Rusty

Red Mountain Creek, off US 550 (the "million-dollar highway") between Ouray and Silverton, Colorado. The rusty stain is just that--iron oxyhydroxides--that's mostly a result of acid mine drainage (AMD). Sulfide minerals oxidize in the presence of water and oxygen, yielding sulfuric acid which in turn puts metals, including iron, into solution. The dissolved iron then oxidizes further from atmospheric oxygen and precipitates. Because it's just rust, it's harmless if not particularly attractive. Other dissolved metals, tho, can be considerably more of an environmental hazard. Over the last few decades there have been efforts to isolate the drainage from old tailings heaps upstream to minimize further AMD. The inset shows a close-up of the staining along the creek.