Testing a New Tool to Delineate Valley Bottom Margins and Measure River Confinement in the Waipā River Catchment, New Zealand

Abstract

Emerging technologies provide remarkable capacity to generate insights into controls upon variability in geomorphic river form, in space and time. Accurate delineation of valley bottom margins (VBMs) and measurements of valley confinement are critical parts of this process. The Valley Bottom Extraction Tool (VBET) is more accurate than other recent GIS-based tools that apply simplistic catchment-scale assumptions and more comprehensive than those that are limited to reach-scale applications (Gilbert et al., 2016). Building on VBET, the Confinement Tool (CT) takes into consideration the channel position on the valley floor and quantifies its degree of confinement, and thus its ability to laterally adjust (O’Brien et al., 2019). This study assesses the performance of these tools in determining the variability of valley confinement in the diverse landscape settings of the Waipā River, a major tributary of the Waikato River on the North Island of Aotearoa New Zealand. Informed by nationally available datasets, CT outputs were compared to manually extracted measures of confinement, to examine how topographic signatures of differing landscape settings influence the effectiveness of these emerging technologies as tools to inform applications in river science and management. Despite a history of volcanic activity (at Mount Pirongia and atop the volcanic plateau), limestone terrains, and various dry or underfit streams incised into former fan and swamp deposits associated with the shifting alignment of the Waikato River post-Taupō eruption demanding unique tool inputs and variables for individual Waipā sub-catchments, the final outputs were consistent across spatial scales. However, with the overall average accuracy of the automated outputs falling below 30%, the quality and resolution of the underpinning data impede the tool’s ability to produce reliable results. This limits the tool’s usefulness to generalistic, broad-scale appraisals of landscape controls on river form and processes. Expert geomorphic insight is required to avoid misinterpretation of findings. The study highlights the limitations of New Zealand’s national datasets to conduct such analyses. Unless this database is updated, it will not be possible to meet the full potential of emerging technologies, negating their capacity to inform systematic management strategies across a range of scales.