Impervious Surfaces Hinder Infiltration and Increase Runoff
What is an Impervious Surface?
Impervious surfaces are surfaces that do not allow water to
infiltrate the soil. They include rooftops and asphalt or concrete
roads, parking lots and sidewalks.
If these surfaces stood alone in a sea of porous soil and vegetation,
they probably wouldn't cause many problems; instead they are
usually connected to one another. Rooftops drain onto driveways,
sidewalks and parking lots that connect to a network of paved
During rain storms or periods of snow melt, these surfaces
channel water down our city streets, into storm drains, and
eventually into our creeks and lakes.
What harm is done?
In a completely natural setting, streams
come to equilibrium with their environment. Water from
storm events erodes some parts of the channel, while
depositing sediments in others. Groundwater slowly
enters the stream from the surrounding watershed long
after storms have past, acting as a constant resupplier.
Urban streams fill more quickly (since water can run off
of pavement quickly) and with more water than their natural
counterparts. All of this water causes greater stream channel
erosion and less of this material is redeposited in the channel
before washing into a larger river or lake. Since
less water infiltrates the soil in urbanized watersheds,
less is available to recharge streams during dry seasons.
What can you do to help?
Help reduce the amount of stormwater
running off your property reducing impervious surfaces, creating
a wet garden or installing a rain barrel. Read more at the
Citizen Action - Reduce Runoff
page of this website.
If you plan to build a new house, or hope to add new driveways
or sidewalks to your existing property, consider using pervious
building materials. Learn about these materials here.
How do Duluth Streams Rate?
The following graph presents the percent of the Duluth Trout
Stream watersheds covered with impervious surfaces. Streams
with greater than 10% impervious surface (the stream impact
threshold) are more likely to have degraded water quality
Technology for measuring impervious surfaces on the landscape have improved dramatically over the past few years. The data shown in the graph is from satellites, and researchers at the University of Minnesota were able to carefully calibrate the data to determine how much "greenness" was in each 30m x 30m pixel. The amount of greenness is directly related to the amount of impervious surface, and so for each pixel, the researchers can determine the % impervious within that pixel. Summing this for the entire watershed provides a highly accurate measure of the total amount of impervious surfaces. More detail on this method and a tool for mapping other statewide watersheds can be found at www.land.umn.edu.
Satellite data is available for different time periods, allowing a comparison over time, to track how impervious surfaces are changing in our watersheds. This can be an important tool for determining which watersheds are most at risk for water quality problems.
The red line follows a convention
used by the National NEMO program (nemo.uconn.edu), and is
associated with studies showing that when impervious surface
values are greater than 10%, streams are likely to have water