Tracing bedload transport in a high-elevation, formerly-glaciated mountain basin

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 three years along the 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 km 2), which flows through a fluvially-dominated hanging valley, and Lower Strimm Creek (LS; 7.5 km 2), 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. Cumulatively, tracers in US have travelled across distances (i.e., inner quartiles) shorter than 2 m, which correspond to over two orders of magnitude less than what observed in LS. These figures translate, after calculations of tracer inter-survey virtual velocities, in estimated bedload volumes equal to about 3 m 3 in US and 600 m 3 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 mobilised by mass wasting process 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.