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Alb the explicit zone is the least biased. Figure 16 illustrates the difference between implicit and explicit zonation. BRAIDED 50/50 RIFFLE-POOL 7 2T3__? EXPLICIT ZONATION 9 IMPLICIT ZONATION STRATIFIED RIVER SEGMENT Figure 16. Implicit and explicit zonation of a stratified river segment to generate a population of candidate representative reaches. If the technique of explicit zonation is used, the candidate reaches may or may not be of the same length. However, with implicit zonation each candidate reach must be of nearly equal length to avoid sampling bias. Candidate reaches should be large enough to encompass two riffle and pool sequences, or meander crossing-meander pool sequences. This distance is a function of the high discharges occurring in the stream, which also largely determine the channel width. As a general rule, the spacing of successive riffles in straight channels, and crossing bars in meandering channels, is about. five to seven times the channel width (Leopold, Wolman, and Miller, 1964). Thus the length of candidate representative reaches encompassing two such features should be 10 to 14 times the average channel width. Having dissected the stratified stream segment into candidate reaches by the explicit- and/or implicit zonation processes, it is now necessary to obtain a sample or number of samples (representative reaches) from the population of candidate reaches. A random sampling of the candidate reaches is preferred, and several techniques may be used. Each candidate mach is numbered sequentially on a reap through the entire population of candidate reaches. A random sample may be obtained through some type of random number generator; a table of random numbers, a deck of cards with a number of cards equal to the number of candidate reaches, the rolling of a single die if there are six or less candidates, etc. Eanie, meanie, miney, mce is not considered a randomly generated number. ?r