The stream segment is defined as that part of a stream bounded by tributary junctions or discontinuities, such as major waterfalls, landform features, significant changes in gradient, or point-source discharges. A segment should be a distinct stream unit that is relatively homogeneous with respect to physical, chemical, and biological properties. Data at the segment scale are collected using GIS data bases and(or) 7.5' maps. Segment characteristics include gradient, sinuosity, stream order, downstream link number, and the presence of water management features such as dams, canals, lakes, or point-source discharges.
The length of a segment often prohibits effective field collection of habitat data. Therefore, the stream reach is chosen as the sampling unit to collect data on habitat. The reach is defined on the basis of the type and distribution of geomorphic channel units (pools, riffles, and runs) that are present in a segment. If the stream is composed of only one geomorphic channel unit (for example, a run), the length of the sampling reach is 20 times the channel width. Regardless of the geomorphic channel units present or the channel width, minimum and maximum length criteria are used. Minimum reach lengths of 150 m in wadeable reaches and 500 m in nonwadeable reaches are recommended. In most wadeable reaches, a maximum reach length of 300 m is sufficient for adequately characterizing habitat and providing representative samples of biological communities. However, in relatively wide (greater than about 30 m) wadeable streams, a maximum reach length of 500 m should be considered. A maximum reach length c riterion of 1,000 m should be considered for nonwadeable reaches.
The type of reach characterization conducted depends on the type of site--fixed or synoptic. At fixed sites, two levels of reach characterization are considered: a first-level reach characterization that provides for relatively rapid, consistent data collection within and among study units; and a second-level reach characterization that provides a balance between detailed descriptions of stream habitat and study unit flexibility in data-collection efforts.
The first-level reach characterization consists of field collection of instream and riparian habitat data based on six transects established perpendicular to streamflow. The second-level reach characterization consists of an analysis of channel geometry, plus additional quantitative evaluations of channel and riparian characteristics that are selected locally to enhance the understanding of temporal changes in the environmental setting and stream habitat at fixed sites.
At synoptic sites, the approach used to collect habitat data must be (1) flexible to accommodate the variety of questions being addressed at a synoptic site, (2) relatively rapid to facilitate data collection at a large number of sites, yet (3) capable of providing an adequate description of stream habitat at one point in time. The reach characterization conducted at synoptic sites, at a minimum, should consist of a subset of the first-level reach characterization conducted at fixed sites.
Microhabitat characterization is an evaluation of the local set of conditions that describe the location of benthic invertebrate and algal communities. Procedures for collecting microhabitat data are described in documents detailing the collection of invertebrate and algal samples, and are not within the scope of this report.