Differential bed sedimentology and the maintenance of riffle-pool sequences

Valley width variation influences riffle–pool location and persistence on a rapidly incising gravel-bed river

“…It can account for the role of bed particle size differences in scour location and 138 timing when sufficient data is available to be used in conjunction with a 2D or 3D numerical 139 model. As a result, it may be consistent with essential aspects of the particle queuing hypothesis 140 (Naden and Brayshaw, 1987;Clifford, 1993a). Table 1 relief was enhanced and high-quality fish spawning habitat on riffles increased.…”

“…No reference to alternate bars, 609 meanders, bar instability, bed material differentiation, or particle queuing was necessary to 610 explain reach-scale riffle persistence (Wilkinson et al, 2008;Clifford, 1993a). The only way to 611 describe and comprehend the fluvial geomorphology of the reach is to frequently refer to aspects 612 of the multiple scales of landform nonuniformity, especially valley width oscillations.…”

“…The 93 abruptness of channel change at these forcing elements causes localized, high-frequency vortex 94 shedding around the constricting elements. In turn, that yields turbulent fluctuations in velocity, 95 lift, and drag that are all thought to be capable of scouring pools and routing sediment (Matthes,96 1947; Lisle, 1986;Clifford, 1993a Taken together, the literature cited above conclusively refutes the conjecture that uniform 102 flow is an appropriate assumption in analyzing physical mechanisms associated with the fluvial 103 geomorphology of riffle-pool sequences in gravel-bed rivers or in re-engineering gravel-bed 104 rivers. Knowing the single set of values defining bankfull dimensions has much less predictive 105 utility than presumed, because these values have not been tied to the key mechanisms associated 106 with channel nonuniformity.…”

Stage dependent variability in tractive force distribution through a riffle–pool sequence

“…More frequently pools have been reported to contain finer-grained sediments than adjacent riffles Keller, 1971;Lisle, 1979;Bhowmik and Demissie, 1982a;Milne, 1982;Hirsch and Abrahams, 1984;Clifford, 1993;Sear, 1996 , possibly as a result of the coarser lag gravel being buried by fines winnowed from the upstream riffle.…”

Nitrogen dynamics in rural streams : differences between geomorphologic units

“…Spatial variation of water velocity, with high and low current areas, controls the necessary alternation of erosion and deposition (Yalin 1971). Changes in near-bed turbulence induced at the beginning of shallow-depth sequences create differences in sediment entrainment (shear stress); these enhance and maintain the sequence through a form of feedback process (Clifford 1993a(Clifford , 1993b. Thus, the longitudinal succession of different geomorphologic units is related to the longitudinal oscillations in the velocity field of turbulent flow (Knighton 1998).…”