Abstract
Abstract The gravel-sand transition (GST) is commonly observed along rivers. It is characterized by an abrupt reduction in median grain size, from gravel- to sand-size sediment, and by a shift in sand transport mode from wash load–dominated to suspended bed material load. We documented changes in channel stability, suspended sediment concentration, flux, and grain size across the GST of the Karnali River, Nepal. Upstream of the GST, gravel-bed channels are stable over hundred- to thousand-year time scales. Downstream, floodplain sediment is reworked by lateral bank erosion, particularly during monsoon discharges. Suspended sediment concentration, grain size, and flux reveal counterintuitive increases downstream of the GST. The results demonstrate a dramatic change in channel dynamics across the GST, from relatively fixed, steep gravel-bed rivers with infrequent avulsion to lower-gradient, relatively mobile sand-bed channels. The increase in sediment concentration and near-bed suspended grain size may be caused by enhanced channel mobility, which facilitates exchange between bed and bank material. These results bring new constraints on channel stability at mountain fronts and indicate that temporally and spatially limited sediment flux measurements downstream of GSTs are more indicative of flow stage and floodplain recycling than of continental-scale sediment flux and denudation rate estimates.
Highlights
Downstream of mountain ranges, river bed sediment fines as channels flow onto lower gradient and laterally unconstrained landscapes (Sternberg, 1875)
Sediment fining is a key component of sediment transport that underpins the dynamic nature of rivers, and is central to fluvial geomorphology and the depositional record it constructs
Downstream of the gravel-sand transition (GST), high rates of channel migration driven by lateral meander migration are enhanced by the presence of a poorly consolidated, low clay-content floodplain material that is devoid of deep-rooted vegetation
Summary
Downstream of mountain ranges, river bed sediment fines as channels flow onto lower gradient and laterally unconstrained landscapes (Sternberg, 1875). Downstream fining is attributed to sizeselective sediment sorting (e.g. Ashworth and Ferguson, 1989; Paola et al, 1992a; Ferguson et al., 1996) and the mechanical breakdown (abrasion) of particles Rivers commonly exhibit an abrupt transition in bed grain size, from gravel to sand, over a short downstream distance Shaw and Kellerhals, 1982; Ferguson et al., 1996), termed the gravel-sand transition (GST). The development of GSTs has been attributed to a combination of size-selective sorting (e.g. Paola et al, 1992b; Wathen et al, 1995; Ferguson et al, 1996; Seal et al, 1997; Parker and Cui, 1998), abrasion of particles There is no generally accepted or universal theory for why GSTs develop
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