Water falling from a height appears white is due to a phenomenon called non-selective scattering. When water falls from a height with high velocity, lot of tiny water droplets are formed around the falling mass of water due to the bombardment of water with the rocks.These tiny droplets are about 100-1000 times bigger in size when compared with the wavelength of visible light which human eyes can see.

The light coming from the sun which the humans can see has three primary colours red, green and blue. When light rays confront the water droplets, they scatter the entire visible range. When these three primary colours red, green and blue are mixed in equal proportions, and produce white colour due to scattering of light. Hence, the colour we see after scattering by the water droplets is white.

When water falls with a low velocity, very less or even no such tiny droplets are formed around the water falling from height, hence no scattering takes place and we do not see white falling water.