Avulsion (river)
River channel diversion process via bifurcation.
Avulsion (river) is the process in sedimentary geology and fluvial geomorphology by which flow is diverted out of an established river channel into a new course via a bifurcation, or splitting of the channel. Avulsions range from full, where all flow is diverted from one channel to another, to partial, where only a portion of flow is diverted. While once attributed primarily to a slope advantage—where a new course offers a steeper gradient than the existing channel—it is now understood that multiple factors influence avulsion rates and progression. These include upstream channel planform, the cross-sectional geometry of the channel at the bifurcation point, and obstructions such as wood or beaver dams.
Avulsions are especially common in river deltas, a process also known as delta switching. As a river deposits sediment upon entering the ocean, it builds a deltaic lobe, such as the bird’s-foot delta of the Mississippi River. As this lobe extends offshore, the channel lengthens while maintaining the same elevation drop, reducing its slope. This lower slope becomes unstable: gravity drives water toward a more direct, steeper route to the ocean, and reduced shear stress on the bed causes sediment to accumulate within the channel, raising the bed relative to the floodplain. During a flood, the river may breach its natural levees and cut a new, steeper channel. The abandoned lobe then subsides. Repeated avulsions create a distributary network in a mature delta. Subsidence or sea-level rise can further promote backwater and deposition, filling channels and leaving a geologic record. On average, an avulsion occurs when the channel bed aggrades enough to become superelevated above the floodplain by about one channel-depth, providing sufficient hydraulic head for any levee breach to trigger an avulsion.
Erosional avulsions can also happen during large floods when a new, straighter channel offers a significantly steeper slope. If the new slope is similar to the old, a partial avulsion may occur, with flow occupying both channels. An example is the 2006 avulsion of the Suncook River in New Hampshire, where heavy rains caused flow to back up behind an old mill dam, overtopping a sand and gravel quarry and cutting a new channel at a rate of 25–50 meters per hour. The mobilized sediment later forced a meander cutoff downstream by superelevating the bed. Another example i
- field
- Sedimentary geology, fluvial geomorphology
- known_for
- Process of river channel diversion via bifurcation
- types
- Full avulsion, partial avulsion, erosional avulsion, meander cutoff
- occurrence
- Common in river deltas and during large floods
- factors
- Channel slope, upstream planform, cross-sectional geometry, obstructions (e.g., wood, beaver dams)
Lore & Background
Avulsions were once thought to occur solely due to a slope advantage, where a river could take a steeper, more direct course. However, it is now recognized that many factors influence avulsions, including upstream channel planform, channel cross-sectional geometry at a bifurcation, and channel obstructions such as wood or beaver dams. In deltaic and net-depositional settings, avulsions are common as sediment deposition reduces channel slope, making the river unstable and prone to breaching its levees during floods to find a steeper path to the ocean. This process, also known as delta switching, builds new deltaic lobes while abandoned lobes subside.
Reader's Guide
Avulsion is a key process in the evolution of river deltas and floodplains, shaping distributary networks and leaving a geologic record in sedimentary basins. In deltas, avulsion occurs when channel bed aggradation superelevates the channel above the floodplain by about one channel-depth, providing enough hydraulic head for a breach to result in avulsion. Meander cutoffs are minor avulsions that occur when the ratio of channel slope to potential slope is less than about 1/5. Avulsions typically proceed downstream to upstream via head cutting erosion and can be triggered by floods or dam removal.
Did You Know?
- Avulsions range from full, where all flow is diverted, to partial, where only a portion of flow is diverted.
- On average, an avulsion will occur every time the bed of a river channel aggrades enough that the river channel is superelevated above the floodplain by one channel-depth.
Frequently Asked Questions
What is river avulsion in geomorphology?
River avulsion is the event where a watercourse abandons its existing channel and carves a new path through a bifurcation point. It is a core process studied within sedimentary geology and fluvial geomorphology.
What are the different types of river avulsion?
Geomorphologists distinguish between full avulsions (the entire flow shifts to the new channel) and partial avulsions (only a fraction of the flow diverts). Erosional avulsions and meander cutoffs represent additional mechanisms by which a river can redirect itself.
Where do river avulsions most frequently occur?
River deltas are the classic setting for avulsion events because of their low-gradient, sediment-rich environments. Large flood events also trigger avulsions in other river systems when water overtops banks or breaches natural levees.
What factors determine whether a river will avulse?
Key controls include the channel's longitudinal slope, the upstream planform geometry, and the cross-sectional shape of the channel. Physical obstructions such as fallen timber or beaver dams can also build up enough head to force flow into a new path.
How does a river avulsion differ from a simple meander cutoff?
A meander cutoff is one specific mechanism that can produce an avulsion, but the broader term covers any diversion of flow out of the established channel via a bifurcation. So every meander cutoff counts as an avulsion event, yet not every avulsion involves a meander loop being cut off.
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