Impacts of land use change on patterns of sediment flux in Weraamaia catchment, New Zealand

Forest clearance between the 1890s and the early 1920s, subsequent scrub growth, and commencement of an afforestation program in 1979, modified the pattern and rate of sediment delivery to valley floors via shallow landslides and gully complexes in a steep headwater catchment (4.8 km2) in New Zealand. Analysis of the historical record, air photograph interpretation, and field survey indicates that both erosion types occurred across the catchment in the 1938 storm, aggrading channel beds and widening the active channel zone. In contrast, a 1 in 100 year event in 1988 (Cyclone Bola) induced numerous shallow landslides, but erosion of gully complexes was largely restricted to subcatchments that retained pasture, and the geomorphic impact of this event on channels was small. The changing volume and calibre of materials delivered to the valley floor, and the distribution of gully complexes, altered patterns and rates of channel adjustment after the events, and the resulting sediment flux. Development of gully complexes maintained coupling processes with channels for periods up to 102 years, forming wide channels in downstream reaches. Upstream-downstream connectivity along the trunk stream was altered by the formation of a large debris fan at the confluence with a tributary subjected to gully complex erosion. In contrast, slopes subjected to shallow landslides became decoupled from channels within 10 years, accelerating channel degradation and narrowing. Effective conveyance of a large volume of fine-grained materials promoted immediate aggradation of gentle-gradient channels downstream. As gully complex areas stabilized following an increase in forest and scrub cover, channel courses became significant sediment sources. Although shallow landslide activity will continue to induce intermittent aggradation in the future, it is inferred that average sediment yield will continue to diminish to levels approaching those experienced prior to clearcutting, and the pattern of sediment flux will recover by 2030.