A river colored by mountains
The Katun River's striking turquoise color is not caused by tropical warmth, but of immense cold and pressure. The river begins at the Gebler Glacier, which sits on the southern slopes of Mount Belukha, the highest peak in the Altai Mountains at 4,506 meters. As this and other glaciers grind their way down the mountain, they pulverize the bedrock into a fine silt known as glacial flour or rock flour. These silt-sized particles become suspended in the glacial meltwater.
The resulting cloudy water gets its color from the physics of light scattering. The fine particles of rock flour suspended in the water absorb the longer wavelengths of light (reds and yellows) and scatter the shorter wavelengths (blues and greens). This gives the water its milky, blue-green appearance. The intensity of the color changes with the seasons. It is most bright in the late spring and summer when melting glaciers release large volumes of this sediment-laden water. In the autumn and winter, when the river is fed more by groundwater, the flow decreases, the sediment settles, and the water becomes clearer.