Equilibrium and transient morphologies of river networks : discriminating among fluvial erosion

Abstract

We examine the equilibrium and transient morphology of alluvial and bedrock river networks. We apply analytical methods and an iterative model to solve for equilibrium slope-area and texture- area (in alluvial networks) relationships under different tectonic and climatic forcings. Transient morphology resulting from a change in uplift or precipitation rate is simulated using the CHILD landscape evolution model. In alluvial networks, it is well recognized that both channel slope and mean grain size usually decrease downstream. These variables play an important role in determining sediment transport rates, and their mutual adjustment to a change in the forces that drive erosion can yield surprising results. Adjustments in grain size can lead to spatially variable channel concavity and larger trans port rates on shallower slopes. As a consequence, equilibrium channel slopes may decrease under higher uplift conditions (or, similarly, faster base-level lowering). Selective erosion and deposition can cause transient channel slopes to both increase and decrease and surface texture to both coarsen and fine, all in response to a single change in forcing. In bedrock rivers, increasing attention has been given to the role of sediment flux on incision processes. We find that all applied erosion rules (stream-power and three sediment-flux models) produce similar equilibrium morphologies, although some details lead to differences in sensitivity.

(cont.) On the other hand, the transient response can be much more complicated than a simple knickpoint migration when the integrated response of the sediment flux is considered. Both increasing and decreasing channel slopes can result from a single change in forcing. Although some of the processes described by the different erosion models in this study represent conditions in very different types of rivers, two important common principles hold. First, concave graded river profiles appear to be a robust element of the landscape and fairly insensitive to the details of the erosion process. However, downstream variations in channel erodibility can alter equilibrium sensitivity to boundary conditions in ways that had not previously been considered. And second, transient conditions in the main channel are highly dependent on the entire network response. The results can be complex and counter-intuitive, highlighting that rivers are not independent of the tributaries that feed them.