An excursion to the Val Costainas whitening river (Graubünden, Schweiz)

This is an incredibly rare phenomenon. The main river is acid and dissolves aluminum salts in the surrounding rocks. Flowing down the valley the main river joins several pH-neutral tributaries that let the main river pH increase gradually. At neutral pH the aluminum salt are not soluble anymore, therefore they precipitate and stick on the stones of the river bed. The water remains crystal clear.
Both scientific and didactic publications on this topic are available, for instance:
www.srf.ch/art...
• Il lat dals ovels alpi...