Displacement and entrainment behavior of bedload clasts in mountain streams
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Understanding how individual grains and populations of grains move through alluvial systems is important for predicting how landscapes adjust to changes in climate, tectonics and watershed management. Mountainous terrain covers over 20% of Earth's landscape and channels running through these steep slopes deliver significant volumes of sediment to lowland systems. However, most historic sediment transport studies were conducted in lowland rivers and laboratory experiments. Recently, upland stream studies have had limited success in monitoring the bedload fraction of sediment transported during floods exceeding bankfull flows. To address the scarcity of bedload observations in natural streams, I characterized bedload displacement and entrainment behavior in a mountain stream over a range of temporal and spatial scales. I designed and developed new fluvial geomorphology tools, including active tracers that were clasts embedded with accelerometers to record the timing of motion relative to discharge. I also deployed passive tracers which were bedload clasts embedded with radio frequency identification (RFID) tags. The passive tracers were used to determined flood-scale displacement lengths. Additionally, I installed RFID antennas on the channel bed to record the times that tracers passed through a reach. Flow strength during flood events was estimated using discharge records and numerical modeling. Datasets collected by the active and passive tracers demonstrated that probabilities of transport, average step lengths and cumulative displacement distances scale with discharge. The heavy-tailed measurements of rest times from the active tracers suggested that bedload transport is superdiffusive in mountain streams. Transport, deposition and re-entrainment records showed that thresholds of motion are best represented by a distribution rather than a constant value, are influenced by channel width and bed slope, and can be lower at re-entrainment than deposition. Historic discharge records and field-modified transport formulas predicted that a broad range of discharges contribute significant fractions of the total bedload volumes, and that magnitude-frequency analyses are highly sensitive to common scaling and extrapolation techniques. The rare field-based observations from this research provide new insights into the complex mechanisms that drive bedload dispersion in mountain streams.