recognized the importance of placing streams and stream habitats in a geographic, spatial hierarchy (Godfrey, 1977; Lotspeich and Platts, 1982; Bailey, 1983).
A spatially hierarchical approach to describing environmental settings and evaluating stream habitat was proposed by Frissell and others (1986). The approach used by NAWQA is a modification of the spatial hierarchy as developed by these authors. The approach proposed by Frissell and others (1986) included five spatial levels--stream, segment, reach, pool/riffle, and microhabitat--referred to as "systems." The modified approach used in the NAWQA Program consists of a framework that integrates habitat data at four spatial scales: basin, stream segment, stream reach, and microhabitat (fig. 1). This approach differs from the scheme proposed by Frissell and others (1986) in that (1) the term "system" is not used, (2) basin is used to refer to stream system , and (3) the pool/riffle system is omitted as a scale to be evaluated but is incorporated into the definition of stream reach.
Figure 1.--Spatial hierarchy of basin, stream segment, stream reach, and microhabitat (modified from Frissell and others, 1986).
Sampling sites are generally chosen to represent the set of environmental conditions deemed important to controlling water quality in the study unit. Sites should represent combinations of natural and human factors thought to collectively influence the physical,