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Summer 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

September 20, 2011

Troubled urban streams


When hurricanes or tropical storms pass through the Washington area, flash flooding can become a problem, not just for houses and businesses in the flood plain, but also for the organisms that live in the waterways being flushed with storm water.

The flow can cause serious disturbances in streambeds, "uprooting aquatic plants, rolling rocks and killing many bottom-dwelling organisms," says N. LeRoy Poff, an ecology professor at Colorado State University.

Heavy rains can also flush pollutants and soil sediments into streams. Although too much sediment in a stream can cause problems, too little can also be detrimental.

"Many streams inside the Beltway have been urbanized for a long time, and there is no longer available sediment from the watershed due to all the impervious surfaces," says Poff. A loss of sediment cripples an urban stream's ability to deposit new banks and bottoms, so the stream becomes incised, cutting an increasingly deeper groove for itself.

Another problem: Roofs, streets and parking lots prevent rainwater from soaking into the ground, sending it as a "flash" through urban waterways. Sensitive species have trouble finding refuge during such an event.

Only species that are pollution-tolerant, have a fast life cycle and are able to handle extremes in flow can survive in such an environment, says Poff.

Asian clams, an exotic invasive species found in local streams, "do very well in extreme flashy environments," says Daren Carlisle, an aquatic ecologist with the U.S. Geological Survey.

Carlisle describes urban-stream fishes, such as the common carp, as generalists: "They can reproduce anywhere and eat anything."

SOURCES: Joel Guyer, USGS Virginia Water Science Center; USGS National Water Information System

Watts Branch, an urban stream in Washington, D.C.