Watershed management is the basis for many environmental protection decisions. The Environmental Protection Agency (EPA) even has a Watershed Academy. The Federal agency has responsibility for implementation of many water pollution control laws, overseeing all states to ensure no locality attracts business unfairly by allowing excessive amounts of pollution.
The differences in boundaries between watersheds and states/counties complicates the efforts to control pollution. "Everyone lives downstream" - but sometimes the factories or urban areas upstream who generate the pollution are in another state and are not harmed by the bad water flowing away, while those affected by the pollution may have limited political influence to ensure upstream problems are fixed.
For example, a major chemical plant in Saltville (now a Superfund site) degraded the water quality for Kingsport, Tennessee for 80 years. Virginia politicians were well aware that Virginia residents received benefits from operating the plant without paying the costs of controlling the excessive chlorides from salt dumped into the North Fork of the Holston River. Those who suffered from the pollution... lived out of state, and could not vote in Virginia.
The state of Tennessee finally went to court to force Virginia to impose realistic pollution control standards on the company - which then closed down, eliminating 1,000 jobs in a community of about 2,000 people.1
Source: Environmental Protection Agency, What is a Watershed?
Three terms in particular are essential in understanding the physical geography of Virginia - watershed, divide, and tributary. No matter where you are on land in Virginia, you are in a watershed, or "drainage basin." EPA says:2
Elsewhere, EPA uses the definition created by John Wesley Powell for a watershed:3
A watershed divide is the ridge on the ground that separates watersheds. If you look at a roof, you will see a divide at the top where a ridge directs rainwater towards one side of the roof or another. (Many houses built since the 1990's have complex rooflines, so those roofs have multiple watersheds separated by different ridges/divides.)
the Blue Ridge is a major watershed divide - rain falling on the east slope flows to the east
Source: National Oceanic and Atmospheric Administration (NOAA), State Topography Image: Virginia
A tributary is a stream that flows into a larger stream - which might be called a branch, stream, creek, run, or river. The concept is the same, despite whatever names has been adopted for the streams.
Tributaries are upstream, and water from tributaries flow downhill on their journey to sea level. (In Virginia, all land and water is at or above sea level, unless someone has dug a unnatural deep hole. There is no natural equivalent in Virginia to the Dead Sea in the Middle East, which is in a depression that is over 1,400 feet below sea level.)
There is a water fountain near the Johnson Center on the Fairfax Campus. Some spray can flow away from the fountain, and - if the water does not evaporate - the droplets flow down to the Potomac River. The Potomac River flows into the Chesapeake Bay, so by definition the Potomac is a tributary of the bay. If you fling a cigarette filter on the ground at the Fairfax Campus, it could float all the way to Virginia Beach.
According to the Chesapeake Bay Program, the bay has 100,000 separate tributaries - and the4
To understand how watershed boundaries affect pollution, first think of a raindrop. It falls to the ground and runs downhill - you know that, of course. Now think of the paved parking lot at the grocery store or Wal*Mart. Does all the water run towards one drain? How many watersheds could you define in that one parking lot? If all the water drains to one outlet, then the entire parking lot is in the watershed of that one drain. The water that falls anywhere on that lot will run downhill to one stream, via that drain and its outlet pipe.
But what if the rainwater on one side of the parking lot runs towards one drain, and rainwater on the other side flows towards a different drain? On a very small scale, you are seeing two watersheds in that parking lot with a divide separating them. After a rainstorm, the divide gets driest first as the water drains away... that's where you tip-toe through the puddles easiest at the end of a storm.
The "divide dries first" pattern is why Route 60 goes down the middle of the peninsula between Richmond and Hampton, and Route 13 goes down the middle of the Eastern Shore. The ridgetops on those peninsulas might be low, but farmers who wanted to avoid getting stuck in the mud understood that the original dirt roads would dry quickest at the top of the ridge.
In Northern Virginia, large parts of Route 7 and Route 50 between Fairfax City and the Blue Ridge follow the watershed divide. The initial road builders chose the relatively-high ridges in part to avoid the cost of building bridges over large creeks, and in part to locate the dirt roads where they would dry quickly after a rain so farm wagons would not get stuck in the mud.
Sometimes, a parking lot looks flat and there's not a sharp "knife's edge" of mountains separating the valleys... but if you just follow the water downstream, you can tell easily what land belongs in what watershed. Next time you're watching a football and soccer game, see if the field has a slight rise in the middle, so water will drain away rather than pool in the center of the field. If so, then there's a tiny watershed divide in the middle of the field. Athletic fields and roads are now designed to have a "crown" in the middle, to facilitate drainage.
A watershed is the area drained by a stream. By that definition, all the land that drains into Pohick Creek, including the majority of the Fairfax campus of George Mason University, is in the Pohick Creek watershed. Rainfall flows off the impervious surfaces on campus into Rabbit Run, which joins Pohick Creek, which flows into the Potomac River.
The Prince William campus is in the Occoquan River watershed. Rainwater that falls on the roof of the GMU buildings at the Prince William campus will end up flowing under Freedom Center Boulevard, through the stormwater pond at the Northern Laboratory of the Department of Forensic Science, and down Cannon Branch to the stormwater ponds at the Route 234-Route 28 intersection near the Manassas airport.
When rainfall is sufficient, those ponds will release water into the downstream portion of Cannon Branch, which flows into Broad Run. Broad Run will join with Cedar Run to form the the Occoquan River, which has been dammed downstream to create Lake Jackson. The raindrops that flow over the dam will continue downstream to the next dam which formed the Occoquan Reservoir, then over that dam and under I-95 to the Potomac River, and ultimately get to the Chesapeake Bay and the Atlantic Ocean.
watershed divide north of GMU campus separates Broad Run and Bull Run watersheds; GMU is located in the Broad Run watershed
Source: Prince William County Mapper
The Arlington Campus is in the Four Mile Run watershed - though with all the concrete and asphalt in the area, it's hard to see the natural topography.
The boundary between watersheds are called a divide. Divides are ridges - if you hike across a divide, you will go uphill to the top of the divide and then downhill on the other side. Rainwater that lands on the ridge will flow down one side into one watershed, ending up in a particular stream. Rain landing just a little bit away on the divide will flow down the other side of the ridge, and end up in a different stream.
For example, rainfall that lands at the top of the Blue Ridge does not sit there, accumulating on the surface in natural lakes. Rainfall flows downhill to the east or to the west. Under the force of gravity, raindrops move, initially in "sheet flow" across the surface (most obvious in parking lots...) or by sinking underground and then flowing downhill. Ultimately, the water that soaked into the soil will emerge at springs, join the rivulets flowing across the surface, and form small creeks/streams/runs.
Small "first order" creeks/streams/runs flow together, creating larger branches/creeks/streams/runs and ultimately rivers. In Virginia, there is no consistent logical distinction in size or other characteristics between branches, creeks, streams, or runs. Early explorers and/or local residents chose those labels almost at random, though anything called river will (almost always...) be larger than anything nearby called branch, creek, stream, or run.
At Big Meadows in Shenandoah National Park, the raindrops that land in the actual meadow across Skyline Drive from the Byrd Visitor Center will flow to the east and form Hogcamp Branch.
at Big Meadows in Shenandoah National Park, raindrops landing in the meadow will flow to Hogcamp Branch - not into Hawksbill Creek
Source: US Geological Survey (USGS), National Map - National Hydrography Dataset
Hogcamp Branch is a tributary of Rose River, which flows into the Robinson River. The Robinson River flows into the Rapidan River, which flows into the Rappahannock River, which flows into the Chesapeake Bay, which flows into the Atlantic Ocean. Hogcamp Branch is in the headwaters and in the watershed of each of those rivers downstream. A raindrop landing at Big Meadows will move 3.500 feet downhill on its 100-mile trip to sea level at Fredericksburg, just east of I-95.5
Rainfall that lands on the parking lot for the Big Meadows lodge, campground, and picnic area west of Skyline Drive will flow westward across the pavement and the land into small rivulets that have been etched into the hillside after eons of erosion. The small rivulets ultimately join to form Little Hawksbill Creek or Hawksbill Creek. Those creeks flow down the west face of the mountain, into the Page Valley, and then into the South Fork of the Shenandoah River at Luray. The junctions of the creeks and the South Fork of the Shenandoah River are the "confluences."
Of course, gravity does not quit working just because the mountain stream has reached the valley - the South Fork of the Shenandoah River flows down the valley to Front Royal. There, the South Fork and the North Fork join to form the main stem of the Shenandoah River... which keeps flowing downhill to Harpers Ferry. The water from the parking lot at Big Meadows will end up reaching sea level when it flows past the Fall Line at the District of Columbia and, ultimately joins the Chesapeake Bay and finally the Atlantic Occean.
By definition, the confluence is where streams join, where they flow together and at least one stream loses its name. Below the confluence at Harpers Ferry, there is no Shenandoah River. The rain that landed at Big Meadows and washed down the western slope of the Blue Ridge will continue to flow downhill/downstream past Harpers Ferry, but that waterflow below Harpers Ferry is labelled "Potomac River."
We do not hyphenate names of streams that flow together, the way we might hyphenate last names when two people get married today. Imagine the challenge of residents far downstream in Hampton Roads, if our custom was to list all the streams through which water flowed before reaching a place. If we hyphenated river names to acknowledge all the streams up to the watershed divide, the stream flowing past Jamestown could be called the Jackson-Cowpasture-Bullpasture-Calfpasture-Maury-Rivanna-Appomattox-James River... or worse.
In some areas of the western US, the streams flowing out of the mountains may dry up before reaching the ocean. In Utah, many streams flow into the Great Salt Lake or equivalent small lakes where the water evaporates, rather than flows on to the Pacific Ocean. As the stream leaves the mountains, the volume of the water in the stream channel may get smaller rather than larger, until the stream dries up completely at a salt flat or sink. In Virginia, however, nearly every stream will get larger as it flows downhill and other streams flow into it, ultimately forming rivers and reaching the Gulf of Mexico or the Atlantic Ocean. The one Virginia exception is in karst topography, where streams might disappear or "sink" into the ground and flow underground through porous limestone.
Sinking Creek west of Virginia Tech in Blacksburg is in karst topography. Near Sinking Creek, water on the surface may flow into sinkholes in the limestone. On the surface, there is no stream connecting the sinkhole with any creek, but underground the water is is still flowing downhill. It will reappear in a spring near the creek, or bubble back to the surface in the streambed itself.
During the winter, when water flows are higher, the surface flow in Sinking Creek goes all the way to the New River. In the summer, water may flow in the channel of Sinking Creek upstream of US 460, but near the mouth of the stream there will be nothing but river gravel in the streambed. The water has disappeared underground, and will emerge from springs in the bed of the New River.
James River/Shenandoah River Divide
Follow Interstate 81 north from Staunton to the divide between the Shenandoah and James Rivers. The climb to the divide at the Raphine/Steele's Tavern exit on the boundary of Augusta and Rockbridge counties is a subtle rise; there is no clear dividing line indicating the traveler has literally "crossed the divide." The headwaters are etching away at the same bedrock and the relative energy of the two streams is basically the same at this site, so there is no clear ridgeline visible to identify the watershed boundaries.
Curiously, two streams flowing in opposite directions, away from the watershed divide separating the Shenandoah/James rivers at this spot, are both called the South River. One "South River" flows north to Port Republic, where it joins with the Middle and North rivers to form the South Fork of the Shenandoah River. The water flows north to Harpers Ferry, the veers east to cut through the Blue Ridge and go east to Washington DC.
The other "South River" flows south into the Maury River near Lexington, then joins the James River and cuts through the Blue Ridge at Balcony Falls on its trip to Richmond. Water traveling south or north down either South River will go 230-250 miles to reach Tidewater. A more-direct path to the Potomac River would be only 135 miles, but the circuitous paths of Virginia's creeks, runs, streams and rivers reflects eons of erosion. Straight lines on maps almost always reflect some intervention in nature by humans.
Watershed Boundaries in Southeastern Virginia
The watershed divides in southeastern Virginia are subtle, but significant. Most of Dismal Swamp is naturally in the Roanoke River watershed, though ditches and canals have altered local hydrography and some water from Lake Drummond could drain to the Chesapeake Bay now. The southern part of the cities of Virginia Beach, Chesapeake, and Suffolk are not in the Chesapeake Bay watershed.
Watershed boundaries shape modern pollution control efforts. The initiative to "Save the Bay" includes regulations that limit nutrients discharged by wastewater treatment plants, incentives for adopting Best Management Practices in agricultural areas, and controls on stormwater runoff could be applied city-wide... but in the southern portion of the three cities, those water quality tools will impact Back Bay and the Albemarle/Pamlico Sound - not the Chesapeake Bay.
Today, it's a good bet that more "Save the Bay" license plates are sold on the Northern Neck, or even in the Shenandoah Valley, than in the counties west of I-95 bordering North Carolina. None of Virginia on that border drains into the Chesapeake Bay - not even the southern part of Virginia Beach. Drive south on US17 on the eastern edge of the Dismal Swamp in the city of Chesapeake, and you'll see the same signs announcing that you are "Leaving the Chesapeake Bay Watershed" - just like the sign you see on Interstate 81 north of Roanoke...
The North Landing River west of Back Bay runs due south to Currituck Sound. Virginia's rivers in the state's southeastern corner (including the Blackwater, the Nottaway, and the Meherrin) drain into Albemarle Sound in North Carolina, rather than into the Chesapeake Bay.
On the Eastern Shore, US 13 follows close to the watershed divide. East of the highway, most of the streams drain directly to the Atlantic Ocean. Most farmland west of US 13 is located in the Chesapeake Bay watershed, where farming practices are affected by efforts to clean up the Chesapeake Bay. Virginia must meet Total Maximum Daily Load (TMDL) water pollution requirements, reducing runoff of nutrients (nitrogen and phosphorous) and sediments into the bay - so the watershed divide is a key feature on the Eastern Shore.
Most of Southside Virginia does not drain into the Chesapeake, and this was a factor in the slow development of the area in colonial times. Planters on the Eastern Shore, and on the west side of the bay in areas known as the Peninsula, the Middle Peninsula, and the Northern Neck, had direct access to international transportation. English ships could sail directly to wharfs in tidewater Virginia, load tobacco, and return home with minimal delay.
Tobacco farmers in Lunenburg and Brunswick counties, however, had to haul their crops across several rivers, in the days when roads were poor or non-existent. The physical geography of the colony delayed settlement and reduced the value of the land sold in those watersheds, compared to the Chesapeake Bay counties.
Watershed boundaries are almost never straight lines - but many of Virginia's political boundaries are often straight lines, defined by survey coordinates. Virginia's southern border was intended to be straight, forcing surveyors ignore natural contours and march though the Dismal Swamp and across the New River. There is a bump in Virginia's southern boundary east of Bristol, where a boundary dispute was caused in part by the difficulty of surveying in the rugged topography of the highest part of Virginia just south of Mount Rogers, but the political boundary ignores physical features in order to follow a line of latitude written in colonial charters during the 1600's.
Source: US Geological Survey (USGS), Water Monitoring Day