Watersheds and Divides in Virginia

Watershed management is the basis for many environmental protection decisions - the Environmental Protection Agency even has a Watershed Academy. The differences in boundaries between watersheds and states/counties really complicates the political geography at times. "Everyone lives downstream" - but sometimes the folks upstream are in another state, and those affected by pollution have limited influence to ensure a problem is fixed.

For example, a major chemical plant in Saltville (now a Superfund site) degraded the water quality for Kingsport, Tennesse 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 salt dumped into the North Fork of the Holston River. Those who suffered the costs lived out of state. 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 2,000 jobs in a community of about 2,000 people.

	Two terms 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... No matter where you are, you're in a watershed!" Elsewhere, it defines a watershed as "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."

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? In that case, 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, as in the parking lot and the graphic to the right, 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:

Virginia's political boundaries are often straight lines, like the edges of the 1:24,000 scale topographic maps produced by the U. S. Geological Survey or the pages in an atlas. Virginia's southern border, and you can see how surveyors marched straight over the landscape, though the Dismal Swamp and across the New River. There's bump 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.

Watershed boundaries are not straight lines. A watershed is the area drained by a stream. By 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. The Prince William campus is in the Occoquan River watershed - rainwater that falls on the roof of those buildings will end up flowing downstream to the Occoquan. 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 continues downhill, but as part of the Potomac River. In ordinary life, 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 in Hampton Roads, if our custom was to list all the streams through which the water flowed before reaching Hampton Roads.

In some areas of the western US, the streams flowing out of the mountains may dry up before reaching the ocean. As the stream leaves the mountains he volume of the water in the streams gets smaller. 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. 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.