Abstract
Abstract. In formerly glaciated mountain settings, Pleistocene glaciations are responsible for profound spatial reorganization of the landscape structure. By imposing local channel slope and the degree of hillslope–channel connectivity, glacial macro-forms can exert first-order controls on the downstream strength and continuity of the coarse sediment cascade. To estimate quantitatively these controls we trace bedload transport for 3 years along Strimm Creek, Eastern Italian Alps. Specifically, we monitor the travel distance of 490 PIT-tagged particles (b axis: 23–229 mm; weight: 83–6525 g) at two contrasting sites: Upper Strimm Creek (US; 4 km2), which flows through a fluvially dominated hanging valley, and Lower Strimm Creek (LS; 7.5 km2), located downstream, in a relict glacial trough where it experiences periodic colluvial sediment inputs from lateral tributaries. Tracer positioning within the streambed is periodically tracked in the field with a portable antenna in order to assess progressive travel distances, as well as the extent of the channel active layer, in relation to snowmelt and rainfall-driven peak flows. Interestingly, we show that tracer virtual velocities for selected inter-survey periods are independent of tracer weight at both study sites. Cumulatively, tracers in US have travelled across distances (i.e. inner quartiles) shorter than 2 m, which correspond to over 2 orders of magnitude less than what was observed in LS. These figures translate, after calculations of tracer inter-survey virtual velocities, into estimated bedload volumes equal to about 3 m3 in US and 600 m3 in LS, with most of the transport (75 % in US, and 93 % in LS) occurring during snowmelt. A similar contrast in bedload transport rates, even without considering the additional volumes of material mobilized by mass-wasting processes in LS, testifies the extent to which the glacial imprinting can still affect contemporary sediment transfer, and thus postglacial landscape evolution, in mountain drainage basins.
Highlights
The quantification of bedload transport rates and temporal variability in mountain streams is of primary importance in many applications in geomorphology, freshwater biology, and hydraulic engineering (Wohl, 2010), but these still remain very difficult to predict (e.g. Comiti and Mao, 2012)
Inspection of characteristic percentiles (i.e. D50, D84, and D90) of surface grain-size distribution as a function of downstream distance (Fig. 5a), while confirming that surface bed texture is considerably coarser in Lower Strimm Creek (LS) than in Upper Strimm Creek (US), indicates consistent patterns of downstream fining within the two sites
These patterns are interrupted by the rocky valley step that separates the decoupled hanging valley from the glacial trough, downstream of which sediment calibre resets at markedly higher values, as lateral sediment supply from debris-flow-dominated tributaries feeds Lower Strimm Creek
Summary
The quantification of bedload transport rates and temporal variability in mountain streams is of primary importance in many applications in geomorphology, freshwater biology, and hydraulic engineering (Wohl, 2010), but these still remain very difficult to predict (e.g. Comiti and Mao, 2012). Semi-empirical transport capacity equations proposed so far offer results heavily dependent on the experimental setup of the flumes or field conditions on specific study sites in which they were originally developed. When tested in mountain streams, these equations tend to overestimate the actual bedload transport rate by several orders of magnitude, especially when applied to ordinary flood events The way tracers work makes them ideal means for evaluating in-channel sediment connectivity over time (e.g. Bracken et al, 2015). They allow for gathering quantitative information on thresholds for particle entrainment, transport distances and preferential resting sites, sediment sorting by particle size or shape, and depth and width of the active layer (especially when complemented with scour chains), as well as on sediment transport rates
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