Post-rock-avalanche dam outburst flood sedimentation in Ram Creek, Southern Alps, New Zealand

- We examine nearly 30 years of post-landslide dam breach change.
- The system has not attained equilibrium and is transport limited.
- The river is eroding laterally instead of vertically keeping pace with uplift.

Rock avalanches are common in mountainous regions that are tectonically active. They are capable of forming natural dams of uncertain persistence that have significant impacts on the river system over wide spatial scales and possibly over geological time scales. Here we combine field data and digital elevation model (DEM) analysis to show the response of Ram Creek, New Zealand, to 28 years of sediment dispersion following the 1968 emplacement of a co-seismic, rock-avalanche dam that breached catastrophically in 1981. The results show a system that has not attained equilibrium, being unable to move the quantity of dam-derived sediments, and will likely not attain equilibrium before the next major sediment input; it is in a state of persistent disturbance where localised reworking dominates. Erosion in Ram Creek is focussed on lateral bevelling and bedrock gorge widening rather than vertical incision to keep pace with tectonic uplift. Importantly for studies of tectonic geomorphology, this widening - which if sustained will form a strath terrace - does not represent a period of reduced uplift. Stream metrics (concavity and steepness) are unable to differentiate the identified rock-avalanche-induced knickpoint from tectonic and lithological knickpoints.