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ELI5 Yahaaa

Erosion on the outside

Why Do Rivers Bend?

A river cuts its outer bank and drops sand on the inner one, so every bend deepens.

1 Water swingsfaster roundthe outside2 It cuts theouter bank3 And drops sandon the inside
Every bend makes itself bendier.

  1. Step 1 of 5

    A perfectly straight river is almost impossible.

    1 Straight flow2 One smallobstacle3 The flow isdeflected

    The tiniest obstacle nudges the flow to one side. Nothing corrects it, so that small deflection is the beginning of a bend.

  2. Step 2 of 5

    Water moves faster round the outside of a curve.

    Outer bank Fast — erodesInner bank Slow — deposits

    The outer path is longer, so water there travels faster. Fast water carries more grit and cuts harder.

  3. Step 3 of 5

    So the outside is cut and the inside is filled.

    Outer bank Undercut and collapsingInner bank Sand building up

    The outer bank is undercut and collapses; the slow inner side drops its sand as a beach. The bend migrates sideways.

  4. Step 4 of 5

    Which makes the bend bigger, which makes it worse.

    A slight bend Faster outerflow More outer erosion A sharperbendRunsaway

    A sharper curve means a bigger speed difference, which means faster cutting. Bends grow into loops entirely on their own.

  5. Step 5 of 5

    Eventually the loop gets cut off.

    1 The loop nearlycloses2 A flood cutsacross3An abandonedoxbow lake

    When two bends nearly touch, a flood punches straight through. The river takes the short route and abandons the loop as an oxbow lake.

    Satellite photos of floodplains show dozens of old oxbows — a record of where the river used to run.

The short version

Rivers bend because outer banks erode and inner banks build up, and each bend makes itself sharper.

Try it yourself

Trickle water down a tray of damp sand. Within a minute it stops running straight and starts winding.

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