Reach-Scale Adjustments in Alluvial Channel Morphology in Response to Active Folding

Recent interest in the role of channel adjustment in response to tectonic or climatic perturbations highlights the need for field-based calibration of width-scaling relationships for channels in both alluvial and bedrock rivers. Because such adjustments may serve as a primary means for focusing erosive power in fluvial systems, a quantitative understanding of morphologic variation as a function of differential tectonic uplift provides valuable insight into fluvial response and behavior within actively deforming landscapes. In order to document changes in channel morphology in response to tectonic forcing, we have conducted detailed surveys of formerly antecedent outwash channels preserved as wind gaps over actively growing folds along the Ostler fault zone of southern New Zealand. Surveying focused on channel width and the depth of incision for over 30 cross profiles on each channel examined. Results show marked narrowing of channel widths over the proximal flanks of anticlinal uplifts toward minimum values well before reaching the uplift crest. We compare downstream variations in channel morphology with contemporaneous spatial and temporal changes in channel incision. In the context of specific stream-power models for fluvial erosion, we then assess the relative contribution of channel width and slope adjustments in enabling incision to keep pace with uplift for the life of the antecedent stream. Despite difficulties in reconstructing channel slope in deformed wind gaps, comparison with both modern unperturbed drainage gradients upstream of the fault zone and currently oversteepened stream gradients over the active hangingwall anticline indicates that adjustments in slope also play an important role in focusing erosive power through the growing structure and in allowing channels to maintain their course over active folds. In each case, our results highlight the interdependence of variations in channel morphology and imposed changes in slope and elevation through differential uplift. Such data also provides new field constraints on existing width-scaling relationships for alluvial rivers in actively deforming terrains.