Pools and riffles

Alternating flow

Rivers naturally develop alternating zones of fast and slow flow along their course.

Outer bend erosion

Faster flow on the outside of bends causes vertical erosion, creating deeper areas.

Pool formation

Scouring by fast, turbulent water forms deep depressions called pools.

Inner bend deposition

Slower flow on the inside of bends deposits coarse sediment.

Riffle formation

Accumulation of gravel and pebbles creates shallow riffles between pools.

Energy balance

Pools and riffles alternate to help the river maintain a stable energy pattern.

Helicoidal flow

A corkscrew‑shaped flow transfers sediment from pools to riffles, reinforcing the sequence.