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U.S. GEOLOGICAL SURVEY
Data Series 345

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Site Selection

This study was conducted in five study areas that included six NAWQA study units (table 1). The five areas included the combined Apalachicola-Chattahoochee-Flint River Basin (ACFB) and Georgia-Florida Coastal Plain Drainages (GAFL) referred to here as ACFB/GAFL, Central Columbia Plateau–Yakima River Basin (CCYK), Central Nebraska Basins (CNBR), Potomac River–Delmarva Peninsula (PODL), and the White-Miami River Basin (WHMI). A single USEPA Level III Ecoregion (Omernik, 1987) within each area was selected for the study in order to capture similar hydrologic landscapes, soils, climate, land use, and biota in a single map classification. To be selected, a single ecoregion had to contain significant agricultural activities, a range in nutrient conditions, and a sufficient number of suitably sized streams for study (fig. 1).

In each of the five study areas, basin and reach coverages were derived from the Elevation Derivatives for National Applications project using 30-m Digital Elevation Model data. Independent basins between 50 and 2,500 km² were selected and the National Hydrologic Dataset (NHD) was used to verify the locations of streams. A number of independent basins were selected to represent a range in the estimated loadings of nitrogen and phosphorus to each basin. Estimates were based on atmospheric data, animal populations, and fertilizer sales (Ruddy and others, 2006).

The following measurements were made as part of the field reconnaissance to select specific sampling sites:

• nutrient concentrations,
• specific conductance,
• pH,
• water temperature,
• dissolved oxygen,
• estimates of average channel width,
• depth,
• riparian buffer width,
• canopy cover, and
• observations of stream habitat types and adjacent land use.

This process resulted in the selection of 27–30 sites in each study area for a total of 143 sites—ACFB/GAFL, 29 sites; CCYK, 29 sites; CNBR, 28 sites; PODL, 27 sites; and WHMI, 30 sites. Each site (or reach) had to be at least 150 m in length and was defined by a repetition of a geomorphic sequence (for example, 2 riffles and 2 pools) or 20 channel widths if repetitive units were not available (Fitzpatrick and others, 1998). All sites selected and sampled are shown in appendix 1.

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