River deltas are dynamic systems, fed by sediment flows and shaped by tides and wave action over long timescales. They’ve long proved fertile ground for human civilization. Many of the world’s biggest cities are located on river deltas, where easy access to marine transport, fishing, and coastal soils gives them an economic boost. But human activities and climate change have led to the instability of deltas around the globe, threatening the ecosystem services they provide. Reservoirs and demand for sand, for example, have left many deltas starved of sediment: The Nile Delta, the Mississippi Delta, and the Yellow River Delta are all experiencing shoreline erosion as sea level rise, land subsidence, and dwindling sediment supply interact.
Coastal communities have a vested interest in promoting delta resilience, but there’s growing understanding that our conventional and inflexible strategies for managing deltas—including storm surge barriers and river embankments—are unsustainable in the face of climate change. Managers are turning to nature-based solutions, but those solutions require deep knowledge of stable delta behavior. There is almost no place on Earth left undisturbed by human activities, which makes modeling and predicting modern delta behavior all the more challenging. It’s critical to develop natural solutions to contest the challenges deltas face.
Here Hoitink et al. synthesize recent research on river deltas to identify the ways that human activities interact with natural processes to create instability and discuss the tools that researchers can use to better understand those processes and render deltas resilient. The study area spanned the globe, using the best-studied delta systems, such as the Mississippi in the United States, the Rhine-Meuse in the Netherlands, and the Yellow River of China, to inform about process evolution of deltas.
The team identified four main processes revealing delta instability that relate to both natural dynamics and human activities: riverbank failure, channel incision and siltation, river avulsions, and a shift to hyperturbidity. These processes take place over a wide range of spatial and temporal scales. Riverbank failure, or a dredging-induced hyperconcentration of suspended sediment, can occur over relatively short timescales (years to decades), whereas channel bed erosion and the formation of a new deltaic landscape may require centuries.
The authors call for the development and improvement of a host of analysis tools to better understand these complex processes, including numerical modeling, network and dynamic system theory, and direct and continuous observations. Empirical data sets on the behavior of past and present river delta systems can help inform predictions of delta behavior in the future.
The major challenges that remain, according to the authors, are determining the annual sediment balance, predicting the impacts of dwindling sediment supply and sea level rise, and combining approaches to monitoring and modeling river deltas to optimize our understanding of river delta resilience in our rapidly changing world. (Journal of Geophysical Research: Earth Surface, https://doi.org/10.1029/2019JF005201, 2020)
—Kate Wheeling, Science Writer