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The phenomenon of downstream fining has been attributed to both particle abrasion and selective particle sorting; the latter is generally considered to play the dominant role within resistant lithologies. It has been recognized that tributaries can disrupt fining patterns; however, few downstream-fining studies have considered the entire fluvial network structure. Here we combine a theory for selective transport with a model of river-basin evolution in order to simulate the dynamics of selective sorting throughout a drainage network. Previous numerical modeling studies of single-thread or braided channels have treated downstream fining as a phenomenon driven by differential deposition rates. We show, however, that in an eroding drainage network, downstream fining emerges as a natural dynamic adjustment to variable water, sediment, and energy inputs, even under conditions of uniform size distribution in sediment flux. Thus, although selective deposition and abrasion clearly can and do play a role in some fluvial systems, neither is necessary to produce downstream fining within a drainage network.

A change in tectonics affects erosion rates across a mountain belt, leading to a period of non-steady sediment flux delivery downstream. The nonlinear relationship between tectonics and transient sediment delivery from an erosional catchment makes extraction of paleo-tectonic signals from stratigraphy challenging. We use a numerical landscape evolution model to explore how sediment flux from an erosional watershed responds to non-steady rock uplift. We focus on the time lag between the onset of a rock uplift change and the onset of a corresponding change in the sediment flux and the magnitude of the sediment flux relative to the steady rate. We observe that (1) sediment flux does not always record changes in the rock uplift rate when the duration of a rock uplift interval is less than 25% of landscape response time, or time for a landscape to transition from one steady state to another after a perturbation; (2) sediment flux response to variable rock uplift is positively correlated with the duration of rock uplift intervals; and (3) a nonlinear response between erosion rates and tectonic perturbations can result in increasing sediment flux through time even after rock uplift rate decreases. How quickly the sediment flux signal responds to a perturbation depends on how close the landscape was to steady state before the perturbation. These results illustrate conditions under which tectonic signals have the potential to be stored in the stratigraphic record or lost in an erosional system, and the importance of network dynamics for understanding signal propagation. [ABSTRACT FROM AUTHOR]