Wild, scenic, and toxic: Recent degradation of an iconic Arctic watershed with permafrost thaw

Abstract

The streams of Alaska's Brooks Range lie within a vast (~14M ha) tract of protected wilderness and have long supported both resident and anadromous fish. However, dozens of historically clear streams have recently turned orange and turbid. Thawing permafrost is thought to have exposed sulfide minerals to weathering, delivering iron and other potentially toxic metals to aquatic ecosystems. Here, we report stream water metal concentrations throughout the federally designated Wild and Scenic Salmon River watershed and compare them with United States Environmental Protection Agency (EPA) chronic (4-d) exposure thresholds for toxicity to aquatic life. The main stem of the Salmon had elevated SO₄^2- concentrations and elevated SO₄^2-: Ca relative to a predisturbance baseline for most of its length, consistent with increased sulfide mineral weathering. Most of the tributaries also had elevated SO₄^2- concentrations and elevated SO₄^2-: Ca, especially those in the upper watershed. The Salmon River mainstem consistently exceeded EPA chronic exposure thresholds for total recoverable iron, total recoverable aluminum, and dissolved cadmium from its first major tributary to its mouth. Nine of ten major tributaries that we sampled exceeded EPA thresholds for at least one metal on at least one of three sampling dates. Our findings indicate that habitat quality for resident and anadromous fish has been severely degraded in the Salmon River watershed. Loss of important spawning habitat in the Salmon and many other streams in the region might help explain a recent crash in chum salmon returns, which local communities depend upon for commercial and subsistence harvest.

Keywords: acid rock drainage; aquatic toxicity; salmon.

Fig. 1.

The Salmon River mainstem (67.40, −159.74) on September 6, 2020, when discharge was likely well below the long-term median (SI Appendix, Table S1). The Kobuk River is out of view at the Top of the frame. Photo credit: Ray Koleser (A). Shaded relief map of the Salmon River watershed in Kobuk Valley National Park, northwest Alaska (B). Photographs of the sampling locations at the mouths of each tributary are available in SI Appendix, Figs. S3–S12.

Fig. 2.

Visual and chemical characteristics of the Salmon River mainstem before and after degradation in 2019. Photographs were taken just upstream of the confluence of “Noname Creek” with the Salmon River in late June of 2016 (A) and in mid-July of 2023 (B) (67.5215, −159.7671, photo credits: Roman Dial). Water chemistry samples were collected, filtered, and acidified just above the confluence of the Salmon with the Kobuk River in mid-August of 2013 and 2023 (C). The 2013 samples were collected as part of routine National Park Service water quality monitoring (3).

Fig. 3.

Metal concentrations, along with SO₄²⁻ concentration and the molar ratio of SO₄²⁻ to Ca, along the length of the Salmon River main stem, when sampled in August of 2022 and in July and August of 2023. Red symbols highlight metal concentrations that exceed the chronic (4-d) exposure threshold set by the United States Environmental Protection Agency. The dotted vertical lines indicate the mouths of the major tributaries (Tables 1 and 2), while the dashed horizontal line indicates the SO₄²⁻ concentration and SO₄²⁻: Ca in a sample collected by the National Park Service at the mouth of the Salmon River on August 21, 2013. The confluence of the Salmon with the Kobuk River is slightly downstream of the 120 km sampling location.