Abstract:
Identification of sediment sources, delivery pathways and storage sites can support the management of sediment issues in a catchment. The combination of high sediment yield in mudstone dominated tributaries and headwater reaches, and efficient delivery pathways have resulted in sedimentation issues in the lower reaches and beaches feeding from the Waimatā River at Gisborne. Analysis of patterns and processes was conducted using the River Styles Framework. Areas of high capacity for adjustment were differentiated as transfer and throughput reaches. Erosional and depositional features with a high capacity for adjustment were characterised in the transfer reaches. At the same time, throughput had a low capacity for adjustment and effectively flushed sediment through the system resulting in the Waimatā behaving like a sediment flume. Connectivity modelling highlighted the influence of land use on the potential for sediment generation and transport. High connectivity is exhibited in steep slope and low vegetated areas, whereas low connectivity is evident in low slope or densely forested reaches. These patterns were further illustrated using a sediment delivery model (CASCADE), which showed that highly connected reaches were often entrainment dominated, whereas low connectivity areas represented depositional areas. Lastly, the influence of changing land use was demonstrated for three land use scenarios. The forest removal scenario highlighted the influence of forest cover on sediment generation, as recently deforested areas had a dramatic shift from low to high connectivity. The two regeneration scenarios showed contrasting patterns, where reforestation promoted hillslope stabilisation, and the introduction of barriers, such as wetlands, resulted in sediment sinks. Ultimately, assessments of connectivity and River Styles demonstrate a multifaceted approach to assess catchment scale linkages of reach characteristics to support management applications.
