The control of grain-scale mechanics on channel form, landscape dynamics, and climatic perturbations in gravel-bedded rivers.

摘要

Landscapes evolve over millions of years, through the complex interplay of climate and tectonics. Mountains in particular represent a staggering range of spatial and temporal scales, challenging our ability to understand how the landscape is sculpted. Mountains do not simply disappear by bulk denudation. The key process of river incision results from the entrainment, displacement, and collision of coarse particles with the bed; a phenomenon known as bed load transport. This dissertation seeks to elucidate how bed load transport in natural rivers is driven by floods, to provide a mechanistic connection between climate and landscape evolution. Field surveys of coarse particle displacement and channel geometry are combined with hydrological time series, to study the interaction between floods and bed load dynamics, and their implications for channel form. Results from tagged cobbles demonstrate that mean particle displacement is proportional to applied fluid momentum in excess of the threshold of motion, while dispersion of tracers is superdiffusive due to the burial and excavation of cobbles. These field surveys reveal that particle motion remains in a state of partial transport for a diverse population of flows, and that particle sorting and transport distances closely match theory developed from small-scale laboratory experiments. Analysis of hydrological time series shows that the threshold of particle motion truncates the distribution of applied stress, resulting in thin-tailed distributions of forcing for flows above the threshold of motion. This analysis further shows that, because a coarse-grained river adjusts its geometry so that the flow at the banks is at the threshold of motion, the probability of experiencing larger stresses diminishes exponentially. Field surveys of channel geometry and particle size reveal that the geomorphological impacts of urbanization are reduced for coarse-grained channels adjusted to frequent sediment transport events. Taken together, these observations indicate that the threshold of particle motion represents a first-order control on the influence of climate on river dynamics, and the landscapes through which they flow.