Nearly pristine rivers in northern Alaska are suddenly turning orange, with water sometimes carrying more metals than water from mines.
The phenomenon is occurring in the Brooks Range, an Arctic region where the ground normally remains frozen for several years. This permanently frozen ground is called permafrost. As temperatures rise, its thawed layer can become deeper. Water then seeps into layers of the underground that it previously reached much less often.
An orange waterway in the Brooks Range, northern Alaska. Credit: U.S. Geological Survey / Josh Koch
This water encounters minerals containing iron and sulfur, among other elements. Chemical reactions then release sulfuric acid and various metals. Iron gives the waterways their highly visible rust color. The process is called acid rock drainage. It can therefore occur naturally, without a mine or industrial activity nearby.
To measure its extent, researchers studied six watersheds located in parks and reserves in northwestern Alaska. Between 2022 and 2024, they collected water from the main rivers, their tributaries, and small flows coming from the slopes. The analyses revealed highly acidic water heavily enriched with sulfates and metals.
The most surprising result appears farther downstream. The main river manages to limit acidification when these waters mix into it. Yet some metals continue downstream, and their concentrations may keep rising, even sharply. Researchers measured concentrations about 70 times higher as far as 97 km downstream from where the contaminated water entered.
Orange water joins the blue water of the Kutuk River in Gates of the Arctic National Park, Alaska. Credit: Ken Hill / National Park Service
Water-quality records also make it possible to date a major change. In 2019, sulfate and zinc concentrations suddenly increased in several watersheds. That year saw the hottest summer ever recorded in Alaska, followed by a very snowy winter. A thick layer of snow can insulate the ground and slow its refreezing.
In other words, more water may have penetrated deeply into the thawed ground and then reacted with the minerals. The concentrations subsequently declined gradually, without immediately returning to their previous levels. The phenomenon now extends beyond these six watersheds: more than 200 affected rivers and streams have already been documented, particularly in Arctic and boreal regions.
No harmful effects on residents or wildlife have currently been established for these isolated waterways. Researchers also report no mass fish die-offs or known drinking-water problems in the villages. Further studies must now determine the risks to aquatic ecosystems and track how these rivers change as thawing continues.