Abstract
Solutes, suspended sediment and bedload calculations are important for understanding the dynamic processes of flow pathways and geomorphic change. Large amounts of solute and sediment studies have focused on alpine environments within New Zealand with little attention given to temperate forests, an important and valued component of the New Zealand’s land use. This aims of this study is to quantify all three loads (solute, suspended sediment and bedload) for a pristine temperate forest, suitable as a future reference catchment. Over the ten-month study, 14 event flows were evaluated utilising 15-minute stage, specific conductivity and turbidity data as well as grab samples and post event reach surveys. Annual solute load was 14.85 t km-2 yr-1 (53% of total load), slightly higher than the annual suspended sediment load of 12.79 t km-2 yr-1 (45%). Bedload was a smaller proportion of total load with 0.57 t km-2 yr-1 (2%), 4% of suspended sediment. The chemical composition demonstrated a schist-dominated catchment with calcium bicarbonate waters travelling through subsurface pathways. Hysteresis analysis suggests that solutes do not respond to initial flushing of old water and are, therefore, completely dominated by dilution effects of incoming new water for each event. Suspended sediment hysteresis, however, showed variation over events. Clockwise hysteresis was dominant, which indicated the majority of events showed mobilisation of in-stream sediment. Anti-clockwise hysteresis was also evident in following peaks coupled with reduced sediment loads indicating temporary catchment exhaustion of fine sediment. Seasonality effects were observed for solute loads whereby winter showed the lowest rate of solute accumulation and autumn the highest. Sediment loads were in comparison purely dominated by event flows. Bedload surveys showed evidence of deposition and erosion over the study period. Deposition was attributed to sediment build up behind upstream large woody debris which was subsequently removed and sediment deposited within the reach. Erosion occurred due to prior catchment stress and through threshold discharge evacuating sediment.