Water flows downhill due to gravity, obviously. Rivers flow downhill - but "downhill" in the past may have been dramatically different than today. For example, the gaps in the Blue Ridge were carved by rivers that used to flow from western Virginia to the Atlantic Ocean. It may require a little effort on your part to see how, millions of years ago when the Shenandoah Valley was much higher in elevation, ancient rivers flowed downhill and carved out valleys where I-66, Route 50, and Route 7 now cross over the Blue Ridge. Later, the Shenandoah River "pirated" their waters and erosion lowered the depth of the valley, reversing the relative elevations.
Similarly, the Potomac River once flowed over the current location of Tysons Corner, before shifting to the north and carving the modern channel. Today Tysons Corner is one of the highest spots in Fairfax County - that's why you see water towers and telecommunications towers at the intersection of Route 123 and Route 7. However, go back a few million years and the site of Tysons Corner was once surrounded by higher land and a river flowed (downhill...) through Macy's, Nordstroms, and the movie theaters. The rounded river cobbles were left behind and the river bottom became a ridgetop, when the Potomac River shifted directions and etched its modern valley elsewhere.
Very few maps include an arrow showing that a river flows east, or south, or whatever. All rivers will flow down from the higher elevation to the lower elevation - but a "planimetric" map of the rivers of Virginia will show only the horizontal features; such a map will not show vertical topographic contour lines that could help you determine the direction of flow.
If you live in an area, there's a simple way to figure out the way the water goes: look out the window when you cross a bridge. Otherwise, to understand the flow pattern in an area unfamiliar to you, get skilled in identifying the high ground where rivers start in order to determine what direction is downhill.
For example, the James River flows between the Blue Ridge Mountains and Richmond, Virginia - but if you are new to Virginia, how do you determine whether the river flows west-to-east (from the Blue Ridge towards Richmond) or east-to-west (from Richmond towards West Virginia)?
On a map showing topographic contours, you can determine the elevation of two places along a river. The elevations marked on contour maps show that Glasgow (where the James River cuts through the Blue Ridge) was built on a flat flood plain, not on the steep hills at that site - and that the river there is 650' higher than in Richmond:
Water flows downhill, of course. The elevation difference between Glasgow and Richmond means the James River is flowing west-to-east, downhill from Glasgow towards Richmond.
Once you know the direction of flow, you can follow the line of the river on the map towards the east, downstream of Richmond, and see the water in the James River will flow into the Chesapeake Bay and the Alantic Ocean.
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.
|Before reviewing in more detail the three sections highlighted on that map, orient yourself.
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.
Potomac River mouth
| 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 shows the channel of the James River through the Piedmont, from the Blue Ridge to the Fall Line. The color scheme communicates 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 a distinctly darker blue. The higher elevations at Massanutten Mountain (circled in white) are tan. 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, of course. East of the Blue Ridge in Virginia, downhill always will be towards the southeast and the Atlantic Ocean. West of the Blue Ridge, and south of Roanoke, some Virginia rivers flow west to the Mississippi River and the Gulf of Mexico.
The Coastal Plain is relatively flat compared to the Piedmont, Blue Ridge, Valley and Ridge, or Appalachian Plateau physiographic provinces, so 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. The Southern Branch is connected to the North Landing River by the Intracoastal Waterway. (Back Bay - and the barrier island separating it from the Atlantic Ocean, the location of False Cape State Park - are nearby at the southeast corner of Virginia.)
In Richmond, the James River and Kanawha Canal had to blast through granite just west of downtown, almost bankrupting the company because of the costs and delays. In southeastern Virginia, 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.
West of the Blue Ridge, rivers might flow to the Gulf of Mexico. Two forks of the Shenandoah River flows northeast from either side of 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 may not be obvious on small-scale images. Refer to the contour lines in the Virginia 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.
In southwestern Virginia, most rivers flow south (Holston River, Clinch River, Powell River) or west (Levisa Fork, New River) towards the Ohio River, then on into the Mississippi and the Gulf of Mexico. The Shenandoah River flows north, just like the Nile River in Egypt.
If you always thought north was "up the map" because that's the way the maps hung on the wall in elementary school - well, don't be confused and think north is "uphill." Staunton and Waynesboro are located at the southern, upper end of the Shenandoah Valley. The Shenandoah River flows north to Harpers Ferry, flowing downhill the entire time it flows north.
Don't say the Shenandoah River flows "up" to Harpers Ferry, unless you can repeal the law of gravity. When you go upstream on the Shenandoah River, you are going southwest. When you go from Harpers Ferry to Harrisonburg or Staunton, you are going "up the Valley" - at least technically.
The mountain ranges (or low hills) that define the edge of a watershed are watershed divides, where the rain divides to flow in one direction or another from the top of the hill towards the bottom. All of Virginia is east of the Continental Divide in Colorado, which splits water so some raindrops flow to the Pacific Ocean and other raindrops flow to the Guf of Mexico.
There is an Eastern Continental Divide in southwestern Virginia, similar to that more-famous Continental Divide in the Rocky Mountains out west. On one side of the Eastern Continental Divide, Virginia rivers flow to the Atlantic Ocean. On the other side of the divide, the rivers flow to the Gulf of Mexico.
Drop a stick into the South Fork of the Shenandoah River near Luray, and the stick will float down the Shenandoah River to Harpers Ferry, then down the Potomac River to the Chesapeake Bay, then through the bay past Cape Charles/Cape Henry into the Atlantic Ocean. If you lose your inner tube (or beer can...) at McCoy Falls in the New River near Virginia Tech at Blacksburg, it will float westward through West Virginia, down the Mississippi River, past New Orleans to the Gulf of Mexico - and will never come close to the Chesapeake Bay.
NOTE: There are other continental divides in North America. The more-famous Great Divide/Western Continental Divide is marked by the Rocky Mountains, separating the Gulf of Mexico watershed from the Pacific Ocean watershed. The Northern Continental Divide runs just north of the Great Lakes/St. Lawrence River in Canada, separating the Arctic Ocean/Hudson Bay from the other watersheds to the south.
the Eastern Continental Divide separates rivers flowing to Atlantic Ocean vs. Gulf of Mexico
Source: US Geological Survey (USGS), Continental Divides in North Dakota and North America