Virginia has been shaped by its rivers. Storms have carved away at the land since the continents and the atmosphere emerged, and the rivers have carried "Virginia" mountains to the oceans before the current mountains (or oceans) even existed. What we see today is a brief snapshop, in geologic time. Come back in a billion years - old mountains will be flattened by erosion, new mountains will be uplifted by tectonic forces, and rivers will move their channels as the perpetual process of erosion continues to reshape the landscape.
Today we view the Blue Ridge and Allegheny Front as the primary topographic features directing water towards the east or west. However, Virginia's rivers have been carrying sediments downstream, reshaping the surface of the state, for eons. The New River was flowing even before the continents smashed into each other 200 million years ago, before "thundering lizards" left their dinosaur tracks in the sandstones of Triassic Basins.
Most Virginia rivers were redirected by the Appalachian Orogeny, 200 million years ago, and subsequent creation of the Atlantic Ocean. The New River is unique because it continued to flow through its old channel, and cut through the Appalachian Mountains as they rose. The current New River Gorge in West Virginia is just the most recent demonstration of how the energy in the New River's falling water can defeat the strength of rising rock...
A map of the hydrography of Virginia shows the rivers run in different directions - they don't all flow south, or "down the map." Rain falling in Southwest Virginia ends up flowing though Tennessee, Kentucky, and West Virginia to the Ohio River, then down the Mississippi River to New Orleans and the Gulf of Mexico. When the clouds move just a little further to the north or east, the rainfall will run downhill in the other direction. It can follow the Roanoke River watershed to Albemarle/Pamlico Sound, or the James River/Potomac River watersheds to the Chesapeake Bay.
When it quits raining, the rivers don't dry up immediately. Water in the ground seeps into the drainages, sometimes emerging in a defined location called a "spring" and sometimes adding more flow in less obvious ways to streams and rivers. A river will run even during a drought, as groundwater from previous rains seeps through the soil to the low spots. In a drought, those groundwater levels will gradually drop, just as a wet sponge left out in the sun will get dry at the top.
When the groundwater level drops below the level of the stream, then the "bed" of the stream - the sand, mud, or rocks in the bottom - will be exposed. In the summertime, many areas have edges of their streambeds showing rounded rocks and, occasionally, flopping fish in remaining pools of water. The rocks had rough edges when they first reached the earth's surface through erosion, but got rounded edges as the rocks were bounced around and washed downstream.
It's the natural cycle for rivers to etch the landscape, until ultimately (if left unaffected by other forces) the mountains are transformed into hills and then into a flat level plain. In the continental collisions before the Appalachian orogeny, 25,000-feet high mountains were located where Emporia and Richmond are located today. In the last 200 million years, Virginia's rivers have eroded those mountains away. Today you can see the incredible sediments carried westward, in the layers of sandstone and shale stretching from western Virginia through West Virginia to the Great Lakes.
Thousands of feet of sediments eroded westward off the mountain ranges pushed up by the Taconic and Acadian orogenies, filling the lowlands in the center of the continent and forming today's Ohio, Indiana, and Illinois. Perhaps an equal amount eroded to the east, prior to the creation of the Atlantic Ocean. After the continents split again and the Atlantic was formed, the sediments on the Coastal plain and the Coastal Shelf were deposited primarily by the the Potomac, Rappahannock, York, and James rivers (as they are known today, in their current locations).
Much of the largest estuary in the Western Hemisphere is located in Virginia. The Chesapeake Bay is the drowned mouth of the Susquehanna and James rivers. Both rivers once ran separately to the Atlantic Ocean, before sea level rose after the last glacial period and formed the modern bay about 3,000 years ago. You can still see evidence of the separate river channels at the mouth of the bay.
artifical stream to drain stormwater behind Research 1 building on Fairfax Campus of GMU
When the Chesapeake Bay Bridge-Tunnel was built, the U.S. Navy required a portion of the span be constructed as an underwater tunnel. This was intended to prevent an enemy from collapsing the bridge, blocking the shipping channels, and trapping the aircraft carriers at Norfolk. There are two tunnels now - the southern one in the deep channel that was carved by the ancestral James River, and the northern tunnel in the channel of the old Susquehanna River.
The southeast portion of modern Virginia is known today as both Hampton Roads and as Tidewater, but Tidewater is also applied to all portions of Virginia where the water level is affected by the tides - which is most of the land east of I-95 and north of US 460. Tidewater is not necessarily saltwater. Alexandria, Fredericksburg, and Richmond are tidewater ports. After a heavy rain upstream, the water in those ports may be completely fresh, with no brackish/salt water at the surface, but the tides will still cause the fresh water to rise and fall at the docks.
Virginia requires separate fishing licenses for saltwater and freshwater, and separate agencies enforce regulations. Back Bay, Northwest River, North Landing River, Dismal Swamp Canal south of Deep Creek Locks, and the Intercoastal Waterway upstream to Great Bridge Locks are defined as "freshwater." The boundary between salt/fresh is defined as:1
|Potomac River||Rt. 301 Bridge|
|Rappahannock River||Rt. 360 Bridge|
|Piankatank River/Dragon Run||The 1st set of power lines immediately upriver of Anderson Point|
|York River System (including the Mattaponi and Pamunkey Rivers)||Rt. 33 Bridges|
|James River||A line connecting Hog Point on Hog Island (Surry County) and the downstream point of the mouth of College Creek (James City County).|
salinity varies from fresh (less than 0.5 parts/thousand salt) to saline (35 parts/thousand) at the ocean
Source: Draft Tidal Wetlands Guidelines submitted to Virginia Coastal Zone Management Program (Figure 5)
1. "Freshwater/Saltwater License Lines on Tidal Waters," Virginia Department of Game and Inland Fisheries, http://www.dgif.virginia.gov/fishing/regulations/tidalwaters.asp (last checked August 15, 2012)