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Random controls on semi‐rhythmic spacing of pools and riffles in constriction‐dominated rivers

Average pool spacing between five and seven bankfull widths has been documented in environments throughout the world, but has limited theoretical justification in coarse‐bedded and bedrock environments. Pool formation in coarse‐bedded and bedrock channels has been attributed to bedrock and boulder constrictions. Because the spacing of these constrictions may be irregular in nature, it is difficult to reconcile pool‐formation processes with the supposedly rhythmic spacing of pools and riffles. To address these issues, a simulation model for pool and riffle formation is used to demonstrate that semi‐rhythmic spacing of pools with an approximate spacing of five to seven bankfull widths can be recreated from a random distribution of obstructions and minimum pool‐ and riffle‐length criteria. It is assumed that a pool–riffle couplet will achieve a minimum length based on dominant‐discharge conditions. Values for the minimum‐length assumption are based on field data collected in New England and California, while the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation. Results from the simulations show that the location of pools can be primarily random in character, but still assume an average spacing between four and eight bankfull widths for a variety of conditions. Field verification data generally support the model but highlight a highly skewed distribution of pool‐forming elements and pool spacing. The relation between pool spacing and bankfull widths is attributed to the common geometric response of these features to dominant‐discharge conditions. Copyright © 2001 John Wiley & Sons, Ltd.