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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.
The Fairfax County Stormwater Planning Division has begun
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.
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.
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 there was an unreliable,
incomplete stream map and that many perennial streams were not
being protected under the County’s Chesapeake Bay Preservation
Ordinance (CBPO).
Under the current CBPO, perennial streams are only those
depicted as perennial on the U.S. Geological Survey (USGS) maps.
These streams as well as wetlands, tidal wetlands, and tidal
shores are included within Resource Protection Areas (RPAs)
that perform certain ecological and biological processes indicative
of maintaining good water quality. RPAs are vegetated riparian
buffer areas, which include land within a major floodplain and
land within 100 feet of the aforementioned water bodies. These
buffer areas are important in the reduction of sediments and
nutrients, as well as the other adverse effects of human activities
that could potentially degrade these systems and those downstream.
If streams are not identified as perennial on the USGS map,
they do not warrant an RPA under the current CBPO.
The Chesapeake Bay Local Assistance Department (CBLAD) has
revised the state’s regulations and requirements to determine
a consistent methodology for identifying perennial streams.
Since the state regulations allow localities to “exercise
judgment” in determining some requirements, Fairfax County
has some latitude in developing its own definition of perennial
streams and protocols to identify them. The County is currently
revising its CBPO to ensure compliance with the state’s
regulations.
The amended CBPO is scheduled to take effect on March 1, 2003.
The revised ordinance will protect all perennial streams by
including them in RPAs. One of the major goals of the perennial
stream project is to consistently field-identify and map all
perennial streams using a scientifically valid protocol and
include them in RPAs. Once the map is completed some time
in 2004, it will go to the Board of Supervisors for consideration.
Until the official map is adopted, the County will create and
use an interim map that will reflect the portion of perennial
streams that will have been field-identified through the end
of this year. It will also include 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 come in for
review, County staff will require that developers perform
field studies to determine if streams are 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 will continue over the next two years on a watershed
by watershed basis, coinciding with the Fairfax County Watershed
Master Planning Program.
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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What
are perennial streams?
