Paving over
the landscape

More than 1.5 million acres of land are developed each year in the United States. Development alters the surface of the land by replacing natural cover with rooftops, roads, parking lots, and sidewalks. These hard surfaces are impermeable to rainfall and are collectively known as impervious cover.

Recent watershed research has shown that impervious cover has a profound and often irreversible impact on the quality of our nation's aquatic resources. More than 30 different scientific studies have documented that stream, lake and wetland quality declines sharply when impervious cover in upstream watersheds exceeds 10 percent. The strong influence of impervious cover on aquatic systems presents a major challenge to communities interested in sustainable development.

Here are some of the impacts of impervious surfaces on water resources:

• Higher peak discharge rates and greater flooding
• More frequent bankfull flooding
• Lower stream flow during dry weather
• Enlargement of the stream channel
• Greater streambank erosion
• Increased alteration of natural stream channels
• Less large woody debris in streams
• Loss of pool and riffle structure
• Increased number of stream crossings, with greater potential to affect fish passage
• Degradation of stream habitat structure
• Decline in stream bed quality (imbedding, sediment deposition, turnover)
• Fragmentation of the riparian forest corridor
• Warmer stream temperatures
• Greater loads of stormwater pollutants
• Bacterial levels that exceed recreational contact standards
• Lower diversity of aquatic insects and freshwater mussels
• Lower diversity of native fish species
• Loss of sensitive fish species (e.g., trout, salmon)
• Lower spawning success of fish
• Decline in wetland plant and animal diversity

From Better Site Design: A Handbook for Changing Development Rules in Your Community by the Center for Watershed Protection

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