Avulsion

Avulsion is the sudden abandonment of a river channel as flow shifts to a new course, often across its floodplain. It redistributes sediment and can reshape deltas and river plains.

Channel aggradation: The buildup of sediment within a river channel, raising its bed relative to surrounding land. Bed raising can make a channel less favorable than a lower route across its floodplain.
Delta lobe switching: The repeated shift of delta-building river flow among different distributary routes. Avulsions redirect sediment supply and help move active deposition to new parts of a delta.
Kosi River: A Himalayan river that flows through Nepal and India before joining the Ganges. Its large historical channel shifts illustrate the scale of avulsion on an alluvial plain.
Meandering river: A river that follows a sinuous path with bends that migrate across its floodplain. Meanders shift gradually, unlike the abrupt channel abandonment characteristic of avulsion.
Crevasse splay: A fan-shaped deposit formed when floodwater and sediment escape through a breach in a natural levee. A breach can enlarge into a new channel that captures the river.
River delta: A sedimentary landform formed where a river deposits material as it enters standing water. Channel avulsions help distribute sediment among delta lobes and reshape the delta surface.
Mississippi River Delta: The complex delta built by the Mississippi River at the Gulf of Mexico. Its successive delta lobes record long-term shifts in the river’s principal sediment route.
Channel migration: The gradual lateral movement of a river channel across its floodplain. It changes a river’s position over time without necessarily producing a sudden new course.
Natural levee: A raised ridge of sediment deposited beside a river channel during floods. Levees confine ordinary flow, but their height can promote escape toward lower floodplain areas.
Alluvial fan: A fan-shaped sediment deposit formed where a stream exits a confined upland onto flatter terrain. Both systems shift depositional pathways, but avulsion is a channel change rather than a landform.

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