A meander migrates through outer-bank erosion and inner-bank deposition, linked by the three-dimensional movement of water and sediment.
A meander is a sinuous bend in a river channel. As water follows the bend, the fastest flow and deepest channel commonly lie towards the outer bank. Greater shear and sediment impacts erode that bank, creating a steep river cliff or cut bank.
Secondary, helical circulation moves near-surface water towards the outer part of the bend and near-bed water towards the inner bank. This helps transfer sediment across the channel. Lower transporting power near the inner bank encourages deposition, building a gently sloping point bar.
Repeated outer-bank erosion and inner-bank deposition shift the channel sideways and often downstream. During high flows, erosion can narrow a meander neck; a shorter route may cut through and abandon the old loop. An oxbow lake is a possible result, not a necessary stage in every bend.
Discharge, sediment supply, bank cohesion, vegetation, channel slope and available floodplain space influence the pattern. Cohesive or rooted banks can help preserve a single channel while allowing gradual migration. Very resistant banks or confined valleys restrict lateral movement.
Bank protection and channel straightening can prevent local migration or cut off bends, but may move erosion downstream and reduce floodplain habitat. A river plan should therefore consider a reach and its connected floodplain, rather than one threatened bank alone.
Step by step
Outer bank erodes
Faster, deeper flow applies greater stress to the bank.
Inner bank builds
Secondary circulation and weaker local transport favour point-bar deposition.
Bend migrates
Repeated erosion and deposition move the channel across the floodplain.
Worked example: Reading a plan-view image
A crescent of exposed sediment on the inside of a bend and a steep bank on the outside support a point-bar and river-cliff interpretation. A single image identifies features; repeated dated images are needed to measure migration.
Watch out for this
The inner bend erodes most because it has the shortest route.
Flow through a bend is three-dimensional. The fastest flow is commonly displaced towards the outer bank, while sediment builds on the inside.
Check your understanding
Where should a point bar normally be identified?
- On the inside of a meander bend.
- Only at a river source.
- At the fastest outer-bank flow.