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.

The white bars (adapted from Prey et al 1996) show estimates based on examining maps and driving around neighborhoods to assess the land use in each watershed. Each different type of land use was assigned an estimated amount of imperviousness, and these were summed for all the areas in each watershed.

Technologies have been rapidly advancing, and in 2003 a separate analysis used aerial photographs, digital maps, and software called ISAT: the Impervious Surface Analysis Tool developed by the National Oceanic and Atmospheric Administration’s Coastal Services Center to more accurately quantify the amount of impervious surfaces in Duluth’s watersheds. Additionally, estimates for 6 watersheds were calculated using manual digitization of aerial photos, including image analysis, which uses the spectral properties (colors) of the photo to pick out impervious surfaces.

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.