The signature of an extreme erosion event on suspended sediment loads: Motueka River catchment, South Island, New Zealand.

Abstract

Five years of continuously monitoring turbidity and suspended sediment (SS) at four sites in the Motueka River catchment, northern South Island, New Zealand, has characterised the downstream and temporal dispersion of high SS inputs from an extreme rainfall event. The rainstorm, of>50 year recurrence interval, was concentrated in the upper Motueka and Motupiko tributaries and delivered high sediment outputs from re-activated gully complexes and landslides. These only appear to activate when a rainfall threshold is exceeded. Monitoring stations in these tributaries captured a ∼20- to 30-fold increase in SS concentrations and event sediment yields, whereas the monitoring station at the coast recorded only a 2- to 5-fold increase. The high concentrations and event yields decayed exponentially back towards normal levels over -2-3 years at both upstream and downstream sites. Field observations suggest that this erosion recovery trend relates more to the exhaustion/stabilisation of transient riparian sediment storage than to healing of the primary erosion sites by surface-armouring and/or re-vegetation. The downstream decay relates both to dilution (from other tributaries carrying lower SS concentrations) and dispersion processes. Similar space-time patterns have been observed in other New Zealand catchments and have implications for the reliability of sediment yield estimates based on short monitoring records. A method is suggested for incorporating such extreme erosion events into estimates of long-term average sediment yield.