Flow structure and sustainability of pools in gravel-bed rivers
ABSTRACT: Pool-riffle channel morphology in gravel-bed streams creates a range of micro-habitats that are important for maintaining ecological diversity. This study is motivated by the need to restore sustainable spawning and over-wintering habitat for salmon as a means of mitigating for dam construction. The velocity reversal hypothesis (Gilbert, 1914, Keller 1971, 1972) has been used as a potential mechanism for the sustainability of pools. Velocity reversal implies that the velocities are smaller through the pool than across the riffle at low and intermediate flows. At high discharges this condition becomes reversed and velocities through the pool may exceed the velocities across the riffle. Recent criteria have been developed that utilize the bathymetric characteristics of the channel to ascertain whether or not velocity reversal will occur. However, it is unclear whether the velocity reversal criterion predicts pool sustainability under all flow conditions and if limitations to this approach exist. The role of a high velocity core in controlling sediment deposition and scour processes was investigated under different flow discharges in addition to conditions when velocity reversal may not recreate the original pool riffle morphology.