|picture taken on 14 June 2011|
On lakes, in bathtubs and in mugs of beer, the cause for foam is the same. Agitation at the surface causes air to get under the surface film of the water. Weakened surface tension is unable to force the air out, but rather keeps water wrapped around a volume of air, creating a bubble. If this happens for long enough, foam is created.It was windy when I took this picture yesterday evening, so maybe that's why there was foam at the shore of the lake?