Streams through the City: Water Quality and Quantity

Adapted from Impacts of Urbanization on Stream Water Quantity and Quality

Impervious surfaces like pavement can cause water to flow into streams more rapidly, carrying pollutants such as sediments along with it. Photo by NRCS.
Impervious surfaces like pavement can cause water to flow into streams more rapidly, carrying pollutants such as sediments along with it. Photo by NRCS.

Since the 1950s, urban areas have increased by more than 400 percent, and are now home to 80 percent of Americans. Water resources in these areas are threatened, and understanding how urbanization affects water quantity and quality is increasingly important.

U.S. Forest Service scientists Ge Sun and Peter Caldwell recently reviewed urbanization and its impacts on water for the Water Resources Impact journal. Sun and Caldwell are research hydrologists at the Southern Research Station Eastern Forest Environmental Threat Assessment Center, and the Coweeta Hydrologic Laboratory, respectively.

Urbanization affects streams by altering microclimate, surface water dynamics, groundwater recharge, stream geomorphology, biogeochemistry, and stream ecology. These changes affect both water quantity and quality (nutrient, sediment and pollutant levels).

Key findings from the article include:  

  • When forests are cleared for urban uses, the amount of water flowing through streams increases. The magnitude of the increase depends on how much of the area is covered by impervious surfaces such as roads and parking lots, how many trees remain, and other factors.
  • Some researchers have found that when approximately 40 percent of a watershed is impervious, the amount of runoff can double.
  • Urbanization affects water quality because more pollutants are produced in urban settings, and the watersheds lose the ability to hold and retain water because of the increase in impervious surfaces. Some studies suggest that water quality and stream integrity may be negatively affected when as little as 5 percent of surfaces are impervious to water.
  • Converting watersheds from forest to urban areas often elevates sediment and nutrient concentrations in surface waters by tens to hundreds of times.
  • Urban waters often contain pharmaceuticals such as antibiotics, analgesics, narcotics, and psychotherapeutics, as well as pesticides, heavy metals, pathogenic microbial populations, and organic pollutants such as PCBs.
  • The abundance, diversity, and health of fish, amphibians, reptiles, and invertebrates are declining due to water pollution. Aquatic animals are affected by increased stream flow, changes to the stream bed substrate, and pollution.
  • Protecting water resources in urban landscapes requires best management practices (BMPs) and protecting source headwaters.

The contemporary watershed management goal is to influence land management practices within a watershed and prevent development from occurring or otherwise minimize impact in critical locations that are particularly sensitive to water quality and quantity.

As more areas are urbanized,  supplies of clean water could be threatened. Cities and towns with periodic droughts or wide ranges of temperature and precipitation are also more likely to be threatened, and climate change will exacerbate these threats. A combination of factors such as infrastructure renovation, improved designs for new water and sanitation systems, and wider implementation of watershed management principles will be needed to provide clean water for expanding human populations.

Read the full text of the article.

For more information, email Sun at

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