White water occurs in the upper course of a river when the gradient and obstacles disturb the fl ow of water, causing it to churn and create bubbles. These bubbles refl ect back much of the light that hits them, making the water appear white. Whether a river fl ows smoothly often depends on its speed, and the steeper the riverbed, the faster the water will fl ow. The combination of fast-fl owing water and obstacles like rocks causes the fl ow to become turbulent, with unpredictable variation in the speed and direction of the Discover which part of a river’s course provides the setting for some of the world’s most dangerously turbulent water Why does water turn white? water. This creates a variety of features in the river. Where water doubles back on itself, pockets fi lled with bubbles open up; these provide much less buoyancy and feel like ‘holes’. Objects lodged in the river can act as strainers, allowing water to pass through, but blocking larger debris. In areas where the water moves rapidly, it wears away at the surface of rocks underneath, creating undercuts. The challenges of navigating the features of white-water rapids – whether jutting rocks or whirlpools – attract kayakers and rafters every year.
