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 streets.

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 and habitat.

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 color coding follows a convention used by the National NEMO program (nemo.uconn.edu), and are associated with studies showing that when impervious surface values are greater than 25%, streams are likely to have water quality problems, and values from 10-25% indicate the watershed is "at risk" for water quality problems.