From 2000 to 2004, the USGS National Water-Quality Assessment (NAWQA) program conducted studies to determine the effects of urbanization on riverine ecosystems in nine major metropolitan study areas with varying physiographic settings across the U.S. (Couch and Hamilton, 2002; Tate et al., 2005). Study areas included Portland , Ore. (POR); Salt Lake City , Utah (SLC); Denver , Colo. (DEN); Dallas-Fort Worth, Tex. (DFW); Milwaukee-Green Bay , Wis. (MGB); Birmingham , Ala. (BIR); Atlanta , Ga. (ATL); Raleigh , N.C. (RAL); and Boston , Mass. (BOS). In each study area, 28-30 watersheds were selected to characterize a gradient of urbanization (low to high), while holding relatively constant the key environmental factors that might influence a biological response, such as basin size, soil characteristics, and stream gradient. Depending on the study area, the gradient of urbanization was generally associated with the loss of either primarily agriculture or naturally vegetated land (e.g., forested).
The urban intensity of each watershed was assigned from an urban intensity index (UII) derived using basin-scale variables measuring housing density, road density, and urban land cover percentage. These variables, respectively, represented the extent of population, infrastructure, and developed land cover, that together served as a proxy for the comprehensive changes associated with urbanization. The functional concept of the UII is that it would characterize many disturbances associated with urbanization, and would relate to changes in biological assemblages better than any single environmental factor. The biological assemblages investigated in theses studies were algal, invertebrate, and fish; the environmental factors that were investigated were multiple physical and chemical variables that could affect these assemblages. This project specifically addressed the algal component, and the primary objective was to determine if changes in their assemblages (i.e., algal response) could be directly related to urbanization as characterized by the UII, or if the algal response was more clearly related to environmental factors at either the watershed (distal) or reach (proximal) scale.