Bedload transport dynamics in supply-limited versus supply-excess channels in a small upland catchment

Bedload transport - the movement of material along a channel's bed by rolling or saltating rather than in suspension - is a large component of sediment budgets in mountain streams but is poorly understood and notoriously difficult to measure. Instantaneous bedload measurements are hampered by significant uncertainty due to temporal and spatial variability in transport rates and long-term bedload datasets are rare. Here we use a unique 36-year record of bedload flux from South Branch of Birch Brook in Western Massachusetts, USA, to show that annual bedload flux trapped by a weir can be accurately modeled via the 'virtual velocity' of RFID-equipped tracer rocks (n = 178) to within ~25%. We leverage this field-validated method to compare the transport dynamics in proximal reaches on the South and Middle Branches experiencing sediment-limited and sediment-excess conditions, respectively. Our tracer rock mobility data span two years, record at least seven bed-mobilizing events, and show that, despite experiencing similar hydraulics and in-event virtual velocities, the supply-excess channel transported tracers farther annually (a higher annual virtual velocity) than the supply-limited channel. Put more clearly, supply-excess reaches move sediment through the system more quickly than supply-limited reaches, a finding that implies that supply-limited reaches store sediment for longer periods than supply-excess reaches. This result runs counter to the traditional transport-limited vs. supply-limited concept where supply-limited reaches transport much of the available sediment. We hypothesize that this effect results from morphologic differences in these small (1-3m wide) upland channels experiencing dichotomous sediment loads. Grains in supply-limited reaches can more easily 'hide' from flows in pools or behind framework clasts, whereas in sediment-excess reaches, the amount of sediment in the channel results in a morphology that more often directly exposes individual grains to bed shear stresses.