The criterion of what differentiates megafans from typical alluvial fans is an artificial one of scale.
The scale divide varies in the literature, with the most common being a 100-km apex-to-toe length.
Through complex processes like avulsion, the stream changes course over time occupying different areas of the fan.
Over long periods of time, sediment builds up creating a fan formation.
Generally the three-dimensional architecture of megafan deposits consists of multi-storied sandsheets, gravel in upper reaches, interbedded with overbank muddy layers, thickness and facies distribution vary from upstream to downstream reaches [2] Rivers forming large fans happen in various settings around the world most notably, in foreland settings (e.g. Kosi, Gandak, Pastaza), intracratonic basins (e.g. Pantanal, Taquari, Cuiaba), and in complex settings like in the case of Pilcomayo.