We got in touch with Austin Chadwick, a recent graduate from Caltech who did his PhD research under the supervision of Professor Mike Lamb, to talk about how his work has connected with the community studying river sustainability, and what his career plans are.
Tell us about river deltas and why understanding their dynamics is so important.
River deltas make up only about one percent of Earth's land surface, but host more than ten percent of the human population and some of the most biodiverse ecosystems on Earth. Understanding the dynamic nature of these landscapes is important for sustaining these people and ecosystems in the face of climate change. Because deltas are found where rivers meet the sea, they sit at a unique interface between climate pressures. Global mean sea level threatens to rise by up to one meter in the next century, posing flood risks to hundreds of millions of people and billions of dollars in assets. Flood risks are compounded during large storms when rivers pour over their banks. Periodically, these rivers will even shift their course to the sea entirely, in catastrophic floods called river avulsions. River avulsions have been responsible for dangerous floods and civil unrest over human history, but also nourish wetlands with sediment and build land to counter land loss due to sea-level rise and coastal subsidence.
What was the goal of your PhD research here at Caltech? What did you learn from your research?
Despite the central role avulsions play in river delta evolution, the processes controlling their occurrence remain poorly understood compared to inland landforms. Improving our understanding of the river avulsion process was the goal of my PhD research at Caltech. This involved three main steps. First, I developed the first theoretical framework for when and where river avulsions take place on deltas, based on the physics of coastal river hydrodynamics and sediment transport patterns. Second, I tested this theory using global remote sensing, field observations of the Yellow River delta, and laboratory flume experiments of small-scale deltas. Finally, after theory validation, I explored the theory's implications for future delta dynamics.
Perhaps the most important of these implications was our improved estimates of land loss and land building on deltas in the face of twenty-first-century sea-level rise. By resolving river avulsion dynamics with sediment budgets and sea-level projections, we were able to quantify how much sediment resources are spent building sustainable land, and how much is wasted building unsustainable land that is doomed to drown between avulsions. We found that sustaining modern coastlines will require roughly two times more sediment than previously estimated—or frequent engineered river diversions.
After you finished your degree, it seems that the work you and Mike did has started to influence the way people model river deltas and river dynamics. How is your work continuing to impact the field?
It is still early to say for sure, but I hope that our work has helped stimulate a paradigm shift in our scientific community's approach to river delta modeling. By highlighting the importance of river avulsion patterns on delta evolution and fate, we have shown that the classical approach of modeling deltas as homogenous landforms is inadequate. Instead, we need to model deltas as what they are: heterogenous landscapes forged at the largest scale by cycles of river avulsion and sediment deposition. The research community is starting to apply these concepts to provide more robust models of the river as an evolving system, as referenced in this feature in Science in 2021. Such modeling will deliver a groundwork to plan engineered diversions at a pace that mitigates flood hazards and land loss in the face of future climate change.
What have you been doing since your PhD and what are your plans for the future?
Since my PhD, I have continued to pursue my interests in river dynamics through consulting and research positions in Earth and Environmental Sciences. I am currently a postdoctoral scholar at the UC Santa Barbara Earth Research Institute, where I am using state-of-the-art image processing techniques to quantify river channel dynamics captured in historic satellite imagery from around the world. In the future, I hope to continue my work in river dynamics as a university professor or research scientist.