Abstract
Monitoring of river morphology is essential in assessing erosional and depositional areas, the stability of the riverbed, and is important to understanding the complex fluvial processes key to river management. The step pool morphology of alpine catchments creates a complex environment to study the interlinking processes of river morphology and bedload sediment movement. However, research creates an opportunity to identify the interconnectivity and scalability of morphological bedload response to infrequent high flow events. The aim of this research was to quantify the spatio-temporal dynamics of step pool morphology and examine the scalability of the processes between catchments. This thesis presents a study of the step pool morphological response to variations in discharge levels of two distinct New Zealand rivers: Birch Hill Stream and Silver Stream. The reaches were surveyed approximately every two months from February 2018 to December 2018. Additionally, data was collected of bedload sediment transport through the use of RFID tags. Of the two catchments Birch Hill Stream had a more developed step-pool morphology, which resulted in limited morphometric changes throughout a period of low to moderate flows, as a result of bed armouring by large keystone boulders. However, this period also showed large travel distances for the RFID tracer samples and exemplified the clustering of sediment within the step pool sequence. Comparatively, Silver Stream showed a greater morphological response to low and moderate flows due to the higher proportion of smaller clast sediment, which resulted in a large erosion event of 87.83 m3. High flow discharges of approximately 45 cumecs, caused a break down and reorganisation of step sequences in Birch Hill Stream with 52.31 m3 of bedload erosion. A deposition event in response to similar high flows in Silver Stream indicated that the high proportion of finer sediment created a smoothing effect and removed the step sequence through infilling of pools. Fine sediment present in Silver Stream was consistent with the higher abrasion and attrition rates and associated with higher friable sediment compared to Birch Hill Stream. Overall, the significant differences in the catchments, similarities in morphological low flow responses, and differences in morphological high flow responses indicate that the high flow events are one of the key drivers of morphological development. Additionally, the identified differences in morphometric responses are determined by the scale and physiographic elements of the catchments.