![]() reach-length controls, is it the artifact of highly localized perturbations, or some combination of both conditions? (5) Are PRS maintained by the same processes that form them, or do ensuing processes dominate feature persistence once the morphology is initiated? (6) Are free-formed and forced PRS created by the same process or are they analogous bedforms with different origins? Presently, we cannot address these issues with confidence. Pool filling ultimately reduces deep-water habitat and impacts sensitive aquatic species (Robert. ![]() Sedimentation focused in wake zones, and recirculating eddies is especially important (Kieffer, 1985 Schmidt, 1990 Lisle and Hilton, 1992 Cluer, 1995 Wohl and Cenderelli, 2000 Jansen and Brierley, 2004 MacVicar and Roy, 2011). Hilton (1992, 1999) introduced the V* method to estimate low-flow sedimentation, which relies on elevation differences between buried coarse lag deposits relative to overlying fine layers (Fig. Pool volume declines in response to increased sediment supply (Keller, 1978 Lisle, 1982 Madej and Ozaki, 1996 Lisle and Hilton, 1999), flow regulation, deforestation, road corridor construction and mining (Sear, 1992 Wohl and Cenderelli, 2000 Goode and Wohl, 2007). Pool volume is a function of high-flow scour and subsequent filling, which increases sediment continuity (Andrews, 1979 Lisle, 1982 Lisle and Hilton, 1992 Wohl et al., 1993 Sear, 1996 Wohl and Cenderelli, 2000 DeVries et al., 2001). transport through PRS is intricately linked to infilling of pool volume, often by finer bed sediments and usually at lower flows, which can create unique pool depth versus discharge relations (Lisle, 1982 Wilcock et al., 1996). Essentially, hydraulic conditions in pools and riffles can be similar and still create variations in sediment mobility if sediment characteristics differ. Similarly, when riffles contain coarser bed sediments than pools, variations in localized bed resistance create sediment mobility patterns that could preserve bedforms (Carling and Wood, 1994 Milan et al., 2001 Vetter, 2011a). Meanwhile, in situ vibration created by turbulent fluctuations and infilling of fine sediments results in pool sediments that are less interlocked and more unstable relative to more tightly packed riffle sediments (Clifford, 1993a Sear, 1996 MacVicar and Roy, 2011 Hodge et al., 2013). Dispersive forces could raise riffles sediments and depress pools because of underlying differences in grain size (Yang, 1971). Variations in grain-size distributions and dissimilarities in bed-sediment structures, including discrepancies in particle size, shape, protrusion, pivoting angle, and packing, lead to variations in entrainment rates that could maintain variations in depth (Clifford, 1993a Carling and Wood, 1994 Sear, 1996, 2015). maintenance may also depend on differences in sedimentological properties of bed material in pools versus riffles. 2 Typical hydraulic features in a sinuous pool-riffle. Reversal of localized velocities in jets or near-bed regions is possible without reversals of cross-sectional-averaged velocities ( Table 1) Fig. Flow convergence at channel obstructions and width reductions produces backwater, locally increased water-surface slopes, and narrow regions of high momentum fluid called jets (Kieffer, 1985(Kieffer,, 1989Lisle, 1986 Sear, 1992Sear,, 1996Thompson et al., 1998Thompson et al.,, 1999Jansen and Brierley, 2004 Wilkinson et al., 2004Wilkinson et al.,, 2008MacVicar and Roy, 2007a). Data from some field and model studies demonstrate at least weak mean water-surface-slope, velocity, or shear-stress reversals between pools and riffles, but other results show no reversals ( Table 1) Flow convergence is accompanied by mean-flow acceleration and results from physical narrowing of flow (Fig. Although originally applied only to near-bed velocities, the hypothesis was expanded to evaluate possible reversals in mean-flow conditions partly because hydraulic theory predicts increasing velocities with distance above the channel-bed (Jarrett, 1984 Robert et al., 1992 Robert, 1997). to the velocity-reversal hypothesis, bed velocities in existing pools increase at faster rates with stage than corresponding velocities in riffles, which creates higher shear stresses in pools that are capable of maintaining and perhaps establishing the features (Keller, 1971).
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