The location and length of the sampling reach are determined by a combination of geomorphic characteristics (that is, repeating geomorphic channel units (Meador, Hupp, and others, 1993) and fish sampling considerations (Meador, Cuffney, and Gurtz, 1993). Composite qualitative and semi-quantitative samples are collected within each sampling reach to characterize the benthic invertebrate community. Typically, a single sampling reach is established at each site. However, three sampling reaches are established at a subset of sites (intensive ecological assessment sites) in order to assess variability among sampling reaches.

The primary determinant of the length of the sampling reach is the presence of repetitions of two geomorphic channel units such as a sequence of pool, riffle, pool, riffle. Only those geomorphic channel units (riffle, run, and pool) that cover greater th an 50 percent of the active channel width are considered when determining the length of the reach. When repetitions of geomorphic channel units are not present or occur at intervals of more than 1,000 m, then the length of the reach is determined to be 20 channel widths, based on the width of the channel at the boundary of the reach. Theoretically, this length represents at least one complete meander wavelength (Leopold and Wolman, 1957). Regardless of the method used to establish sampling reach length, the minimum and maximum acceptable lengths are 150 m and 300-500 m, respectively, for wadeable sites and 500 m and 1,000 m for nonwadeable sites (Meador, Hupp, and others, 1993).

The location of each sampling reach is related to a durable reference point, such as a stream gage or bridge pier (Meador, Hupp, and others, 1993), that is used to permanently define the location of the sampling reach. Sampling reaches are located where conditions, primarily instream and riparian habitats, are representative of the local area and support objectives for which the site was chosen (for example, representativeness of a certain land use, agricultural practice, or reference condition). In order to meet these objectives, the sampling reach might be located above, below, or adjacent to the site location as long as the water chemistry and hydrologic data collected at the site accurately reflect conditions within the sampling reach or reaches.

A hypothetical intensive ecological assessment (IEA) site consisting of a basic fixed site associated with multiple sampling reaches is shown in figure 1. Each sampling reach is composed of repeating geomorphic units, two pools (shaded areas) and two riffles (unshaded areas). In this example, sampling reach "A" is located above, sampling reach "B" is located at, and sampling reach "C" is located below the basic fixed site. Alternatively, the study-unit biologist might decide to locate all three sampling reaches above or below the basic fixed site, so long as there are no significant intervening changes in water chemistry, hydrology, or habitat conditions among sampling reaches. Where possible, multiple sampling reaches are separated by a minimum of 15 0 m.