River Meander Stage1 — free printable diagram

How a river meander forms

A meander is a bend in a river, typical of its middle and lower course. It forms and then grows because of one simple asymmetry: the water flows fastest round the outside of the bend and slowest round the inside. Fast water erodes and slow water deposits, so the outside of the bend is cut back while the inside is built up, and the bend becomes steadily more pronounced.

What a meander is

A bend in a river's course

Where they are found

The middle and lower course of a river

Outside of the bend

Fastest flow, deepest water — erosion

Inside of the bend

Slowest flow, shallow water — deposition

Landform on the outer bank

A river cliff — a steep bank

Landform on the inner bank

A slip-off slope, or point bar — a gentle shelf of sand and shingle

Erosion processes

Hydraulic action and abrasion

If the neck is cut through

An oxbow lake is left behind

• The speed difference is the engine of the whole landform. The channel is deeper on the outside of the bend, so there is less friction and the water moves faster there; on the inside it is shallow and slowed by friction with the bed.
• Eroding one bank while building up the other means the bend does not stay put. A meander migrates sideways across the valley floor and slowly downstream, which is why the cross-section is asymmetric — steep on the outside, gently shelving on the inside.
• Hydraulic action is the sheer force of the water against the bank; abrasion is the load the river carries being scraped against it. Both act hardest on the outer bank.
• As erosion continues on both outer banks of a tight loop, the neck of land between them narrows. Eventually, usually during a flood when the river is at its most powerful, the water takes the short route straight across.
• The abandoned bend is then sealed off by deposition at both ends and left as a crescent-shaped oxbow lake, separate from the river, which gradually silts up and dries.
• Diagrams of meanders at different stages are a sequence, not alternatives: they show the same bend developing over time, so reading them in order is the exercise.