Quantitative assessment of landform equifinality and palaeoenvironmental reconstruction using geomorphic models

We present results from a study of post-glacial alluvial fan evolution in New Zealand, in which a series of > 83,000 numerical simulations of fan formation were conducted, within a Monte Carlo framework, in order to evaluate the potential for reconstructing past environmental conditions by matching simulated and observed fan morphologies. Our results indicate that simulations based on a wide range of model boundary conditions and internal process parameterisations are able to reproduce the observed fan characteristics. Such equifinal behaviour is a product of an imposed limit on fan progradation and associated autogenic feedbacks, which drive fan entrenchment even in the absence of external changes in water or sediment supply. We find little evidence that the majority of preserved fan terraces that formed post-entrenchment did so during periods of higher than average discharge or sediment supply to the fan apex, or that evidence for significant changes in sediment supply associated with major climate shifts are likely to be preserved in the current fan morphology. These results illustrate the difficulties that are inherent in the interpretation of alluvial landforms as records of past environmental conditions. They also highlight the need for more rigorous quantification of uncertainties in field and model-based interpretation of landscape responses to environmental change.