Pools and riffles are genetically-linked in alluvial rivers. Pool-infilling subsequently occurs, as pools act as areas of deep, low flow velocity and near-standing water conditions. In a flowing stream, a riffle-pool sequence develops as a streams hydrological flow structure alternates from areas of relatively shallow to deeper water. Subcritical flow occurs during divergent flow at low flow stage. Indeed, they may be removed and replaced during extreme floods, as they reform at lower flow stages (velocity reversal hypothesis).Īt high flow stage, when flow converges through pools, decreased roughness and greater bed shear stresses induce scour and flushing of sediment stored on the bed. Although very stable, with 5-10 % of the stream area in supercritical flow and some small hydraulic jumps over obstructions, riffles may be mobile at and above bankfull stage. Riffles are commonly dissected during the falling stage of floods, when the water surface is shallow and steep, and the stepped long profile is maintained. At high flow stage the water surface is smooth, as bed irregularities are smoothed out. In subsequent high discharges, deposition occurs as the resistance of these features induces a reduction in velocity over the riffle surface. Concentration of coarser fractions at high discharges (bankfull and above) produces incipient riffles, while lower flows (up to bankfull) may be sufficiently competent to amplify and maintain the initial undulations once they have reached a critical height. Physical and chemical parameters should be measured at the same time as fish sampling to specifically characterize the habitat. Prior to sample collection, a habitat assessment should be performed. Riffles are zones of temporary sediment accumulation which increase roughness during high flow stage, inducing deposition. Each reach should contain riffle, run, and pool habitat samples, if possible. The less harsh pool environmental conditions may allow both the run and riffle morphs to utilize them without any further plastic adaptation. They form at characteristic locations, typically along the concave bank of bends in sinuous alluvial channels. Pools, when compared to run and riffle habitats, are characterized by small substrate, non-turbulent and low water velocity and greater water depth (Fig. Pools tend to be narrower than riffles and act as sediment storage zones. Alluvial pools are alternating deep areas of channel along an undulating reach-scale longitudinal bed profile. Pools may span the channel, hosting tranquil or standing flow at low flow stage. These sediment storage zones tend to comprise tightly imbricated bed materials, suggesting the action of local sorting mechanisms. Alluvial riffles are alternating shallow step-like forms that span the channel bed. Clusters of gravel (up to boulder size) are organised into ribs, typically with a rippled water surface at low-stage. They occur at characteristic locations, typically between bends (the inflection point) in sinuous alluvial channels. Topographic highs along an undulating reach-scale longitudinal profile.
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