A Catchment Framed, Process Based Approach to Analysis of the Evolutionary Trajectory of the Tongariro River

Abstract

Major degradation has occurred to rivers world-wide, as channels and habitats are simplified and ecosystems impaired. Although river rehabilitation has been suggested as the best tool to stop this decline, efforts thus far have been marred by failures. A lack of process-based, geomorphic understanding is commonly given as a key reason. Multi-scalar understandings of space and time seek to address this shortcoming. This thesis applies analysis of landscape form at catchment, reach and bar scales to appraise process relationships within the Tongariro catchment, New Zealand. At the catchment scale, a DEM is used to analyse sediment conveyance within, and between, landscape units by calculating erosion indices, slope categories and drainage line stream power. Field-based analysis of the character and behaviour of streams across the catchment grounds this work. A sediment budget describes the bulk transfer of material since 1.8 ka, following the eruption of Lake Taupo. As this volcanic activity reset process zones in the catchment this presents a useful time constraint for this study. Mechanisms of channel adjustment and temporal patterns of response are assessed using 80 years of aerial photography for the 15 km reach of the lower Tongariro River. A framework to characterise river sensitivity is presented. Analyses of controls (i.e. bed material, transport capacity and valley confinement) within the wandering cobble bed reach are assessed. This provides a process-based explanation of within-reach variability in the pattern and rate of channel adjustment. At the bar scale, the distribution of bed materials was mapped using terrestrial laser scanning. Sediment entrainment during different magnitude/frequency floods was modelled. A conceptual model describing the geomorphic effectiveness of each flood event on within-bar surfaces is presented. Insights across multiple scales were synthesised using a catchment-framed, process-based, evolutionary trajectory approach. This is used to predict future pathways of geomorphic adjustment. Tongariro-specific findings include the resilience of the wandering, cobble bed reach, as a lag of lahar material retains steep slopes within the lower reach, flushing the active gravel fraction. Directly downstream, the highly sensitive braided reach captures this gravel, causing high rates of channel adjustment and widening. The meandering sand and delta reaches are narrowing towards a threshold condition, beyond which avulsion is likely. Tools which combine different scales and types of insights (i.e. qualitative landscape evolution and quantitative process based analysis) are necessary to provide a more comprehensive underpinning to rehabilitation schemes, allowing management strategies to ‘work with’ the underlying processes.