Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models

Abstract

We have used early Miocene valley‐filling basalts to reconstruct fluvial long profiles in the Upper Lachlan catchment, SE Australia, in order to use these as well‐constrained initial conditions in a forward model of fluvial incision. Many different fluvial incision algorithms have been proposed, and it is not clear at present which one of these best captures the behavior of bedrock rivers. We test five different formulations; the ability of these models to reproduce the observed present‐day stream profiles and amounts of incision is assessed using a weighted‐mean misfit criterion as well as the structure of the misfit function. The results show that for all models, parameter combinations can be found that reproduce the amounts of incision reasonably well. However, for some models, these best fit parameter combinations do not seem to have a physical significance, whereas for some others, best fit parameter combinations are such that the models tend to mimic the behavior of other models. Overall, best fit model predictions are obtained for a detachment‐limited stream power model or an “undercapacity” model that includes a river width term that varies as a function of drainage area. The uncertainty in initial conditions does not have a strong impact on model outcomes. The model results suggest, however, that lithological variation may be responsible for variations in parameter values of a factor of 3–5.