As Alaska's permafrost thaws, its rivers are running rusty.

Across Alaska, thawing permafrost is releasing iron into the meltwater that feeds streams and rivers — enough, in some places, to turn the water an oxidized orange and reshape the ecosystems fish depend on. A new U.S. Army contract is aimed at understanding it.

The Cold Regions Research and Engineering Laboratory at Fort Wainwright, near Fairbanks, is looking for a lab to run isotope tests on up to 40 water samples gathered from thaw areas this summer. The isotopes act like a fingerprint, letting researchers trace where the iron is coming from and how it moves through a watershed.

The stakes run past the science. When permafrost gives up its iron into the water, the communities most affected are the ones that fish those waters to eat — a link that Alaska Native tribal health researchers have been working to document. Quotes on the contract are due July 21.