Thursday, June 02, 2011

Natural froth

click photo to enlarge
Speaking of being puzzled (as I was in yesterday's post), something that made me scratch my head as a child was the froth that I saw regularly on the River Ribble. It wasn't always positioned at the base of waterfalls (or "forces" or a "foss"), but was sometimes floating along on a quiet stretch, making patterns of whirls, curls and blobs, until it was rushed down some rapids to be broken up and spread throughout the torrent.

I remember that my initial thought was that the paper mills upstream must be responsible for the froth and foam. Perhaps, I thought, it was a result of the discharges that they made into the river. But, as one by one, the mills closed down, and the froth continued to be evident I speculated on other causes. Was it farm run-off? Or maybe it was caused by natural agents in the rocks, soil and plants.

On my recent visit to the Settle area I took this photograph of froth on the river just below Stainforth Force. I have no doubt that the turbulence of the flow seen in yesterday's image had a part in creating the froth. But, since not all agitated river water produces it there must be additional factors involved. A little research shows that this phenomenon is widely found, and does often have a natural cause. It seems that organic compounds dissolve in the water and act as natural surfactants allowing fine bubbles to form and coalesce as foam or froth on the surface of the river. The plunging action of a waterfall or rapids is usually the stimulus for its creation.

Looking down on the froth from a small cliff above the river I took this shot, pleased by the patterns that were being formed and then slowly changed as the water moved on. Looking at my image on the computer I was reminded of some of the photographs that I took of patterns in frozen puddles last December.

photograph and text (c) T. Boughen

Camera: Canon
Mode: Aperture Priority
Focal Length: 60mm
F No: f8
Shutter Speed: 1/500
ISO: 100
Exposure Compensation:  -0.67 EV
Image Stabilisation: On