River Rapids

Water speeds up as it squeezes over a submerged rock. When the current runs faster than surface waves can travel upstream (Froude number > 1, "supercritical"), the waves can't escape — they get pinned in place over and behind the rock as a stationary wave train, even though the water itself races through them. Where the flow slows back down it often snaps back in a foamy hydraulic jump.

Try it: drag a rock shallower or crank the flow and watch a smooth bump steepen into a breaking standing wave. The pattern is stable; the water is not. Drop in a rubber boat (B) and watch it surf the standing wave or get caught in the churn behind a rock — then grab it and throw it back upstream.