water over roadway

Stormwater Basics

Rain and snowfall in Washington provide our forests, crops, and ecosystems with vital water they need to thrive.

Natural systems manage rainfall and water runoff through interception, evaporation, transpiration, infiltration, and gradual runoff into surface waters. But built features such as roads, roofs, sidewalks, and parking lots, can alter these natural processes. As a result, water quality can be degraded, erosion and flooding can occur, and stream flows can be impacted during dry periods.

When the amount of rainfall and snowmelt exceed the land’s ability to absorb it, the result is stormwater runoff. The amount of runoff and the rate at which it flows varies with the intensity and duration of the rainfall as well as with the type of land surface. While a moderate rainfall on permeable soils might produce little to no runoff, the same amount on an impervious parking area can produce substantial runoff.

If runoff is not properly managed and flows unimpeded, it can transport contaminants such as heavy metals, bacteria, pathogens, viruses, organic compounds, and other pollutants directly into creeks, lakes, rivers, and marine waters. Not to mention cause erosion and produce flooding.

It’s important to recognize that stormwater isn’t just an issue in developed, urban areas. It can also cause problems in less-developed, rural areas.

Are storms getting worse?

One result of human-induced climate change is the altered rainfall patterns in the Pacific Northwest. Climate researchers predict an increase in the frequency and intensity of heavy rain events in Western Washington, leading to an increase in winter streamflows. This may lead to more flooding in the future.