Mountains, monsoons, and megafans

Abstract

In certain cases, the rivers draining mountain ranges create unusually large fan-shaped bodies of sediment that are referred to as fluvial megafans. We combine information from satellite imagery, monthly discharge and precipitation records, digital elevation models, and other sources to show that the formation of fluvial megafans requires particular climatic conditions. Specifically, modern fluvial megafans in actively aggrading basins are produced by rivers that undergo moderate to extreme seasonal fluctuations in discharge that result from highly seasonal precipitation patterns. The global distribution of modern megafans is primarily restricted to 158-358 latitude in the Northern and Southern Hemi- spheres, corresponding to climatic belts that fringe the tropical climatic zone. No rela- tionship exists between megafan occurrence and drainage-basin relief or area. The ten- dency of rivers with large fluctuations in discharge to construct megafans is related to the instability of channels subject to such conditions. Because of the correlation between sea- sonal precipitation and megafan occurrence, the recognition of fluvial megafan deposits in ancient stratigraphic successions may provide critical information for paleoclimate reconstructions.