Bridge BR Channelized area CA Diversion DV Feedlot FL Hydropower HP Impoundment IM Industrial outflow IO Low-head dam LH Natural lake NL Storm sewer SS Streambank stabilization SB Thermal discharge TD Waste-water treatment WT Other OT
A stream reach (fig. 1) is the least clearly defined unit in the spatial hierarchy (Frissell and others, 1986). Although a stream segment is a discrete unit that should represent a uniform set of physical, chemical, and biological conditions within a stream, its length (often more than several kilometers) prohibits effective field collection of data. Therefore, the stream reach is chosen as the principal sampling unit to collect physical, chemical, and biological data. The type of reach characterizatio n conducted is dependent upon the type of site (fixed or synoptic). However, the factors considered in selecting a stream reach are the same for all sites.
The type and distribution of geomorphic channel units are the most important factors in selecting a stream reach. Geomorphic channel units are fluvial geomorphic descriptors of channel shape and scour pattern that are widely used in habitat assessment surveys (Orth, 1982; Ohio Environmental Protection Agency, 1989). The development of specific sequences of geomorphic channel units is a fundamental stream process (Ying, 1971; Beschta and Platts, 1986). Identification of geomorphic channel units is important because it classifies stream habitat at a spatial scale relevant to most biota in streams (Frissell and others, 1986). Three types of geomorphic channel units are considered when selecting a stream reach--pools, riffles, and runs (fig. 4).
The stream reach should include at least two examples each of two types of geomorphic channel units. Only those geomorphic channel units that are greater than 50 percent of the channel width are considered when the length of the reach is determined. The