The location and length of the sampling reach are determined on the basis of a combination of repeated geomorphic channel units (Meador, Hupp, and others, 1993) and fish sampling considerations (Meador, Cuffney, and Gurtz, 1993). Composite qualitative and quantitative samples are collected within each sampling reach to characterize the algal community. Typically, a single sampling reach is established at each site; however, three sampling reaches are established at a subset of 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 t han 50 percent of the active channel width are considered when determining the length of the reach. If repetitions of geomorphic channel units are not present or are present at intervals of greater than 1,000 m (for example, in large rivers), 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 the length of the sampling reach, the minimum and maximum acceptable ranges are 150 to 500 m for wadeable sites and 500 to 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 instream and riparian-habitat conditions are representative of the local area and support NAWQA study-unit objectives (for example, representative of a specific land use, agricultural practice, or reference condition). In order to meet these objectives, the sampling reach may be located upstream, downstream, 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 site (basic fixed site with multiple sample 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 upstream of the basic fixed site; sampling reach "B" is located at the basic fixed site; and sampling reach "C" is located downstream from the basic fixed site. Alternatively, the study-unit biologist might decide to locate all three sampling reaches upstream or downstream from the basic fixed site as 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 150 m.
Qualitative periphyton samples are collected to document the occurrence of algal taxa in as many available periphyton microhabitats within the sampling reach as possible. The purpose of qualitative sampling is to develop a detailed list of the taxa prese nt in the reach at the time of collection. This type of sample, referred to as a qualitative multihabitat (QMH) periphyton sample, is prepared by compositing collections of periphyton from microhabitats present in the sampling reach.