What is an impervious surface?
An impervious surface
is any surface such as pavement, buildings, patios, and sidewalks
that does not allow water to seep through to the soil below. On average,
about 65% of impervious surfaces are used for roads and parking lots
and the remaining 35% are buildings.
What impacts do impervious
surfaces have on water quality?
When a watershed
is paved, it sheds water like a raincoat. A watershed with minimal
impervious surfaces absorbs water like a sponge. As a greater percentage
of the area within a watershed is covered by roads and parking lots,
buildings, patios, and sidewalks, the volume and velocity of storm
water runoff carrying sediments, nutrients, and pollutants to lakes
and streams increases. This not only affects water quality but also
can increase flooding and erosion. Water flowing into streams and
lakes from parking lots or rooftops is often much warmer than water
flowing in through natural pathways and can harm aquatic plants and
animals, particularly in streams. Pollutants and nutrients move quickly
into surface waters when storm water and snowmelt cant percolate
into the soil. Impervious surfaces also may reduce groundwater recharge
and promote flashy streams (water levels fluctuate rapidly,
What can I do to minimize
Water is channeled
from buildings, roads, parking lots, and sidewalks into gutters or
directly into lakes or rivers. Careful landscaping in your yard, near
roads and driveways, and along shorelines will help reduce the amount
of runoff flowing directly into lakes or rivers or through storm sewers.
Minimize the amount of hard surfaces or covered areas that prevent
water from seeping into the ground. For example, use gravel instead
of asphalt or concrete for driveways and walkways. Choose stepping
stones or patio blocks instead of solid concrete sidewalks and patios.
Divert rain from paved surfaces into gravel-filled trenches or areas
of vegetation to collect water and allow it to filter gradually into
soils. Direct flow from drain spouts and roof gutters into water gardens
or rain barrels instead of directly down the sidewalk or driveway.
Vegetated shoreland buffer zones by the waters edge reduce the
amount and force of the runoff from impervious surfaces reaching the
lake. Vegetation also helps by intercepting rainfall before it reaches
What kind of walkway would
be best near the shore?
Wood decking, bricks,
bark mulch, or interlocking stones set in sand make good walkways
and patios. These walkways allow water to filter into soil more readily
than concrete or asphalt. Walkways and paths should follow the natural
contour of the land to minimize erosion. Well-worn paths that are
bare of vegetation often have soils that are so compacted that they
act like asphalt, channeling water into a fast-moving stream. If you
choose to use wood decking, buy untreated, rot-resistant wood such
as cedar or redwood. Treated wood contains toxic chemicals that can
leach out of the wood and impact your lake. Your city, township, or
county may regulate docks and piers, so check with them before purchasing
or building one. Also, check with your local zoning officials since
most zoning ordinances restrict the amount of impervious surface allowed
near shore. If you need to remove vegetation or move more than 10
cubic feet of soil to create a walkway to the water or a dock, check
with the Minnesota Department of Natural Resources, Division of Waters.
How can we protect our lake
from polluted runoff?
shoreland development projects allow water and its associated nutrients,
sediments, and contaminants to seep into the ground or adsorb to the
soil before they reach the water. A buffer strip of vegetation along
the shoreline helps filter sediments and nutrients before they reach
the water. A network of diversions, terracing, and contouring can
help control runoff problems.
Who can I contact if I have
questions or a problem related to impervious surfaces?
Check your local telephone
listing, the Who to Contact
section of the Minnesota Shoreland Management Resource Guide Web site,
or the Web sites listed below for:
What are some additional resources
related to impervious surfaces?
- Lakescaping for
Wildlife and Water Quality. 1999. C.L. Henderson, C.J. Dindorf,
and F.J. Rozumalski, Minnesota Department of Natural Resources
- A Guide for
Buying & Managing Shoreland. 1988. Minnesota Department of
Natural Resources, Division of Waters
- Streambank Erosion
a Greater Understanding. 1991. Minnesota Department of Natural
Resources, Division of Waters
A Guide for Waterfront Property Owners. 1994. M. Dresen, Wisconsin
Lakes Partnership, Stevens Point
- Protecting Our
Waters, Shoreland Best Management Practices Fact Sheet #8: Minimizing
Runoff from Shoreland Property. 1998. University of Minnesota
Extension Service, St. Paul
- Better Site Design:
An Assessment of the Better Site Design Principles for Communitties
Implementing the Chesapeake Bay Presercation Act. 2000. www.cwp.org