Stormy Weather and Stormwater Impacts - MSU Center for Water ...

The Problem of Stormwater
According to the US EPA, stormwater runoff is the leading cause of water quality
impairment in the United States. Urban stormwater runoff occurs when rain or
snowmelt flows over impervious surfaces, such as buildings, roads, driveways,
sidewalks, and compacted soils. Along its way, stormwater picks up debris, chemicals,
bacteria, and other pollutants and carries them directly to rivers, lakes and streams.
Polluted stormwater runoff can have many adverse impacts on plants, animals, and
humans. For example, most waterborne disease outbreaks in the United States are
associated with large storm events.
Impervious surfaces are a key component of the stormwater problem. Impervious
surfaces prevent precipitation from naturally infiltrating into the soil and, as a result,
precipitation falling on these surfaces increases stormwater runoff. Many studies have
examined the relationship between impervious surface area in a watershed and
biological indicators of river and stream health. From these studies we have learned
that as the amount of impervious area in a watershed increases, the biological
integrity of rivers and streams declines. This pattern is found across the United States
with a variety of aquatic organisms used as environmental health indicators, from fish
to aquatic insects. We start to see negative impacts on aquatic ecosystems when 10%
of a watershed is covered in impervious surfaces and once the level of impervious
surface area reaches 25%, stormwater impacts seem unavoidable.
Photo: Erin Dreelin
Parking lots are just one example of an impervious surface
that does not allow water to infiltrate into the soil.
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