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We have just come back from completing the Otter (9-13 Feb 2010) which has to rank as one of the best hikes in SA. Unfortunately we were not able to complete the much talked about Bloukrans River crossing due to the fact we encountered over 2m of foam / scum at the first river crossing just before the 4km mark on day 4. One of the members of our party managed to get through the foam but we nearly lost her as it was extreamly dangerous as you have no idea what you are standing on nor can you see where you are going. The rest of us managed to bypass the foam by crossing the river further up and scrambling up a steep slope to intersect with the escape route but encountered more foam on the next beach we got to. We decided to abort - all covered in green, slimy, dead-fish smelling foam residue and took the escape route we had intersected earlier. We walked back to the Bloukrans river from Andre hut later in the day to find the Bloukrans exit also covered in the foam.
Has anyone else encountered this foam on the hike? The seas were very rough with high swells the day before. Also, what is in the foam/scum? Some of us are now covered in a rash where the foam touched our skin.

The foam is actually a natural occurrence due to protein content in the water, and precipitated by heavy wave activity, which places much air bubbles in to the water. The proteins binds to the air bubbles to create this foam.

Interestingly, in marine aquariums, precisely this technique is used to remove excess protein, before it breaks down in the water, polluting it. An air stone is placed inside a tube and the water is forced in the opposite direction through the tube, than the rising mist of fine bubbles. This friction causes the bubbles to scrub the water form excessive proteins, creating a similar foam which is captured and removed.

This type of filter is called a protein skimmer.

Thanks - always nice to know what it is and what causes it!