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SENT BY:DPRA St.Paul, MN ; 4:_12-33 ; 9:31AM 612 227 592- 612 407 41914 5 CHESTER L. ARNOLD, JR. AND C. JAMES GIBBONS intensive land uses that do generate pollution; (3) pre- vent natural pollutant processing in the soil by pre- vencing percolation; and (4) serve as an efficientconveyance system transporting pollutants into the waterways. It is not surprising, then, that research from the past 15 years consistently shows a strong cor- relation between the imperviousness of a drainage ba- sin and the Acalth of its receiving scream (I4ein 1979; Griffin 198o; Schueler 1987; Todd 1989; Schueler 1992; Hooch and Reinfelc 1993; Schueler 1994a)- Figure 2 is a stylized graph of this general rela- tionship, showing stream health decreasing with in. creasing impervious coverage of the watershed, or drainage basin, of the stream The horizontal lines mark average threshold values of imperviousncss at which degradation first occurs (10%), and at which degradation becomes so severe as to become almost unavoidable (30%). These thresholds serve to create three broad categories of stream health, which ran be roughly characterized as "protected" (less than 10%)i "impacted" (10%-30%), and "degraded" (over 30%). Thresholds are always controversial and subject to change, yecit is important to note char to date, the threshold of initial degradation in particular seems to be remarkably consistent. The scientific literature in. cludes studies evaluating scream health using many different criteria-polluaanc loads, habitat quality, aquatic species diversity and abundance, and other furors. In a recent review of these studies, Schueler (1994a) concludes chic "This research, conducted in many geographic areas, concentrating on many dif- ferent variables, and employing widely differentc meth- ods, has yielded a surprisingly similar conclusion - scream degradation occurs at relatively low levels of imperviousness (10-20%)" (100). Recent studies alsr+ suggest chat this threshold applies to wetlands health. Hicks (1995) found a well-defined inverse relationship between freshwater wetland habitat quality and im pervious surface area, with wetlands suffering impair• ment once the imperviousness of their local drainng- basin exceeded 10%. Impervious coverage, then, k both a reliable and integrative indicator of the impact of development on water resources. The second factor in favor of the use of impervi ousness is that it is measurable- This enhances its uril ity both in planning and regulatory applirarions. (Examples follow in a later section.) Depending on rl it - size of the area being considered and the parriculw application being applied, a wide range of Tech niques-with a wide range of price tags -exists for rhr measurement of impervious coverage. For site level applications, on -site measurement FIGURE 2. Stylized relationship of imperviousness co stream health Modified (mm Schueler 1992 245 APA JOURNAL • SPRING 1996