Assessing the Geomorphic Response of the Wairoa Catchment to Post-Settlement Land Use Changes: An Environmental History Investigation

Abstract

Successful catchment-wide river management strategies require well informed decision making that builds upon appropriate understanding of how the cultural and physical history of a place creates the contemporary landscape. To comprehensively understand a natural system it is necessary to appreciate its' legacy, the formative processes at work, and the anthropogenic imprint. An appraisal of the environmental history of the Wairoa River, located within the wider Hunua Range Catchment of East Auckland, is used alongside field based analyses within a multiple lines of evidence approach, to understand the cultural and physical history of the Wairoa. River responses to human disturbance, and their manifestation through the catchment, are analyzed in relation to these patterns of River Styles. Marked differences in river environments are evident above and below the geological knick point afforded by the Hunua Falls in mid-catchment. Antecedent controls have exerted significant influence over the Wairoa River's ability to respond to external environmental disturbance. Since Maori and European arrival in the catchment (1769 and 1850 respectively) there has been an 80% reduction in native forest cover. Today, the catchment is comprised of pasture and exotic forestry. An initial pulse of fine-grained sediment was delivered to the valley floor following deforestation. However, this sediment has not been actively transferred downstream; rather, it has been stored within tributary valley fills. Aerial photographs and historical maps show no significant planform changes in channel positions over time. There is no evidence for substantive alterations to channel geometry, other than local adjustments induced by willows along the lower course of the river. This finding contrasts with many similarly disturbed catchments in New Zealand where high sediment yields have been generated by gullying and landslips. The lack of channel adjustment and sediment flux in the Wairoa Catchment is attributed to the fine-grained, cohesive nature of sediments on valley floors and the limited connectivity of this river system. The Hunua Falls induced base-level control in mid catchment, inducing low slopes in upstream areas. This has limited the degree of incision in upstream tributaries. Low stream power in conjunction with this disconnectivity has resulted in the Wairoa being unable to access and mobilise significant stores of sediment. Seemingly, streams in the Wairoa catchment are able to accommodate disturbance events. Subsequent to the Wairoas' initial post deforestation adjustment, disturbance events have not been of large enough magnitude to significantly impact the morphology of the receiving waterways. This study highlights the importance of catchment-scale and catchment-specific understandings of disturbance as a mechanism to achieve greater rehabilitation success through the application of locally grounded scientific understandings.