Imperviousness of a watershed

New: Measuring the Impact of Development on Maine Surface Waters (off-site)

There is a relationship between the percent impervious area of a watershed, and the water quality of the waterbody to which the watershed drains. In Maine, no streams with greater than 10% imperviousness that have been evaluated for attainment of aquatic life standards have met Class B standards, and many fail to meet Class C standards as well. Notes from a talk given by Jeff Dennis (DEP).

Two means of identifying "at risk" streams are by using monitoring data, when available, and by using the imperviousness of the watershed as an indicator.   In broad terms, a stream is considered at risk if it is not currently meeting standards, or is at risk of degredation based upon the percent of its watershed that is impervious.  Streams are classified as AA, A, B, or C by statute, and there are water quality standards associated with each classification.   Most urban streams are classified as B or C and are required to meet less stringent standards than Class AA and A streams.  The classification system relates back to requirements under the federal Clean Water Act.  For more information about water quality classification, see the Classification Page.  A stream is considered "impaired" if it is not meeting one or more of the water quality standards related to its classification.

Imperviousness as an indicator

"Imperviousness" refers to the area of roads, parking lots, sidewalks, rooftops, and other impermeable areas in the watershed.  The percentage of the watershed that is imperious can be used as an indicator to measure the impact of land development on aquatic systems.   For a good summary on this topic, see The Practice of Watershed Protection, Article 1, "The Importance of Imperviousness", (off-site), which discusses research relating relative stream stability and health to the percent imperviousness of a watershed (off-site, takes awhile to load).   Staff at DEP have also studied many streams in Maine to determine whether the indicator works in Maine, and it does.   No streams with watersheds of over 10% imperviousness, which have been examined in Maine, have been found to meet Class B standards.  In other words, all have shown detrimental impact to the aquatic community of the stream.    Growth in watersheds below 10% can be expected to result in detrimental impacts on streams as imperviousness approaches 10%, unless steps are taken to control the quantity and quality impacts from stormwater runoff.

Summary of the Method Used to Develop an Algorithm to Predict the % Imperviousness of Watersheds.

Maps

Urban Stream Watershed Imperviousness and Attainment Status (large files that take time to load). If the map appears very small on your screen, please use the function on your browser that allows you to enlarge the image.