Rising sea levels and river delta growth

Sea-level rise and subsidence, which can drown river deltas, can also drive the phenomenon of river avulsion, in which rivers rapidly change course, according to a new study.

A US team used field data and modelling to investigate the timing of river avulsions and found that the frequency is related to the rate of sea-level rise, the rate of subsidence, and the supply of sediment that drives delta growth.

The balance of the rates also determines whether a delta advances or retreats, they write in a paper in the journal Proceedings of the National Academy of Sciences.

The work was led by Austin Chadwick, then at the California Institute of Technology, with colleague Michael Lamb and Vamsi Ganti from the University of California Santa Barbara.

“Our results provide a quantitative framework to predict delta response to future sea-level rise, which is valuable for planning engineered diversions to nourish deltas and prevent catastrophic hazards,” they write in their paper.

Deltas counteract sea level rise by building up sediment, which mostly occurs near a river channel itself. Sometimes, however, the river will switch course through an avulsion and begin building up the delta somewhere else.

“So avulsions are the way that the river spreads its sediment out over the whole landscape,” says Chadwick. “The questions we’re asking are how often do rivers naturally change their course, and how is that going to change with climate change and human interference.”

To date, there has been no consensus on how rivers respond to climactic shift.

In their work, Chadwick and colleagues combined observations from the geologic and historical records with a mathematical model of river dynamics.

They discovered there are three ways that deltas can respond to sea level rise, which depend on the balance between the rate of sea-level change and the sediment supplied by the river.

The first occurs when a river has a lot of sediment and sea-level rise is relatively slow. According to the model, these rivers are resilient to sea-level rise, and their avulsion rates remain stable. China’s Yellow River is one example.

The second case occurs when a river has less sediment, or the sea level rises more quickly. In this scenario, avulsions become more frequent, the rising ocean promotes sedimentation, and once a channel fills to a certain depth, the river will jump its course.

The third occurs in the extreme, when sea level rise outpaces a river’s ability to deposit sediment. As the ocean infiltrates the delta, the river will reach its maximum avulsion rate, and the whole system will begin migrating inland.

Scientists hadn’t known about this case before, the researchers say, and it explains the previous inconsistencies in the scientific literature.

“Overall our results indicate lowland deltas will not passively drown but instead will respond to relative sea-level rise through more frequent cycles of sedimentation and river avulsion,” they write.

“While this response acts to mitigate land loss in coastal wetlands, it also heightens flood hazards.”