Nature Watercolors by Mario Reis.

For his art series "Nature Watercolors" German artist Mario Reis soaked his canvas in various rivers to truly capture the "color" of a river:

Slowly, sediment is deposited, minerals are soaked up and pollutants settle out of the surrounding water. After collecting these canvas ‘traps’, the artist unstretches the works and freezes interesting results with spray adhesive.

Link – via Moon River