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Impervious Surfaces & Water Quality
 Impervious surfaces are areas covered with roads, parking lots, roofs and other surfaces that do not allow water to soak into the ground. The result is a significant increase in the volume of stormwater that runs off the land, and significant impacts to local waterways.
According to research from the Center for Watershed Protection and many other sources, streams are impaired when impervious surfaces over just 10% of a watershed. Streams in watersheds where impervious surfaces cover 25% of the watershed area cannot support aquatic life. |
Average % of impervious surface by land use type: |
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What to expect when just 10% of a watershed is covered with impervious surfaces: |
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| Land Use |
Impervious Cover % |
Forest |
1% |
Urban/Suburban Open Land |
3% |
Low Density Residential
(0.5 units/acre)
|
12% |
Low Density Residential
(1 units/acre)
|
20% |
Medium Density Residential
(2 units/acre) |
25% |
Medium Density Residential
(3 units/acre)
|
30% |
Medium Density Residential
(4 units/acre)
|
38% |
High Density Residential
(5-7 units/acre)
|
40% |
Multifamily Townhouse
(>7 units per acre)
|
65% |
Commercial
|
85% |
Parking – Unpaved
|
90% |
Roads/Paved Parking
|
100% |
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Increased floods and flood peaks, leading to stream straightening and streambed erosion;
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Increased erosion, leading to loss of trees and vegetation along the banks (at 8% to 10% impervious surface coverage, streams double in the size of the bed due to the increased volume);
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Increased pollutant loads;
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Increased shell fish diseases and beach closures;
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Increase in stream temperature which interferes with many biological processes;
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Increased bacteria, often as a direct of a high density of household pets;
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Decreased high weather flow;
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Decreased pooling;
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Decreased woody debris, a crucial habitat element for aquatic insects;
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Decrease in substrate quality;
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Decreased fish passage during dry weather flow periods due to the enlarged stream bed;
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Decrease in insect fish and fish diversity. At 12% imperviousness, trout and other sensitive species can no longer survive in the stream.
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Source: Center for Watershed Protection, 1998. “Rapid Watershed Planning Handbook” |
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