megafloodplain

A megafloodplain is the broad, low-lying surface adjacent to a river system that records episodes of megaflooding. The term describes extensive floodplain–scale deposits and landforms produced by megafloods—extremely large, high-discharge floods that inundate areas far beyond ordinary floodplains. Megafloodplains can extend over tens to hundreds of kilometers and preserve a stacked record of extreme-flow events, including channel avulsions, overtopped banks, and temporary lakes or wetlands during flood intervals.

Formation and processes: Megafloods arise when large volumes of water are suddenly released from glacial lakes, breached natural dams, or basin outbursts. The surge scours broad channels, reorders drainage networks, and deposits sizeable volumes of sands, silts, and gravels over wide corridors. Repeated megaflood episodes create successive floodplain surfaces, levees, and widespread overbank deposits, with intervals of relative quiescence allowing vegetation and soils to develop.

Sedimentology and morphology: The sedimentary record of megafloodplains often shows thick, coarse-grained layers overlain by finer overbank deposits. Palaeochannels, levee complexes, and sheet-flood deposits may be preserved across large landscapes. Dating methods such as pollen analysis, radiocarbon dating, and stratigraphic correlation help reconstruct flood timings and magnitudes.

Examples and significance: The Missoula megafloods in western North America produced extensive megafloodplain and terrace surfaces in the Columbia River Basin and the Channeled Scablands. Studying megafloodplains informs interpretations of past climate, river behavior, sediment transport, and landscape evolution, and aids understanding of long-term flood risk in large river systems. See also floodplain and megaflood.