Abstract
Channel sediment transfer patterns in alpine environments are an integrated signal that reflects both channel sediment sources and sediment input from extra-channel slopes. The degree of slope-to-channel sediment connectivity is controlled by several factors, amongst them differences in the magnitude and frequency of rainfall events, particle size, sediment availability and surface maturity. However, there has been limited work on the development of slope-to-channel coupling in deglaciated alpine environments over time. Characterising the controls on slope-to-channel connectivity in this environment may result in a better understanding of how landscapes ‘relax’ from perturbation.
Channel turbidity was measured using custom built iHOBS turbidimeters at two gauging stations, one above a hydrologically connected slope and one below it in the c. 10 km2 Hoophorn Valley, a largely deglacierised basin in the Southern Alps, New Zealand. On this hydrologically connected slope an array of 30 erosion pins was used to measure surface lowering rates on the slope and eight custom built Gerlach troughs were installed in ephemeral slope channels to trap surficial material mobilized during rainfall events. Discharge-suspended sediment concentration hysteresis patterns at the two gauging stations in the main stream channel were used to indirectly infer sediment coupling between the hydrologically connected slope and the main stream.
During the study period, a total of 948 t of suspended sediment was transported through the channel at the upstream site and 715 t through the channel at the downstream site. During rainfall events, episodes of high discharge resulted in transfer of 75% of the total suspended sediment load through the upstream site and 50% through the downstream site. The total mass of sediment eroded from the dissected slope was estimated to be c. 3 t for the study period. Sediment mobilisation was confined to rainfall events with increasing particle sizes trapped with increasing event magnitude. Less than 1% of the particle size distribution of all samples collected during these events was classed as fine sediment, suggesting these sediment sources on the slope were exhausted. Channel suspended sediment transfer patterns during rainfall events were dominated by clockwise hysteresis where sediment concentration peaked before discharge. This pattern was interpreted as representing short distance remobilisation of fine sediment from within the channel, supporting the likelihood of limited input of fines from slopes. The data suggests, therefore, that slope-to-channel connectivity in this environment is dominated by coarse sediment, where there is active coupling. The lack of fine sediment sources on slopes within the basin suggest that since deglaciation there has been a relative decline in fine sediment slope-to-channel connectivity due to source exhaustion and a shift to episodic coarse sediment connectivity. This shift is likely associated with a contemporaneous change from a glacial control on fine sediment production and runoff to a rainfall controlled system, where mechanical weathering of bedrock continues to produce coarse material for transport from slopes to channels.