(Center for Watershed Protection/
Strategies for Suburban Watersheds
Strategies for Rural Watersheds (0 to 10 percent
(10 to 25 percent Impervious Cover)
Impervious Cover)
Principle 1
ne of the major implications of the impervious
Design residential streets for the minimum required
cover model is that it predicts negative stream
pavement width needed to support travel lanes; on-
impacts at an extremely low intensity of watershed
street parking; and emergency, maintenance, and
development, in the absence of watershed treatment. To
service vehicle access.  These widths should be based
put this in perspective, consider that a watershed zoned for
on traffic volume.
two-acre lot residential development will generally  exceed
ten percent impervious cover, and, therefore shift from a
Principle 2
sensitive to an impacted classification. Thus, if a commu-
Reduce the total length of residential streets by
nity wishes to protect an important water resource or a
examining alternative street layouts to determine the
highly regarded species, such as trout, salmon or an
best option for increasing the number of homes per
endangered freshwater mussel, the impervious cover
unit length.
model suggests that there is a maximum limit to growth,
which is not only quite low, but is usually well below the
Principle 3
current zoning for many suburban or even rural water-
Wherever possible, residential street right-of-way
widths should reflect the minimum required to
accommodate the travel-way, the sidewalk, and
It is not surprising, then, that debate has quickly shifted to
vegetated open channels.  Utilities and storm drains
the issue of whether watershed  treatment practices can
should be located within the pavement section of the
provide adequate mitigation for impervious cover model.
right-of-way wherever feasible.
(For example, can stream buffers, storm water treatment
practices, better site design and other forms of watershed
Principle 4
treatment allow greater development density within a given
Minimize the number of residential street cul-de-sacs
watershed?)  Only a limited amount of research has
and incorporate landscaped areas to reduce their
addressed this question, and the early results are not
impervious cover.  The radius of cul-de-sacs should be
reassuring.  At this early stage, researchers are still having
the minimum required to accommodate emergency
trouble detecting the impact of watershed treatment, much
and maintenance vehicles.  Alternative turnarounds
less defining it. Both our watershed research techniques
should be considered.
and ability to implement watershed treatment need to
greatly improve before we can expect a scientifically
Principle 5
defensible answer to this crucial question. Until then,
Where density, topography, soils, and slope permit,
managers should be extremely cautious in setting high
expectations for how much watershed treatment can
vegetated open channels should be used in the street
right-of-way to convey and treat storm-water runoff.
mitigate impervious cover.
Principle 6
As a consequence, land-use plans for rural watersheds
The required parking ratio governing a particular land-
should limit impervious cover to less than 10 percent,
use or activity should be enforced as both a maximum
particularly if monitoring confirms that the streams are of
and a minimum in order to curb excess parking space
excellent quality.  Other key strategies include agricultural
zoning, aggressive land conservation, wide stream buffers,
construction.  Existing parking ratios should be
reviewed for conformance taking into account local
and prohibition of sewers. Any development that does
and national experience to see if lower ratios are
occur should be clustered in village centers, rather than
warranted and feasible.
distributed in large lots across the landscape.