Modelling the variability and dynamics of river plumes in Hawke’s Bay, Aotearoa New Zealand

A quarter of the freshwater entering the global ocean originates from small rivers, yet the dynamics and environmental impacts of plumes generated by small rivers are understudied. Numerous small rivers with varying discharge rates terminate in Hawke’s Bay, Aotearoa New Zealand (NZ) delivering large amounts of terrigenous material to the bay. In this study, a realistic, high-resolution hydrodynamic model configuration is used to characterise the river plumes generated in Hawke’s Bay. River plume variability on continental shelves is driven by river discharge, wind forcing and ambient currents which were realistically modelled in this study. A number of rivers terminating in Hawke’s Bay were tagged with a passive tracer which allows for individual plumes to be identified and treated separately and also allows for all plumes to be evaluated simultaneously. The passive tracers were used to investigate the spatio-temporal variability on seasonal and interannual timescales and to identify the main plume patterns and their potential forcing mechanisms. The river plumes generated in Hawke’s Bay are confined to the inner shelf (inshore of the 50 m isobath). Plumes from the numerous small and irregularly spaced rivers coalesce and on occasion a single large plume is generated. Plume coalescence is most often unidirectional, as observed for other systems; however, opposing alongshore currents can occasionally lead to bidirectional coalescence. Two antithetic plume patterns were identified through Self-Organizing Map (SOM) analysis: (i) two small consolidated plumes confined to coastal areas, typical of low discharges and downwelling-favourable winds and (ii) a single, large consolidated plume, typical of high river discharges and upwelling-favourable winds.