Abstract
Abstract. Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.
Highlights
Knickpoints are zones of locally steepened bed slope in the long profiles of rivers
The position of a knickpoint can stabilize in some cases (e.g. Bennet et al, 2000), a migrating knickpoint invariably recedes upstream
Terrace formation after a sea-level fall in the Mississippi River basin extended to 600 km upstream within
Summary
Knickpoints are zones of locally steepened bed slope in the long profiles of rivers. They are most commonly observed in rivers incising into bedrock (e.g. Crosby and Whipple, 2006) or a cohesive substrate (e.g. Papanicolaou et al, 2008). In the experiments reported here, base level was maintained by weir as a free overfall, whereas on top of which an alluvial reach with a lower slope formed.
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