What's up with the watershed assessment?

By Don Audette
04/28/2005

The map shows part of the Upper Thornton River where a team of people is gathering data related to water quality in Rappahannock County.

It is a pilot study area of three square miles or about 1,900 acres. Located near Sperryville along Route 522 heading toward Culpeper, it includes both the north and south branches of the Thornton River, plus Beaverdam Creek where it joins the Thornton River at Fletcher's Mill.

Anyone on a tract of land can follow the assessment process and imagine its use on their property. Thus, if you had a plat of your land and matched it with a U.S. Geological Survey map you could see from the contours which way water moved on the land, where it came from, and where it went. Rain and snowmelt moving across the property either seeps into the ground to replenish well water; evaporates; or flows into creeks, streams, drainage ditches, or ponds. Stream-flow eventually reaches the Chesapeake Bay via the Rappahannock River.

All rivers in Rappahannock County, the Jordan, Rush, Covington, Piney, Thornton, Hazel, and Hughes, end up blending into the Rappahannock River.

Part of Rappahannock County is in the Shenandoah National Park at an altitude of 3,720 feet above sea level. The lowest part of the county is only 360 feet above sea level. We are talking a maximum vertical drop of over half a mile within Rappahannock County. Rain or snowmelt results in a lot of rushing water moving down slopes, being partially absorbed into the ground along the way, some evaporating, but most running off into streams. The water encounters and impacts much on its trip downward -- the land and its vegetative cover, wildlife, farm animals, humans with their buildings, waste, debris, etc.

Checking it out

The pilot study area on the Upper Thornton River is where all this is being checked out. The north branch flows into the Thornton after passing through woods and farmland. The south branch mostly parallels Route 211 out of the Shenandoah National Park, flows through the village of Sperryville, and runs by the Sperryville wastewater treatment facility before joining the north branch below Sperryville.

The study team is currently testing and monitoring the water in both branches. Their aim is to find any noticeable differences in sedimentation, fecal coliform (or e coli), fish, stream flow, aquatic insects, etc. They are also gathering information on stream buffering; biodiversity; wildlife corridors; wetlands; impervious surfaces such as roads, buildings, parking lots; point and non-point sources of pollution; vegetative cover such as trees, grasses; etc. Assumptions about future development will contribute another set of factors to be worked into the assessment process.

The team is also gathering similar information about the Upper Thornton River downstream from the point where the two branches join, plus below where the Beaverdam Creek runs into the Thornton.

Looking at layering

All the data gathered is being overlaid as "layers" onto maps of the pilot study area. For example, if you were doing the same thing on your land, there might be a wildlife corridor layer. This might show the paths deer usually take across the property, where quail or turkeys make their way, the paths black bears might take on their trips to, say, a nearby orchard. All wildlife, whether fish, animal, or bird, move about. Just as we, as "tame life," are frustrated when encountering more cars on our daily commutes due to development, so wildlife are buffaloed by changes in their pathways.

The layers being mapped onto the pilot study area are: topology, hydrology, existing impervious cover, land use by zoning category, future impervious cover based on land use plans, and aerial photographs. Other layers may include stream mileage and percent in forested condition, existing forest cover, existing and planned stream buffers, floodplains, wetlands, steep slopes, soils and geologic features, hazardous waste dumps, historic sites, developable land remaining, road crossings, storm water systems, existing and planned sewer systems, etc.

Can't get through

Each layer contributes to the assessment process. Take one of the most important layers -- the amount of impervious surface in the area. This is the percentage of an area that is covered with roadways, sidewalks, buildings, parking lots, tennis courts, or anything else that rain or snowmelt cannot penetrate and thus must runoff. Gravel and dirt road and parking surfaces are treated as impervious as they are designed to allow water to run off.

If a watershed has an impervious cover of zero to 10 percent over its total area, then a stream within that area is a "sensitive stream" (good). Such streams run through stable channels, have excellent habitat structure, good to excellent water quality, and diverse communities of both fish and aquatic insects. They do not experience frequent flooding or other detrimental hydrological changes.

If the impervious cover is between 11 and 25 percent, it produces "an impacted stream." There are clear signs of degradation as greater storm flows alter the stream's path, erosion and channel widening occur, stream banks become unstable, and physical habitat declines noticeably. Stream water quality drops to good/fair during storms and dry weather conditions. Stream biodiversity declines to fair, with the most sensitive fish and aquatic insects disappearing.

An impervious cover exceeding 25 percent is not good. It will produce a stream that is called a "non-supporting stream." The stream essentially is a conduit for storm water flow and cannot support a diverse stream community; there are no quiet pools and riffles to support fish; stream banks are eroding and streams are widening; there is no fish spawn, no aquatic insects; and water quality is fair to poor. An example of a land use with a high impervious cover is commercial development. On average, a little over 72 percent is impervious due to the building and its parking lot. A good example is a Wal-Mart.

We have described just one layer, impervious cover, and how it impacts a watershed. Homeowners many times have an intimate knowledge of their property and can aid the multi-talented people working on the pilot study area.

If you have some knowledge of the various layers noted above and wish to join the effort, please see the new RappFLOW web site at: http://www.rappflow.org .

©Times Community Newspapers 2005