Landslide-driven erosion and slope-channel coupling in steep, forested terrain, Ruahine Ranges, New Zealand, 1946-2011

- > 6900 landslides mapped for 221 km2 forested mountain range between 1946 and 2011.
- Spatial and temporal variability of landslides quantified on six dates.
- > 5 million m3 of coarse debris delivered off-slope to river network.
- Legacy sediment from major storms has ongoing impacts on channels.
- Small shallow landsliding is a significant contributor to the sediment cascade.

Landslides are an important means of conveying sediment from slopes to channels in steepland environments, but landsliding is a discontinuous process. This means that any single assessment in time of their contribution to the sediment cascade is of limited value. To better understand landscape dynamics and the contribution of landslides to slope-channel coupling over time, this paper quantifies connectivity over a time span of 65 years in small, steep headwater catchments in the southern Ruahine Range, New Zealand. Temporal variability in landsliding and slope-channel coupling was assessed using six sets of aerial photography flown between 1946 and 2011, from which over 6900 landslides were mapped in ArcGIS, of which up to 78% connected with the stream network. Estimates of the volume of material delivered by landslide erosion to headwater channels were based on ground survey measurements of selected landslide scars and suggest that between 1946 and 2011 over 5 million m3 of sediment was delivered from slopes to channels in the 221 km2 study area. Forest cover is not sufficient to prevent this erosion. These catchment systems are particularly vulnerable to high magnitude storm events, which significantly elevate landslide intensity and enhance sediment delivery, as occurred in the mid-1970s. The legacy of these events remains in these headwater channels, with ongoing consequences for stream and hazard management in and adjacent to the ranges.