title>Impervious surfaces and their impacts on Nature
Connecting People with Nature

Impervious Surfaces

Essential Knowledge for Naturalists

The Influence of Hard Surfaces on Runoff

Impervious surfaces increase the speed and volume of water running off a site, while decreasing the quality of that water. Impervious surfaces like concrete, asphalt, rooftops, brick walkways, and similar structures which do not allow water to penetrate into the soil are a stressor on aquatic life, as well as human communities.

When a new subdivision is built, it usually increases fragmentation, and creates edge effects. Along with these impacts, new roads and rooftops alter natural drainage patterns. Water that would have been filtered by vegetation, soaked into the ground, then infiltrated into the groundwater supply instead drains across a hard surface to a catch basin, falls into a pipe, and is dumped, full speed into a stream. While running across chemically treated lawns and oil-stained driveways, the water picks up contaminants that are not filtered.

Piping the water to get it off-site faster increases the velocity, while the volume is increased by the inability of the water to soak into the ground. The result is, more dirty water enters streams and rivers quicker than it did under undeveloped conditions. Silt-intolerant aquatic life, such as the Eastern Sand Darter, a small fish that is now very rare because of water quality issues, are harmed. This of course has indirect effects. Animals that eat the organisms harmed by excess runoff are harmed as well.

How does this all impact human communities? In more ways than we can imagine. The most notable impact is a marked increase in flooding. Across the developed portion of the United States, there has been a significant upswing in the number of "100 year" or "500 year" flood events. These super-floods are caused by vast amounts of water quickly entering streams with degraded floodplains, and fewer wetlands to filter and store the runoff.

This increased flow and volume brings rivers out of their banks more swiftly, causing property damage, road closures, and disrupting commerce. It also scours mussel beds, realigns streams, and has a tendency to alter stream dynamics.

The next time you have a chance to stand outside in a rain storm, do it. Take a look at a field or forest in the rain, then check out the amount of water running into the storm drains in your local school's parking lot, or at the grocery store. You will be amazed at the extreme difference.

Think about your own home. Your yard, driveway, rooftop. What could you do to decrease the amount of runoff you contribute? Rain barrels? Create a small wetland? Dig up the concrete and install gravel instead?

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