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Selecting Sampling Sites and Habitat Types

Sample site selection involves three elements: (1) locating the basic fixed site where chemical and flow data are available; (2) establishing one or more sampling reaches; and (3) identifying and selecting specific locations of instream habitat types with in each sampling reach from which invertebrate samples are taken. The first element of site selection is a cooperative effort among the discipline groups of the study-unit team to locate sites that represent the set of environmental conditions deemed important for controlling water quality in the basin. Retrospective information is an important component of this element of site selection, as is input from liaison committees and other local experts. This element of site selection is crucial to the success of local, regional, and national synthesis efforts. Sites are chosen to represent combinations of natural and anthropogenic factors thought to collectively influence the biological, physical, and chemical characteristics of water quality in the study unit and to be of importance locally, regionally, or nationally.

The second element involves establishing sampling reaches that are used to characterize conditions associated with the basic fixed sites. The length of the sampling reach is established by guidelines set forth in the stream habitat assessment protocol (Meador, Hupp, and others, 1993). Ideally, each sampling reach includes multiple examples of the major geomorphic features of the stream segment (for example, two pool-riffle sequences) and might be located entirely above or below the basic fixed site, or might encompass it. Major discontinuities in channel or riparian characteristics and intervening point sources within or among the sampling reaches associated with a basic fixed site are avoided. Theoretically, ecological survey sampling is preceded by at least 1 year of antecedent physical and chemical data collection to maximize the integration of these data sets. The reconnaissance effort plays a major role in locating and selecting sampling reaches that represent conditions typical of the areas associated with basic fixed sites.

The third element of site selection involves identifying and locating appropriate instream habitat types. Instream habitat types are broadly defined on the basis of a hierarchical grouping consisting of three tiers: major geomorphic channel units, major channel boundaries, and major channel features (table 1). The highest level of habitat organization is the major geomorphic channel unit: riffle, run, or pool. The second level is based on the influence of margins on the distribution of organisms, particularly in large rivers, and subdivides riffles, runs, and pools into channel, channel margin, and island margin areas (Thorp, 1992). Margins, loosely defined as instream areas associated with the edges of main or secondary channels and islands, are typically depositional, subhorizontal fluvial surfaces, with reduced current velocity as compared with the adjacent channel area. Margins are influenced heavily by the streambanks and typically contain elements directly derived from the streambank, such as root wads, snags, and terrestrial vegetation that trails into the water. Channels tend to be less influenced by the channel banks and represent the main or secondary flow path of the river. The extent of the margin will be influenced by river stage, the size of the river, and channel characteristics. For example, margins might be (1) a substantial proportion of the width of small streams but only a very small proportion of the width of large rivers, (2) greatly reduced on the outside of stream meanders but extensive on the inside of stream meanders, and (3) abundant at low flows but inaccessible at