317 locations of observed orange staining across northern Alaska, mostly in the Brooks Range mountains.

Regional mapping We located 317 locations of orange-stained streams, rusting rivers, and acidic seeps (those colored orange or with dead vegetation) across Arctic Alaska using an opportunistic approach including field-transects, crowd-sourcing, and satellite-image interpretation. Most locations were identified by precipitates that impart an orangish color to sediments and/or orangish hue to water that was formerly clear; brownish streams were not considered stained. During Jun-Aug 2019-2024 we traversed by foot and packraft 3,500 km of the Brooks Range, geolocating stream rusting and acid seeps as we encountered them. Beginning with field-identified locations, we searched on satellite scenes upstream of and nearby for other orange streams, locating where their color emerged by browsing multi-band imagery from 2005-2024, including Ikonos (3m), GeoEye (1-2m), WorldView (1-3m), Sentinel-2 (10m), and Planet (3m). Image interpretation used Google Earth Pro (https://www.google.com/earth/), ESRI Wayback (https://livingatlas.arcgis.com/wayback/), MAXAR (https://evwhs.digitalglobe.com/), Planet Labs (https://www.planet.com/), and Copernicus Browser (https://browser.dataspace.copernicus.eu/). We applied band-filtering to facilitate identification of sites inside a 10,114 km2 area of Sentinel-2 scenes captured 1 and 13 Jul 2023 using normalized pixels calculated as normalized_value = (pixel_value – scene_min_value)/( scene_max_value) where 0.07