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 and divide. EPA says "A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place."2 No matter where you are, you're in a watershed. Elsewhere, EPA uses the definition created by John Wesley Powell: "an area of land from which all surface and ground water flows from higher elevations downhill to a common body of water such as a stream, river, lake, wetland, estuary, or ocean."3
A 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. Most houses have multiple watersheds, with rooftop ridges separating them.
A tributary is a stream, creek, river, etc. that flows into a (typically) larger stream, creek, river... The Potomac River flows into the Chesapeake Bay, so the Potomac is a tributary of the bay. According to the Chesapeake Bay Program, the bay has 100,000 separate tributaries - and the "Chesapeake's land-to-water ratio (14:1) is the largest of any coastal water body in the world. This is why our actions on the land have such a significant influence on the health of the Bay."4
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.
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.
Downstream from Front Royal on the Shenandoah River, there is a small stream flowing down the Blue Ridge on the east bank, as shown on the USGS 1:24,000 scale topographic quadrangle:
The stream in question is not named on the map, but the solid blue lines show where there is water in the streambed of two forks throughout the year. The arrow shows the direction of flow of the Shenandoah River - and you don't need to see the details of the topographic contours to know that the unnamed stream is flowing downhill to the river:
You can judge from the contour lines where the rain falling on the Blue Ridge will drain into the two forks of the unnamed watershed, and eastimate the location of the divide separating the little watersheds from each other on this hill in Warren County:
It's more graphic if you color in all of the land in the watershed:
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. If you fling a cigarette filter on the ground at the Fairfax Campus, it could float all the way to Virginia Beach.
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.
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 flows downhill east and west. The rain falling on the ridgetop does not accumulate in natural lakes on top of the Blue Ridge, but instead forms small streams. At Big Meadows in Shenandoah National Park, rainfall that lands on the parking lot for the 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 Hawksbill Creek, which flows down the mountain, into the valley, and then into the South Fork of the Shenandoah River at Luray. The junction of Hawksbill Creek and the South Fork of the Shenandoah River is their "confluence."
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.
Some watershed boundaries serve as political boundaries, such as the northern and southern sides of King William County.
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.