In rusting Arctic rivers, why can the orange color warn about toxic metals that are not themselves orange?

Answer: Acid keeps metals dissolved

Acid keeps metals dissolved ✓ — Right. The orange iron particles are the visible signal, but acidic water can also mobilize aluminum, zinc, nickel, copper, cadmium, and other metals that may stay dissolved or travel farther. One study found zinc and nickel far higher in impaired reaches than in reference streams. The color is a warning light, not a full inventory.

Only rust flakes matter — Rust flakes can matter because iron and aluminum particles can coat stream beds and interfere with gills, but they are not the only concern. Acid rock drainage can release a suite of less-visible metals at the same time. A river can look like an iron problem while also carrying dissolved contaminants that the eye cannot sort out.

Mud makes metals safe — Mud and particles can sometimes bind metals, but they do not make the problem disappear. In these streams, low pH and sulfate point to reactions that release metals into water before some later precipitate or attach to particles. The safer lesson is that color alone cannot tell you which metals are mobile or biologically available.