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by Jim McGlone, Urban Forest Conservationist,
Virginia Department of Forestry
(Conservation Currents,
Northern Virginia Soil and Water Conservation District, Summer
2006)
Earth is sometimes called the water planet. Huge
areas of the planet are covered with water and that water is
in constant motion. Flowing downhill in streams and rivers;
circling in ocean currents; and cycling through the atmosphere
to come down as rain and start its journey again. This last
motion – coming down as rain – creates an intimate
bond between the land and water. Streams evolve with their landscapes
to handle the storm flows that come off the land. As we change
the land, we change the storm flows and so affect streams.
 Forests are unique
in their ability to moderate storm flows. In a forest, only
ten percent of the rainfall from a moderate storm (of a magnitude
that occurs once every 1.5 to 2 years) actually reaches a stream
as runoff. Tree leaves, twigs, branches, trunks, and stems,
along with the forest floor litter, create an extensive surface
area many times greater than the land area the forest covers.
These surfaces intercept rainwater and allow much of it to evaporate
before reaching the soil. Most of the rain that reaches the
soil is held in the heavy organic layer and soaks into the ground.
This ground water feeds the forest or seeps into the stream
to keep it flowing between storms.
Unfortunately, forests are not compatible with
many aspects of modern life – houses, roads, stores, offices,
and ball fields are part of our communities. So we compromise.
Much of the protection that forests offer to streams occurs
in the 35 feet adjacent to the stream edge, beyond 100 feet
the forest offers little added protection. Forested riparian
buffers – forest strips along streams and other water
areas – filter out sediment, pesticides, fertilizer and
other things that wash off deforested upland areas. They also
help infiltrate some of the runoff from these areas. By filtering
and absorbing runoff, forested riparian buffers protect our
streams. Since we can’t have our forests intact and live
here too, we try to protect forested riparian buffers where
they exist and re-forest buffers where we can.
In rural areas, forest buffers do much to protect
streams and reduce the effects on streams of changes in land
use. However, in urbanized areas like Fairfax County, forested
riparian buffers do not seem to provide as much of a beneficial
effect as we would like.
Much of the pollution in Fairfax County streams
is organic – nutrients, oil and grease, human and animal
waste. Sediment – very fine soil particles – is
inorganic and perhaps the greatest pollutant. Outside of tidal
areas, stream bottoms should be made up of sand, gravel and
other coarse material. The openings between particles are home
to bacteria that consume the organic pollutants and clean the
stream much like a fish tank’s filter keeps the water
in the tank clean. When sediment enters the stream it fills
in these openings, eliminating habitat for the bacteria and
reducing the stream’s ability to clean itself.
Because of the relationship between sediment and
other pollutants and its role in causing stream deterioration,
sediment loads are a crude measure of stream health. Accotink
Creek flows south through central Fairfax County, from the City
of Fairfax and the Town of Vienna to Gunston Bay just below
Fort Belvoir. Although 35 feet of forest buffers the creek for
most of its length above Lake Accotink, the lake still received
35,000 cubic yards of sediment per year or approximately one
dump truck of sediment every 2.5 hours between 1986 and 2002.
 Why are forested riparian buffers failing to protect
stream health in urban areas? Largely it is because urban storm
water management bypasses the buffers. In an agricultural setting,
storm water flows across the entire surface of the riparian
buffer. In urban settings, stormwater is collected in a pipe
and channeled directly to the stream. Because the storm drain
connects our streets and parking lots directly to our streams,
these areas affect our streams’ health. In essence, these
areas have become part of the stream corridor even though they
are at some distance from the stream itself.
If urban forest buffers don’t adequately
manage stormwater, should we abandon them? No! Forested buffers
provide wildlife habitat and critical corridors that connect
larger areas of undeveloped open space. They are integrated
with our stream habitats by providing shade to reduce water
temperatures and leaves, which are food for the creatures at
the bottom of the aquatic food chain. But we must recognize
that past decisions about how we manage stormwater have made
it necessary to broaden our definition of the riparian zone.
Stormwater drains are extensions of the natural
stream network. So when we think about managing storm water
by planting trees in the urban riparian buffer, we must also
think about planting trees along the streets and parking lots
that are drained by our man-made streams. Although trees planted
over turf or pavement don’t manage stormwater as effectively
as forest, their ability to intercept and evaporate stormwater
is greater than no tree at all. To the extent that street tree
plantings can mimic forest settings by including shrubs and
or mulch, their usefulness in managing stormwater can be improved.
The great thing about using trees to manage storm
water is that they do so much more. Trees clean the air; improve
property values; increase retail trade by making shopping areas
more inviting; cool our planet, towns and houses; reduce winter
heating and summer cooling costs; and much more. When it comes
to the environment, trees are the ultimate multi-taskers!
Illustration (top left) reproduced
from Stream Corridor Restoration: Principles, Processes,
and Practices, 10/98, by the Federal Interagency Stream
Restoration Working Group (FISRWG).
Photo (bottom right) courtesy of Fairfax County. |
