River and James River channels)4
As the Laurentide ice sheet advanced to its maximum position, ocean levels dropped again with a rapid, final drop between 30-22,000 years ago.5 The Susquehanna River moved west one more time and etched the Cape Charles Channel - and once again bypassed the expanded Delmarva peninsula by migrating the river mouth further south.
When the last Wisconsin glaciation ended 18,000 years ago, sea levels rose one last time, drowning the mouth of the Cape Charles Channel and forming another estuary. Continued deposition by longshore currents extended the Delmarva Peninsula south and widened it out to Smith Island Shoals. Starting around 6,500 years ago, sediment transported from the shoals (and points north) buried the mouth of the Susquehanna River one last time, blocking the eastern end of the Cape Charles Channel. That forced the Susquehanna River over 5 miles to the south, to carve today's Chesapeake Channel to the Atlantic Ocean.
The mouth of the Chesapeake Bay is not migrating further, for the moment. The currents in the old Susquehanna and James river channels appear to be strong enough to block new sediment deposition, so the southern tip of the Eastern Shore is not growing to the south.6
west of the entrance to the Chesapeake Bay, the North Channel is the former Cape Charles Channel, but the eastern portion has been buried by 160' of sediments (including Fisherman's Island)
Source: NOAA Nautical Chart On-Line Viewer Number 122208 - Approaches to Chesapeake Bay
shape of North American when sea level was 110 meters below the present level 18,000 years ago
(there is no Chesapeake Bay, and the peninsula of Florida extends much further west)
Source: NOAA National Geophysical Data Center, Global Land One-km Base Elevation Project (GLOBE)
Virginia's eastern coastline did not move inland from the Continental Slope until about 12-15,000 years ago, when the ocean began to flood over the wide Coastal Plain of that time. Roughly 9-10,000 years ago, the rising Atlantic Ocean submerged the mouths of the James and Susquehanna rivers, and may have reached as far north as the Rappahannock River.7 Sea level rise tapered off about 6,000 years ago,8 and the bay took on its present shape about 3,000 years ago.9
The creation of the bay may have occurred in stages, rather than in a steady process. Changing chemistry in sediment cores suggest one rapid expansion of the bay occuring roughly 8,200 years ago. The Chesapeake Bay expanded in a quick surge, after a lobe of the Laurentide ice sheet melted and glacial lakes Agassiz and Ojibway rapidly released a pulse of fresh water into the North Atlantic. When the lake water drained off the continent, that event increased the volume of the oceans, triggered both a quick rise in sea level, and altered ocean currents enough to create a brief cooler-than-normal climate.10
Another possibility: the Susquehanna River valley flooded slowly within its channel until sea water finally rose onto the floodplain, creating a sudden one-time increase in the size of the bay. In this scenario, the Susquehanna River floodplain flooded quickly because of its flat topography - rather than because a pulse of meltwater (from the ice sheet covering Canada) suddenly increased the volume of the oceans.11
When the ice sheet reached into Pennsylvania, New York, and Massachusetts, rocks and sand were deposited at the melting edge by the ancestral Hudson River, and even directly by the ice sheet. Terminal moraines still remain today as Cape Cod, Nantucket Island, Martha's Vineyard, and Long Island. South of the melting edge, the Delaware and Susquehanna rivers carried glacial debris to the edge of the Atlantic Ocean.
Sand from the northern coast has been pushed south by longshore currents. In the last 18,000 years (especially in the last 6,000), those sediments were shaped by waves and currents into the beaches/barrier islands along the East Coast shoreline, south to Cape Hatteras in North Carolina. That sand may have been scraped up by the ice sheet and transported all the way from Canada during the last Ice Age, before being deposited on the East Coast. Some sand grains now at Chincoteague, on the bottom of the Chesapeake and Thimble channels, or at Virginia Beach could have come originally from Ontario, Quebec, Ohio, Pennsylvania...
former shape of Susquehanna River valley, before sea level rise created Chesapeake Bay
Source: US Fish and Wildlife Service, Vanishing Lands - Sea Level, Society, and Chesapeake Bay (Figure 2)
The James River carved its own channel towards the edge of the Continental Shelf when sea levels were lower, and even today the Susquehanna and James flow in distinct paths. Their river channels parallel each other, at the mouth of the Chesapeake Bay between Cape Charles/Cape Henry.
The two sets of tunnels for the Chesapeake Bay Bridge-Tunnel mark the Chesapeake Channel (on the north, in the Cape Charles Channel carved by the ancestral Susquehanna River), and the Thimble Shoal Channel (on the south, carved by the James River). Ships sailing to Baltimore use the Chesapeake Channel and follow the path of the drowned Susquehanna River north, while ships headed to Norfolk use the Thimble Shoal Channel and follow the drowned James River west.
The sea level of the Atlantic Ocean has been rising for roughly 18,000 years. When the ice sheets stretched to their greatest extent, the Atlantic was a much smaller ocean because so much water was still trapped in the ice. As the ice melted, fresh water surged into the oceans in various meltwater pulses, and flooded the valleys of the James River, Susquehanna River, and other tributaries to the Chesapeake Bay to create the present pattern of land and water - but that pattern will change.
Today, the Atlantic Ocean is roughly 300-350 feet (100-120 meters) higher than when glaciers reached into Pennsylvania, and sea level has been rising slowly at the rate of 6 inches/century.12
In November 2010, the Virginia Institute of Marine Science (VMS) calculated the trend of Absolute Sea Level rise for Virginia. Absolute Sea Level rise is measured relative to the center of a reference ellipsoid, in contrast to other sudies that calculate relative sea level (RSL) by comparison with fixed points on the earth's surface. Using Absolute Sea Level separates changes in the ocean's height from changes in subsidence of land, both of which affect how Virginia's shoreline will change.
The 2010 VIMS study concluded that absolute sea level rise in the Chesapeake Bay between 1976-2007 is 1.8 mm/yr, less than the worldwide average of 3.1mm/year calculated by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report in 2007. Subsidence ranged from -1.3 mm/yr to -4.0 mm/yr at different locations, causing over 50% of the change in relative sea level.13
Land subsidence is significant enough to suggest that existing coastal communities are at risk of future flooding, no matter how local or state officials may feel about global warming. Even ancient archeological sites will be affected. A 2012 study concluded that:14
However, the rate of sea level rise determines in part if an archeological site will be drowned quickly and preserved, or eroded by wave action. The same study noted:15
If sea level rise predictions of the Intergovernmental Panel on Climate Change16 come true, the Chesapeake Bay could disappear completely and become a salt water gulf comparable to the Gulf of Maine, after the Eastern Shore is covered completely by a rising Atlantic Ocean.
major rivers providing sediment to form Delmarva Peninsula
Source: US Geological Survey, National Atlas