Why Do Sandbars Form on River Bends?

Sandbars (point bars) grow on the inside of river bends due to a classic erosion-deposition cycle:

1. Outside Bank (Cut Bank): Centrifugal force accelerates flow → high velocity → erosion → steep bank retreats.
2. Inside Bank (Point Bar): Flow decelerates → velocity drops → sediment (sand, gravel) drops out of suspension → bar builds outward.
3. Helical Flow: A corkscrew current (surface → outside, bottom → inside) continuously feeds sediment to the growing bar.
4. Migration: As the outer bank erodes and inner bar deposits, the channel slowly migrates laterally—meanders migrate downstream over decades.

This process creates the sinuous, migrating channels of alluvial rivers worldwide. The bar is a dynamic deposit, constantly reshaped by floods.