National Water-Quality Assessment (NAWQA) Program

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Effects of Urbanization on Stream Ecosystems

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Synopsis of Study Design

This study examines the relation between varying levels of urban intensity in drainage basins and in-stream water quality, measured by physical, chemical, and biological factors in 11 metropolitan areas. For each metropolitan study area, stream sites are located in 28-30 basins that have minimal natural variability among them and represent a gradient of urban development intensity.

In this study, in-stream water quality is considered a composite of physical, chemical, and biological characteristics that are influenced by natural factors and by human activities; and, biological communities reflect the integration of these influences.

Where we studied

Map and description of individual study areas

Site selection process:
• Identified similarly sized basins in the study area (basin delineation).
• Assembled basin characterization information (land cover, infrastructure, socioeconomic variables, population, and so on).
• Calculated urban intensity index (UII) to rank sites along a gradient of urbanization.
• Identified an area with minimum natural variability (climate, elevation, ecoregion, etc.)
• Conducted site reconnaissance to assess site suitability for sampling.
• Recalculated information on basin characteristics based on sampling locations.
• Selected final sites that are distributed on the gradient of low to high urbanization.

Representing urban intensity: Urban Intensity Index (UII)
• The urban intensity of each basin is measured using an index that integrates information about the multiple dimensions of human influence on the urban landscape at the drainage basin scale.
• The UII is a multimetric index based on population, infrastructure, land cover, and socioeconomic factors correlated with changes in population density (McMahon and Cuffney, 2000). Some of the variables used in the index may also be important factors for explaining variations in water quality.

Determining variables to be included in the Urban Intensity Index
• Assemble the GIS-derived variables used to characterize population, socioeconomic, infrastructure, and land cover variables for each basin in the study area.
• Run correlation of individual variables with 2000 population density.
• Variables correlated with population density are adjusted to represent a range of urban intensity from 0-100 over all sites within a study area.

What "urban" looks like in each study area -- see “Where we studied

Controlling natural variability
• For each study area, basins were located primarily in a single ecoregion (see table below) to control variability in natural factors that influence water-quality, while allowing the degree of urbanization to vary among study basins.

What we measured

Overview and sampling protocols for data

McMahon, Gerard and Cuffney, T.F., 2000, Quantifying urban intensity in drainage basins for assessing stream ecological conditions: Journal of the American Water Resources Association, v. 36, p. 1247-1261.

Predominant USEPA Level III ecoregions* where basins were located in different study areas.
East Coast:
Atlanta, Georgia Piedmont
Birmingham, Alabama Valley and Ridge
Boston, Massachusetts NE Coastal Zone
Raleigh, North Carolina Piedmont
Great Lakes Region:
Milwaukee-Green Bay, Wisconsin Southeastern Wisconsin Till
South Central:
Dallas-Fort Worth, Texas Texas Blackland Prairies
Mountain West:
Denver, Colorado Western High Plains
Salt Lake City, Utah Central Basin and Range
West Coast:
Portland, Oregon Willamette Valley
Seattle, Washington Puget Lowland

*United States Environmental Protection Agency (USEPA), 2000, Level III ecoregions of the continental United States: Corvallis, Oregon, U.S. Environmental Protection Agency-National Health and Environmental Effects Research Laboratory, Map M-1, various scales.

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