Abstract
Many river corridors in New Zealand, like those globally, are heavily modified. The combination of anthropogenic modification, increased development and a changing climate means that these catchments and river corridors will likely be more prone to a higher frequency of low probability events, such as increased high-magnitude flows. Consequently, there is a need to understand how catchment characteristics are changing in response. The objectives of this study were to determine the long-term trends in flow regime (river flow and rainfall), flood event character, and lower catchment cross-sectional width within the studied Canterbury and Southland catchments. This was achieved using upper catchment rain and mainstem flow gauge records to identify spatially discrete changes to river flow and flood event character. Relative Elevation Models (REMs), and a catchment channel confinement classification were used to determine changes to active channel width within catchment areas with a high propensity for geomorphic change. There was a detected change in the phasing and timing of delivery of river flow and rainfall without a significant change in the monthly rainfall volume, most notably within the Waiau Uwha and Hurunui Rivers which drain the Southern Alps / Kā Tiritiri o te Moana. There was a greater amount of change detected within the three Canterbury braided catchments in comparison to the Mataura River in Southland. The greatest commonality between all catchments was a reduction to lower gauge river flows. The greatest change to flood event character was observed within braided catchments, the most significant of which was a reduction in event length at all gauges. This trend was not observed within the Mataura River gauges. Correlated to this reduction in event length was a comparatively large reduction of active channel cross-sectional width within lower reaches with high geomorphic propensity over the period of imagery. This demonstrated an increased hydraulic efficiency of flood event propagation and the strong primary control of river morphology on river flow characteristics at all flow levels. These findings present insight for how flow regime and flood event character within these river catchments may continue to change, providing a framework for future catchment-based planning.