A river of powdered rock
The intense turquoise color of the Hokitika River is not a trick of the light, but the light itself. The water is saturated with 'glacial flour', a fine-grained silt created by the immense pressure of glaciers grinding against bedrock in the Southern Alps. This powder consists of minerals like quartz and feldspar, pulverized to particles between 2 and 65 microns. Because these particles are so small, they do not settle to the riverbed but remain suspended in the water column.
This suspension of fine sediment is the reason for the river's color. The water itself absorbs longer wavelengths of light—the reds, oranges, and yellows. The suspended rock flour, in turn, scatters the shorter blue and green wavelengths. This phenomenon, known as the Tyndall Effect, is what sends the turquoise hue back to an observer's eye. The shade of the water can change depending on conditions. A higher concentration of particles after significant glacial melt can make the water appear greener, while a lower concentration shifts the color toward a deeper blue. After heavy rainfall, however, the influx of other sediments overwhelms the glacial flour, and the river turns a murky grey-brown.