Influence of Anthropogenic Land-Use Change on Hillslope Erosion in the Waipaoa River Basin

Abstract:

European settlement of the North Island, New Zealand resulted in deforestation of >90% of the landscape followed by conversion to pastureland. The resulting loss of vegetation cover triggered a sharp increase in hillslope erosion as widespread earthflow complexes and gully systems developed on the weak marine sedimentary lithologies in the Waipaoa River basin. However, the rate and volume of hillslope degradation due to land-use change has not been quantified. Using a 1955-2013 decadal sequence of aerial photographs for a ~16 km² sub-catchment, we mapped the spatial extent of active landslides and then created a ‘turf index’ to relate the amount of ground disruption to average downslope velocity. The sub-catchment is predominately comprised of a weak mélange of highly sheared mudstone and riddled with pervasive earthflows that often span from ridgetop to the channel, dissected by gullies. We separated the landslides into three turf classes based on the spectral signature of the landslide, which relates to the style of ground disruption. Based on the movement of trees and other features, we assigned velocities to each class. The resulting three landslide categories are: 1) little to no disrupted ground that averaged ≤1 m/yr, 2) a mix of disrupted ground and intact blocks that averaged 2-5 m/yr, and 3) no intact blocks or vegetation that averaged ≥5 m/yr. We then calculated the average annual sediment flux based on the corresponding turf index, landslide depth, and length along the channel or gully. Using an average toe depth of 4.4 ± 1.3 m (avg ± sd) determined from field measurements of 37 earthflows, we calculated a sub-catchment averaged erosion rate of up to 23 mm/yr (for 1955). Despite evidence that >99% of the terrain is covered by post-LGM landslides, only 38% is active in 1955 (and that proportion is decreasing through time—seemingly correlated with localized reforestation). Compared with long-term (post-18 ka) erosion rates from this catchment (1.6 mm/yr) and the long-term uplift rate (~1 mm/yr), the recent human influence is profound. Previously, we found an increase in erosion over the past 3400 ky (2.2 mm/yr), however this increase could be entirely due to human influence—showing that a few decades of rapid erosion superimposed on a steady state background erosion rate can skew previously calculated long-term rates.