Channel Migration in Alluvial Rivers

An alluvial river flows through sediment that it has transported and deposited, so its bed and banks are readily reworked. When the river's sediment load exceeds its local transport capacity, deposition builds bars or raises the bed, redirects the strongest current and causes channel migration through accompanying erosion elsewhere.

Deposition and flow diversion

Deposition begins where velocity or transport capacity declines, commonly after floods, at bends or where a channel widens. The resulting sandbars, shoals and bed aggradation make flow uneven and shift the thalweg, the line of deepest and fastest flow.

• A bar obstructs or divides the current, concentrating erosive force against another part of the bed or bank.
• Repeated deposition and erosion create a feedback: deposited sediment redirects flow, while redirected flow enlarges a new channel path.

Forms of channel migration

• At a meander, deposition on the inner bank forms a point bar, while erosion of the outer cut bank shifts the bend laterally and downstream.
• In a braided river, mid-channel bars divide the flow into unstable threads that repeatedly form, merge and change position.
• Persistent aggradation raises the channel bed and reduces its relative advantage as a flow path; during a flood, water may breach the banks and adopt a shorter or steeper route.
• Such abrupt abandonment of one course for another is called avulsion, unlike the gradual movement of a meander.

Controls and disaster significance

Deposition does not act alone. Migration also depends on discharge variability, sediment size and volume, bank material, valley slope, vegetation and artificial confinement.

• Rapid channel shifting can erode settlements, farmland, embankments and transport infrastructure even where inundation depth is limited.
• Aggradation can reduce local channel conveyance and increase the likelihood of overbank flooding or avulsion.
• Floodplain planning must therefore account for a migration corridor, not merely the channel occupied at one date.

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