A team of scientists went on an expedition to a river in Alaska to find out why the river suddenly turned bright orange.
Introduction
Alaska is a beautiful place, full of mountains, glaciers, vast forests, and crystal-clear streams. At least, the streams used to be clear. In 2019, scientists were surprised to see that many streams in Kobuk Valley National Park in northern Alaska were suddenly turning bright orange. What could be happening to the water? Where is the color coming from? And is the color a sign that the water is becoming less healthy for fish?
Streams are not the only thing that is changing in Alaska. As the Earth is getting warmer, Alaskan glaciers are getting smaller and winter snow is arriving later. Plus, importantly, the permafrost is starting to thaw.
Permafrost is soil that is frozen solid all year round. Could changing permafrost be to blame for orange streams? Permafrost keeps the water at the surface from going deep underground. When it thaws, water can go deeper than before, carrying oxygen from the atmosphere. The bedrock in Alaska has many minerals that can react with these waters. For example, pyrite (also known as fool’s gold) is made of iron and sulfur. When pyrite is exposed to water and oxygen, it releases iron and sulfuric acid. This is a type of weathering.
We wanted to know why the color of the rivers changed. And we wanted to know whether the changes in water quality were harmful to fish and other aquatic life. We decided to venture into the park to find out!
Methods
The Salmon River watershed is extremely remote! You can’t just drive there. There are no good places to land a plane. We hiked to the headwaters twice, first in August 2022 and then in July 2023. On our third trip, in August 2023, we caught a ride with a helicopter crew that was working in the area. We carried small inflatable rafts and paddled them from the headwaters to the river’s mouth (Fig. 1). We collected water samples from the tributaries and headwater streams. We tested for 22 different elements in the water. We compared our samples to National Park Service measurements from 2013, before the streams changed color.
We used exposure thresholds published by the US Environmental Protection Agency (EPA) to see if the water was unhealthy for fish. Exposure thresholds are the highest amount of a material that can be in the water before it harms living things.
We also needed to test if pyrite weathering was causing the changes in water quality. So, we compared the amount of sulfate to the amount of calcium in the water. Sulfate can result from pyrite reacting with oxygen from the atmosphere. A greater increase in sulfate than in calcium would suggest that the sulfate is coming from pyrite in the bedrock.
Results
There is a lot more sulfate in the Salmon River and most of its tributaries now than there was in 2013 (Fig. 2).
Nine out of ten tributaries that we sampled had toxic concentrations of at least one metal.
Eight out of ten tributaries had high sulfate and sulfate to calcium ratios. This tells us that the changes we measured likely came from weathering pyrite. This leads to the release of toxic metals along with the sulfate.
Even streams that didn’t turn orange had dangerously high levels of metals.
Discussion
Our measurements show that water quality in the Salmon River watershed has gotten a lot worse. It’s very likely that the changes to the water quality are from pyrite weathering. The orange color is from iron. The sulfur released from pyrite weathering turns into sulfuric acid. Sulfuric acid is corrosive. It dissolves other rocks in the bedrock. These rocks can have toxic metals in them, like aluminum and cadmium.
We expect to see more and more permafrost thawing in Alaska and other northern regions. What caught us by surprise is the consequence of thawing permafrost. How will the change in water quality affect salmon? People living in the area depend on salmon for food and income. Salmon are travelers. They spend much of their lives in the ocean, but they travel hundreds of miles up rivers into the mountains to spawn. That’s why the health of mountain streams is so important for salmon. Recently, fishing crews have not been able to catch as many salmon. It’s hard to say for sure whether the low harvest is because of the water quality in the Salmon River. We know that exposure to iron, aluminum, and cadmium in water can harm fish, especially their gills. Although there may be other causes, we think it’s likely that salmon are being harmed by the poor water quality linked to thawing permafrost.
Conclusion
Scientists have known for a long time that Alaska is warming up. But we didn’t expect that permafrost thaw would turn rivers orange and make them less safe for fish! Other streams in Alaska have not seen these effects yet. It’s really important to visit these other sites and make measurements so we can tell when and where changes happen. Climate change is impacting the world around us in lots of ways. What signs can you see where you live? Measuring and tracking these visible changes could be very valuable data!
