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• 21. Water management feature: Record the type(s) of water management feature(s) nearest the segment and the distance from the reference location. Also note whether the feature is upstream of or downstream from the reference location. Include as many water management features as appropriate. Water management feature types are noted using the following two-letter codes:

                    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

  Reach Characterization

  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.

  Selecting a Stream Reach

  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