Description of Study Area

The study area in North Carolina is in the southeastern United States, and includes the five metropolitan statistical areas (MSAs) of Raleigh-Cary (2000 population of 797,071); Greensboro-High Point (population of 643,430); Durham (population of 426,493); Winston-Salem (population of 421,961); and Burlington (population of 130,800) (fig. 2; U.S. Census Bureau, 2003). These five combined MSAs increased in population by 27.7 percent from 1990 to 2000 (U.S. Census Bureau, 2003).

Figure 2. Study basin locations. (See table 1 for key to study basin numbers. Additional information on study basin characteristics is available at http://nc.water.usgs.gov/albe/General/ulug/ULUG_page.html.)

The study area is in the Piedmont ecoregion (Omernik, 1987), specifically in three subecoregions: Northern Outer Piedmont, Southern Outer Piedmont, and Carolina Slate Belt. Landforms in the Piedmont ecoregion are irregular plains with some hills (Griffith and others, 2002). Altitude in the study area ranges from about 50 to 315 meters (m; 164 to 1,033 feet; U.S. Geological Survey, 2005a). Land use in the study area has undergone major transformations from oak-hickory-pine forest to agricultural lands to urban and suburban lands. At one time, the region was heavily farmed in cotton, tobacco, corn, and wheat, and in many areas, the silty-clay soils that are characteristic of the region were moderately to severely eroded (Trimble, 1974).

The climate is warm and humid, with a mean annual temperature of 15 degrees Celsius (59 degrees Fahrenheit) and a mean annual rainfall of 118 centimeters (46 inches; Daymet, 2005). Rainfall is evenly distributed throughout the year; slightly more rainfall occurs in July and August, and slightly less rainfall occurs during October through December. The year prior to sampling (October 2001–September 2002) was a period of low rainfall and drought conditions in the area, although normal rainfall resumed in October 2002 after sampling was initiated in September 2002. Streams in the area typically have low to moderate gradients and gravel to cobble substrates and support warm-water biological communities. Streamflow generally is highest in the winter months and lowest in late summer.

Thirty sites, which covered a range of urban conditions but had similar natural environmental conditions, were selected for study (table 1). The site basins ranged from 5 to 82 square kilometers in size and land use ranged from 3 to 98 percent urban. Sites were selected from a pool of candidate basins based on urban conditions in the basin, minimum required habitat characteristics at the site, and access and accessibility. The method of site selection was the same as that described by Tate and others (2005).