Identifying and Mapping Perennial Streams

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

Did you know that almost all of the water that drains off of your driveway, into the street and down the storm drain, eventually enters the Chesapeake Bay via the network of streams running through your backyard?! This means all of the excess fertilizer that is haphazardly sprayed on yards; the oil leaking from poorly maintained cars; and forgotten dog messes, riddled with fecal coliform bacteria, also get carried along with it!

Like other local governments, Fairfax County has implemented regulations, corresponding with state regulations, to protect local streams and the Chesapeake Bay from further degradation. In 2001, the Fairfax County Stormwater Planning Division began work on a major two-year project to identify and map perennial streams using field protocols that focus on the hydrological (water), geomorphological (channel) and biological (life) characteristics of streams.

Man in streamWhat are perennial streams?

Believe it or not, there is no consistent definition of a perennial stream. Basically, perennial streams carry flowing water continuously throughout the year, regardless of weather conditions. The streambed of these systems lies below the groundwater table and is fed by groundwater sources. The systems also receive input from stormwater runoff. Only periods of hydrologic drought, where the water table recedes below the streambed, will cause the channel to be dry. The biology of these systems includes organisms whose life cycles require a fully aquatic environment for a year or more.

Why did Fairfax County conduct the perennial stream project?

The directive for the perennial stream identification and mapping project came from the Board of Supervisors in response to a request from the County’s Environmental Quality Advisory Council (EQAC). EQAC was concerned that the perennial stream map in use was unreliable, incomplete, and out of date. Before this stream mapping project was undertaken, perennial streams were only those depicted as perennial on U.S. Geological Survey (USGS) maps.

Why is it important for streams to be designated as perennial? Perennial streams, as well as a buffer zone of floodplain land within 100 feet of a stream, make up Resource Protection Areas (RPAs). RPAs are legally protected from development by the County’s Chesapeake Bay Preservation Ordinance.Wetlands, tidal wetlands, and tidal shores also are protected as RPAs because they all perform ecological and biological processes that are crucial for maintaining good water quality. They function as buffer areas, reducing sediment and nutrient content and mitigating adverse effects of human activities.

Prior to the Stream Survey, streams that were not identified as perennial on the USGS map were not given RPA status under the County’s Chesapeake Bay Ordinance. For more information about stream protection and what is and is not allowed within a stream’s buffer zone, see: Resource Protection Areas.

In 2003, the Chesapeake Bay Local Assistance Department revised the state’s regulations about how to identify perennial streams. The new state requirements allowed Fairfax County some latitude in developing its own definition of perennial streams and protocols to identify them. As a result, the County revised its CBPO to ensure compliance with the state’s regulations.

The amended CBPO took effect on March 1, 2003, protecting all perennial streams by including them in RPAs. One of the major goals of the perennial stream project was to consistently field-identify and map all perennial streams using a scientifically valid protocol and include them in RPAs. Once the map was completed in 2004, it went to the Board of Supervisors for consideration.

While the Stream Survey was being undertaken, the County used an interim map that reflecting the portion of perennial streams field-identified through the end of 2003. It included an extrapolation of that information to the unmapped areas of the County based on the average drainage area for perennial streams in hydrologically similar mapped watersheds. When development plans came in for review, County staff required that developers perform field studies to determine if streams were in fact perennial for the extrapolated RPAs.

The perennial stream identification and mapping project was initiated in March 2002. As the County began to develop a protocol to identify perennial streams, staff invited experts from the public and private sectors to provide input. The Stormwater Planning Division field-tested the protocol in five small representative watersheds.

Fieldwork continued from 2001 to 2003 on a watershed by watershed basis, coinciding with the Fairfax County Watershed Master Planning Program.

Does this mean that other types of streams (ephemeral and intermittent) are not important?

Although Fairfax County is focusing on identifying perennial streams, it is important to remember that the headwaters of stream systems begin as small, short-lived ephemeral streams, whose sources of flow originate from storm events. Many ephemeral and intermittent streams begin at storm drain outfalls. Therefore, these systems are extremely important, as they are the direct link between your environmental practices at home and everything downstream.


Perennial Stream:

A perennial stream is a flowing system continuously recharged by groundwater or surface runoff regardless of weather conditions. It exhibits well-defined geomorphological characteristics and in the absence of pollution, thermal modifications, or other man-made disturbances has the ability to support aquatic life. During hydrological drought conditions, the flow may be impaired.

Intermittent Stream:

An intermittent stream is a flowing system under normal weather conditions. During the dry season and throughout minor drought periods, these streams will not exhibit flow. Geomorphological characteristics are not well defined and are often inconspicuous. In the absence of external limiting factors (pollution, thermal modifications, etc.), biology is scarce and adapted to the wet and dry conditions of the fluctuating water level.

Ephemeral Stream:

An ephemeral stream is a short-lived system for carrying stormwater. Under normal conditions, these systems will not flow. The geomorphology is characteristic of flashy and erosive stormwater events. Biology is limited to terrestrial and upland species.

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