The purpose of this project was to understand what sets the shape of river detlas. Throughout the world, river deltas exhibit remarkably similar shapes: some have branching channel networks, others have wave-smoothed shorelines, and some have funnel-shaped mouths with looped channel networks. Despite the ubiquity of these general shapes, it is unknown what processes influence and set these shapes. In this project we conducted three main activities to address this knowledge. 1) We created some of the first global databases of delta location and shape; 2) We developed tool and tecnhiques to measure the branching properties of the channel networks commonly found on some detlas; and 3) We conducted a new suite of numerical modeling that simulates delta formation across the broadest possible range of parameter space.

From these activities have come four important project outcomes. 1) Using our global delta database we have now defined the fluvial and marine conditions that lead to delta formation. Waves and tides are destructive forces that limit deltaic formation, whereas river forces encourage delta formation. With our global model we can predict whether a river will form a delta with 74% accuracy. 2) Using our global delta database we have defined the number of people who live on river deltas. We found that 339 million people lived on river deltas in 2017 and 89% of those people live in the same latitudinal zone as mosttropical cyclone activity. We calculate that 41% (31 million) of the global population exposed to tropical cyclone flooding live on deltas, with 92% (28 million) in developing or least developed economies. This work underscores that coastal flooding is a problem that will disproportionately impact people on river deltas, particularly in developing and least-developed economies. 3) We have provided the first quantitative test of the connection between process forcing and delta morphology. Using these global delta databases, and a series of simulations we were able to show that the process forcing accounts for at least 35% of the variance in the number of distributary channel mouths and 42% of the variance in the shoreline roughness for real-world and simulated deltas. We identify a tipping point in the flux balance where wave influence halts distributary channel formation and show how this explains morphological transitions in real-world deltas. 4) Future work will certainly benefit from the first quantitative delta classification scheme that this proposal has produced. The Galloway diagram is a 50-year old hypothesis for classifying delta morphology or shape, and it has never been suitably defined. Our work in this proposal quantified the Galloway diagram, which allows any delta to be plotted in this space. This paves the way for future research to look at the relationship between delta attributes and position in Gallway space. Importantly, this quantified diagram shows that the key processes that influence delta shape are the sediment fluxes driven by rivers, waves, and tides.

Last Modified: 12/20/2022
Modified by: Doug Edmonds