A river does not wear away both banks equally. On every bend the fast thread of flow presses against the outer, concave bank and cuts into it, while the slack water on the inner bank drops sediment as a point bar, so the meander grows ever tighter until its neck is cut and an oxbow lake is left behind. A dam across the channel exposes the counter-intuitive part: the water below runs clear but sediment-starved, so it eats into the downstream banks while the delta at the mouth shrinks for want of supply.
A river does not wear away both banks equally. On every bend the fast thread of flow presses against the outer, concave bank and cuts into it, while the slack water on the inner bank drops sediment as a point bar, so the meander grows ever tighter until its neck is cut and an oxbow lake is left behind. A dam across the channel exposes the counter-intuitive part: the water below runs clear but sediment-starved, so it eats into the downstream banks while the delta at the mouth shrinks for want of supply.
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