Broken Falls with Foam, Victoria, Australia

(Click image to enlarge)

Further upstream (above this image) the Mackenzie River is broken into many smaller falls and cascades by the irregular rock surface. They cover an area wider than this image shows, before recombining in the main flow seen here on the left. In rare periods of extremely heavy and prolonged rain, the entire bare rock surface seen here can become a single waterfall.

What struck me about this scene is the white foam seen in the lower right of the photograph. The river makes a ninety degree bend here and exits the photograph on the right side. The water's tumble over the washboard like surface of the falls creates the foam. The right angle turn creates a vortex that traps the foam and keeps the ever changing pattern rotating around a relatively stationary axis.

Foam in polluted rivers and streams can be caused by synthetic surfactants found in detergents. In pristine environments like this the foam is primarily caused by the presence of
surfactants from decaying organic matter. Agitation of the water, such as by wind, water currents, or tumbling over a waterfall traps air in these surfactant films, forming the foam that we see.