Perennial Stream Mapping Completed


(Conservation Currents, Northern Virginia Soil and Water Conservation District)

Environmental Scientist records observations in stream“We finished.” So said environmental scientist Shannon Curtis about a two-year stream mapping project that ultimately will affect the way land is, or is not, developed in Fairfax County. Curtis is part of the county’s Stream Protection Strategy team, which was tasked by the Board of Supervisors to identify and map all perennial streams in the county to comply with new directives from the state. Of central importance in the revised state regulations is that Resource Protection Areas now must be designated around all water bodies with perennial flow. The difference between the old language and the amended language is the replacement of “tributary streams” with “water bodies with perennial flow.” A perennial stream carries flowing water continuously throughout the year, except in cases of extreme drought. Tributary streams were only those perennial streams that were depicted on U.S. Geological Survey maps. “Those maps are based on aerial photography,” said Curtis, “and don’t adequately depict perenniality. What we have done is ground-truthing, which gives a truer delineation of what we are looking for.”

The county’s Chesapeake Bay Preservation Ordinance, amended in July 2003, requires that all perennial streams have a protective buffer extending 100 feet on both sides of the stream. That buffer is called a Resource Protection Area. An RPA protects water quality by preserving riparian (streamside) vegetation which filters pollutants from stormwater runoff, reduces the volume of runoff, prevents erosion, provides wildlife habitat, and performs other important biological and ecological functions.

Using a sophisticated protocol modeled after one developed and used by the State of North Carolina, the six-member SPS team went out in pairs to conduct the field surveys. “We were looking for a zone, not a specific spot,” said Curtis. “While property boundaries have a distinct demarcation, streams don’t. As with most things in nature, streams have a gray area where the intermittent channel transitions into a perennial stream.”

The distinction between perenniality and intermittency is where controversy starts. Many developers and property rights advocates don’t want more streams identified as perennial because such a designation limits what a landowner can do with the property. The Chesapeake Bay Preservation Ordinance generally prohibits land clearing and building in an RPA. If a stream is identified as intermittent, there is no RPA designation, and thus there are fewer restrictions on development. Curtis said the new maps will impact not only developers, but also homeowners who want to build home additions—even as small as a deck—within an RPA.

Environmental Scientist looks for aquatic insects in streamThe greatest obstacle to mapping was the weather. “Last year we had a severe drought, and this year was one of the wettest on record,” said Curtis.

During a hydrologic drought, where the water table recedes below the streambed, even a perennial stream may go dry. By the same token, when rainfall hits record levels, every channel is flowing. That’s not to say that flow was the defining factor.

“The protocol has only a couple of factors that are scored directly on the basis of flow,” explained Curtis. “The other 30 or so indicators include evidence of groundwater, condition of the soil, life in the stream, and channel formation. For example, soil that is continuously wet changes to a dull grayish color. The presence of certain indicator species in a stream, especially the species that can’t move downstream easily in drier times, suggests a perennial stream based on their need for an aquatic environment for at least a year. Fish, crayfish, certain salamanders and frogs, and a diverse insect community need water year-round. A stream that has only snails, leeches and scuds (small crustaceans), which all can tolerate brief drying periods, is more likely to be intermittent.”

In addition to the protocol, the team members used information from other sources such as rain and surface water flow gauges, historical data, a soil survey, a drought monitor, groundwater wells, and people’s observations.

“We asked property owners if a stream flows year-round or is it dry in the summer. Perceptions vary, even among family members living in the same house,” said Curtis. “But we used what they said as backup for what we found with the protocol.”
“We believe in the protocol,” said Curtis, “but it’s a very new science. Only time will tell how good it really is.”

“What’s most distressing for us is that in the two years it took us to do this survey, many streams disappeared,” lamented Curtis. “It was too late for the ordinance to protect them from development. But with these maps, we are going to ensure that the streams that are left are protected. Then we can go about the business of fixing the streams that have been degraded.”

For more information about the stream mapping project, call the Fairfax County Stormwater Planning Division at 703-324-5500, TTY 711, or visit the county’s perennial streams Web page.


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