River deltas are home to over half a billion people, and function as economic and agricultural hubs. They host abundant and biodiverse ecosystems, are key nodes in carbon and nutrient cycling, and are climate change hotspots. Recent research in high-resolution remote sensing, modeling, and machine learning have offered new ways to link watershed and marine processes to delta evolution at unprecedented scales. However, the recognition of fluvial fans (also called DFS and megafans) as a distinct fluvial system introduces a new riverine inflow system. This calls for the definition of a new type of delta with distinct spatial and temporal water and sediment distribution pathways, ecosystem support and sensitivity to global change. These deltas are further distinct in their unique morphodynamics and how they construct the sedimentary record.

Fluvial fans are among the largest terrestrial landforms and produced by repeated river avulsions. They are morphologically and genetically distinct from alluvial fans, which are short, steep, and formed by sheetfloods and gravitational processes. Consequently, the deltas discussed here are also distinct from classic fan deltas, both in their formative mechanisms and sedimentary expression.

Where fluvial fans intersect lakes or oceans, their channel networks can resemble those of river-dominated deltas. However, these networks have distinct morphometric characteristics, as they are constructed through nodal river avulsions across low-gradient floodplains. Deltas form at the mouth of the active channel on the fan. When the river avulses, the existing delta is abandoned, and a new one forms at a different location. As a result, fluvial fan deltas are short-lived on the timescale of avulsions and are spatially dispersed across the fan front, which can span hundreds of kilometers in width.

This presentation draws on modern, ancient, and extraterrestrial examples to distinguish the morphodynamics of fluvial fan deltas and to outline their defining characteristics. Fluvial fan formation is driven by high sediment supply and intense, variable rainfall, and is facilitated by low-gradient, laterally extensive floodplains—linking this delta type to specific sediment-routing systems, landscape settings, and climatic regimes. These controls also influence their potential to support biodiversity and their relevance in the search for extraterrestrial life. Their short lifespans, combined with high sediment delivery, shape both their response to global environmental change and the nature of the sedimentary record they produce.