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sequence of geomorphic channel units included in the reach should reflect the sequence of geomorphic channel units in the segment. For example, the geomorphic channel units near the reference location may include a pool and a sequence of riffles and runs. If the pool is present only at the reference location and nowhere else in the segment, the pool is not included in the selected reach.

Repeating geomorphic channel units are not present in all streams. In some cases, the entire stream segment consists of only one geomorphic channel unit. When two examples each of two types of geomorphic channel units are not present, the length of the sampling reach is 20 times the channel width. In meandering streams, 20 times the channel width includes one complete meander wavelength (Leopold and others, 1964). Channel width is measured at a location that is representative of the stream (not an anomalously wide or narrow location).

Regardless of the geomorphic channel units present or the channel width, minimum and maximum length criteria are used. A minimum length of reach is necessary to ensure the collection of representative samples of biological communities, whereas a maximum reach length is needed to limit reach length to a distance that prevents unnecessary sampling and minimizes crew fatigue (and associated reduction of sampling efficiency).

Minimum and maximum length criteria for the reach are dependent upon the influence of stream depth on collection methods when sampling streams. Stream reaches are generally classified as wadeable or nonwadeable. Wadeable reaches are those reaches where an investigator can wade from one end of the reach to the other, even though the reach may contain pools that cannot be waded. Nonwadeable reaches are those reaches where an investigator cannot wade from one end of the reach to the other. Minimum reach lengths of 150 m in wadeable reaches and 500 m in nonwadeable reaches are recommended to ensure the collection of a representative sample of the fish community (Meador and others, 1993) and, therefore, are recommended for a corresponding habitat character ization. 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 (Meador and others, 1993). A maximum reach length criterion of 1,000 m should be considered for nonwadeable reaches (Meador and others, 1993).

The investigator must evaluate a number of factors before selecting the reach. In general, the reach is located upstream of a bridge or other man-made channel alterations to minimize their influence on stream habitat. When compelling reasons dictate that the reach must be downstream from a bridge or other feature, then the reach must be established far enough downstream from the bridge to avoid such hydraulic effects of bridges as scour holes and over-widened channels. In general, the geomorphic featur es of the reach should be representative of the segment. Thus, knowledge of the geomorphic channel units of the segment is important.