Tipping points in river deltas

Abstract

River deltas are susceptible to rapid and irreversible regime shifts in channel morphodynamics, estuarine turbidity and coastal wetland state. In this Review, we discuss the tipping mechanisms and self-reinforcing feedbacks underlying these regime shifts. The propensity for tipping in deltas can be increased by climate and land-use changes, based on projections from published data from 47 deltas worldwide. For example, sand mining, sediment retention behind barriers and increased floods under climate change have led to scour hole formation in the Rhine–Meuse Delta, river bank retreat in the Mekong Delta, and turbidity flushing in San Francisco Bay. Conversely, flood-prone areas with increased sediment influx caused by deforestation might be susceptible to avulsion. Resilience indicators based on observational data quantify the robustness of deltas to such regime shifts and could provide warnings when action should be taken to avoid degradation or yield insight into ways to promote back-tipping. However, resilience indicators alone are insufficient to identify the exact tipping point. Dynamical systems theory could provide a helpful framework to analyse tipping dynamics, but to apply this approach to real-world data, improvements in mathematical models of tipping points will be needed alongside an increase in continuous measurements of river deltas. Land-use and climate changes can reduce the resilience of river deltas to regime shifts in channel morphodynamics, estuarine turbidity and the state of coastal wetlands. This Review discusses how these regime shifts can be triggered, monitored and prevented.