Which Way Do the Rivers Run?
Source: ftp://viva.lib.virginia.edu/pub/gic/states/virginia/dlg/va_hydro.gif
Virginia's rivers don't run due east, straight from the mountains to the Atlantic Ocean. Some run north, others flow west... and all change direction often. The bedrock in different physiographic provinces and specific locations varies in resistance to the power of the water to erode a channel. Rivers shift direction slightly from time to time as they expend their mechanical energy and flow downhill under the influence of gravity to the Atlantic Ocean.
Computerized processing of digital elevation model (DEM) data allows us to view the location of rivers in relation to hills and valleys, without the confusing details of roads and houses that clutter up the view in satellite images. Look at a large image of Virginia topography (from the US Geological Survey Open-File Report 99-11, Color Shaded Relief Map of the Conterminous United States) before we discuss in more detail the three sections highlighted on that map.
Orient yourself first - north is at the top of the map, according to modern tradition. On the right (east) side of the map, the Chesapeake Bay and the main stems of the Tidewater rivers are white. The lower elevations are dark green, the higher elevations are lighter shades of green, and the highest elevations (such as the Blue Ridge) are colored in shades of brown. Note that in the color scheme of this particular map, dark brown corresponds to the highest elevations while dark green corresponds to the lowest elevations.
 | Can you recognize that Box 1 includes the water gap carved through the Blue Ridge at Harpers Ferry by the Potomac River? See the circled area on the right for a close-up. Note that the Potomac River has eroded a wide swath though the soft limestones of the Great Valley west of the Blue Ridge, but has carved only a narrow channel as it cuts through the harder igneous bedrock of the mountains.
The darker green squiggle to the right of Harper's Ferry is the path of the Potomac River. You can trace in the topography where it flows from its headwaters in the Appalachians to the river mouth, where the Potomac disappears as a river and the water flows into the Chesapeake Bay. |  |
Box 2 shows that the junction of the Rappahannock and the Rapidan rivers is also evident. If you look at a different DEM, then compare to the DeLorme Atlas and Gazetteer, you can also identify the channels of Potomac Creek and Accokeek Creek, between Aquia Creek and the Rappahannock River. The asterisk notes the location of Fredericksburg. Another quick peek at the atlas or the state highway map, and you can determine if the I-95 bridge over the Rappahannock is to the east or west of that asterisk...
Box 3 on the large image of Virginia topography shows the channel of the James River through the Piedmont, from the Blue Ridge to the Fall Line. Remember, the color scheme tells you that the higher elevation is in the west, so the James River is flowing to the east. Where Dunlap Creek flows into Jackson's River (now Covington, Virginia) was known as the "head of navigation" for batteaux floating goods down the James River.1
Now that you can recognize the river channel on a DEM, how can you tell the direction of flow? On the DEM's, there is a clear color difference between the area with the highest and lowest elevations. On the image to the right, the channel of the Potomac River is distinct. Colors are not standardized and will vary on computerized maps from alternate sources, but the color differences will always highlight the elevation differences. The rivers will always flow downhill, and east of the Blue Ridge in Virginia downhill will be towards the southeast.
West of the Blue Ridge, you have to think a little harder. Can you see from the color shadings that the Shenandoah River will flow northeast from Massanutten Mountain (in the white ellipse) to the Potomac River? The North Fork of the Shenandoah River flows on the northwest side of Massanutten Mountain, and the south fork flows between the mountain and the Blue Ridge. They both flow northeast, meandering back and forth in the process, and join at Front Royal (at the northeastern tip of Massanutten Mountain).
The channel of the Shenandoah River is not obvious on this image or on the large image of Virginia topography, however. Refer to the contour lines in the DeLorme Atlas and Gazetteer that show elevation, and use them to distinguish "up" and "down." There's no arrow in the Atlas for identifying the direction of flow of the Shenandoah River, but you know it's going downhill...
The James River drops nearly 1,000 meters from its start in Highland County to Hampton Roads. See how far you can trace the James River upstream on the map of Virginia (Source: Global Land One-kilometer Base Elevation (GLOBE) Project of NOAA's National Geophysical Data Center). In contrast, the Coastal Plain is relatively flat compared to the Piedmont, Blue Ridge, Valley and Ridge, or Appalachian Plateau physiographic provinces.
The topography of the southeastern rivers is harder to see with DEM's. The Nansemond River runs through the city of Suffolk. It drops only 20 meters in elevation from the headwaters to the mouth, where it empties into the James River at Hampton Roads.
The Western Branch and Eastern Branch of the Elizabeth River are obviously flowing towards Hampton Roads... but the Southern Branch is connected to the North Landing River by the Intracoastal Waterway. (Recognize Back Bay and the barrier island separating it from the Atlantic Ocean, at the southeast corner of Virginia? False Cape State Park is on that island.)
It was relatively easy to dig a canal connecting the Albemarle Sound to the Chesapeake Bay, since the sediments were soft, unconsolidated sand and mud. They even built two - the Dismal Swamp Canal paralleled by Route 17 in the City of Chesapeake was constructed first, and it's still in operation as another segment of the Intracoastal Waterway.
