Study: River deltas 'self-organize' to withstand human and natural disturbances

BLOOMINGTON, Ind. -- River delta channels that carry water, sediment and nutrients may appear to be random and arbitrary in how they are organized. But research by a team that includes an Indiana University geologist finds there is order to the complexity.

The researchers, using field observations and mathematical modeling, concluded that deltas self-organize according to an "optimality principle," creating networks of channels that increase the diversity of ways in which sediment is transported.

"Channel networks are the blood vessels of a delta system, and their job is to deliver water, sediment and nutrients to the larger delta environment," said study co-author Douglas Edmonds. "We have uncovered an organizing principle that describes how these channel configure themselves to do that job."

The article, "Entropy and optimality in river deltas," is published in the Proceedings of the National Academy of Science.

Edmonds is an associate professor and the Malcolm and Sylvia Boyce Chair in the Department of Earth and Atmospheric Sciences in the IU Bloomington College of Arts and Sciences. Other authors are at the University of California, Irvine; the University of Nevada, Reno; the Swiss Federal Institute of Technology in Lausanne; and the University of Padua in Italy.

The researchers looked for order in river delta channels by using statistics and mathematical modeling along with concepts from information theory. These results will have implications for how to design sustainable deltas.

Looking at 10 major river deltas in North and South America, Europe, Africa and Asia, they determined the probability of flows dividing into smaller channels and merging again at confluences. They discovered that all but one exhibited a high "nonlocal entropy rate," meaning a large diversity of delivery pathways to the sea.

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A collage shows satellite views of river deltas on five continents. Image courtesy of Efi Foufoula-Georgiou

The team confirmed the findings through numerical models. They demonstrated that, after major channel disturbances take place, the channel networks are reorganized and flows tend to recreate diverse water routes. They argue that the diversity of the networks makes deltas more resilient in the face of disturbances caused by weather, human activity or other forces.

Many of the world's deltas have come under threat in recent decades from rising sea levels, local development and the construction of upstream dams that limit the flow of sediment. River deltas occupy only 1 percent of the world's land surface but are home to more than half a billion people and are ecologically important and highly productive. Those circumstances make it urgent to better understand delta processes and how they are affected by climate and human activity, the researchers say.

The University of California, Irvine News office contributed to this post.