Multi-scalar controls on channel geometry and historical conditioning of contemporary riverscapes Whatawhata, New Zealand

Abstract

Physical controls upon downstream trends in channel geometry are analysed across three ~3 km2 headwater catchments in the western Waikato region, New Zealand, at catchment-, reach-, and local-scales. These catchments have different histories of land use change, culminating in variable proportions of differing land cover today (native forest, pasture, Pinus radiata). The systematic catchmentwide appraisal of channel geometry related field measurement of channel crosssectional area, width, and depth to controls such as slope, discharge (using catchment area as a surrogate), valley width (confinement/accommodation space), bank sediment, bedrock outcrops, vegetation cover, wood loading, and channel alignment. Trends in downstream channel geometry were identified and outliers were explained in relation to these different controls. Downstream channel geometry varied markedly in the three study catchments. One catchment (Mangaotama) exhibited more obvious downstream trends than the other two (Kiripaka and Whakakai). This variability was attributed to controls such as discharge and slope (catchment-scale), floodplain pocket formation and bank strength (reach-scale), and in-stream wood and bedrock (local-scale). Controls exhibited varying importance between the study catchments. Top-down hierarchical influences of catchment-scale controls on channel geometry were observed in one catchment (Mangaotama), whereas bottom-up hierarchical influences of local-scale controls were observed in the other catchments. Influences of land clearance on contemporary river character and behaviour were considered in terms of the nature of valley fill (floodplain) deposits. These are identified as post-European settlement materials by undertaking pollen analysis of a sediment core. The greater thickness of these materials has exerted a primary influence upon channel geometry in the cleared catchments. Findings from this study indicate that there are no consistent relationships between channel geometry and the multi-scalar controls, highlighting the inherent complexity and variability of channel geometry in river systems. The ways in which such systemspecific, cross-scalar analyses, tied to appraisal of historical considerations, aid applications of geomorphology to river rehabilitation initiatives are considered.