Competition Between Accommodation Space and River Sediment Supply in New Zealand Estuaries

Abstract This study examines how three New Zealand estuaries with similar geomorphic features but contrasting climates respond to past and projected changes in sea‐level and riverine sediment supply, using an aggregated‐scale estuarine model. Model calibration against historical sedimentation rates demonstrated good agreement, providing confidence in the simulation settings. Results show that estuarine response depends on the interplay between accommodation space created by sea‐level rise (SLR) and the availability of riverine sediment supply, which vary locally due to regional tectonic and climate variations. Under moderate SLR scenarios, some estuaries are able to keep pace or even become shallower through sediment infilling, whereas under higher SLR projections (e.g., SSP5‐8.5), inner estuarine elements consistently deepen, changing the tidal prism and fueling ebb‐delta growth. The influence of river discharge and sediment supply fluctuations further modulates morphological trajectories, generating alternating phases of infilling and erosion. A critical SLR threshold was identified for each estuary, beyond which tidal flats cannot be maintained and the system risks drowning. These findings emphasize the need for site‐specific adaptation strategies, recognizing that estuarine systems may follow divergent evolutionary pathways under climate change.