Warmer summers, melting permafrost and erosion of riverbanks are leading candidates but the exact reason that rivers in northern Alaska have turned orange is unknown, researcher tells Polar Journal. The consequences could be catastrophic for ecosystems.
The Brooks Range in northern Alaska is a remote and isolated place.
Because of this, big changes to the environment can happen without anyone really noticing. But about five years ago, one change was too big to go unnoticed: Rivers in the area that had previously been crystal-clear were beginning to turn orange.
It all started when pilots who frequented the mountains began to notice that something was awry.
“It’s not uncommon to see little bits of orange,” Josh Koch of the US Geological Survey told Polar Journal:
“But when pilots started to notice that they were suddenly landing next to a red river in places where they had been many times before, and when they then flew back home and counted six or seven more, that’s when we knew that it was worth looking into,” he said.
Created database
To get a sense of the problem Josh Koch and his colleagues at the US Geological Survey decided to create a database. This database included satellite images, observations from pilots, and data from a professor at the Alaska Pacific University who regularly took students trekking through the area.
Before long, with help from the database, the problem had been quantified: At least 75 rivers had changed their color.
“We had already been working in this region on other studies, so we knew that these streams used to be crystal-clear. You could see all the way to the bottom. But all of the sudden we would come back and one would have turned orange,” Josh Koch said.
“It was pretty shocking to see,” he said.
The first time he saw the changes with his own eyes, was when he went to the mountains to work on a separate study that followed the area’s ecosystems. In the spring, he had visited a river to set up monitoring equipment and the water had been as it used to be: clear and transparent.
But when he came back in August, things had changed significantly.
“It was bright orange. All the sensors were covered in this orange gunk. It was pretty obvious that something had happened, and because we had these sensors in, we were able to see exactly what had happened.”
“It had been a real step change. It wasn’t a gradual thing,” Josh Koch said.
Four metals in the water
Since then Josh Koch and three colleagues have been trying to figure out the reasons for the sudden color-change. By selecting a subset of the 75 rivers and studying them over a period of time, they have taken a first, important step in finding an answer.
In a study that is still under review but is soon to be published, they have been able to pinpoint the ways in which the chemical composition of the rivers is different from how they used to be, and from rivers that are still clear.
The study points to raised levels of four different metals: Iron, aluminum, manganese, and zinc. Along with this, measurements also showed heightened levels of sulfate, and Ph-levels that are sometimes below three, meaning that the waters are unusually acidic.
But why the chemistry has changed is as yet unknown.
Melting permafrost
Josh Koch and his colleagues have their theories, though. The Brooks Range, like all of the world’s polar regions, is a rapidly warming area. This warming leads to a number of new processes that could all, in theory, lead to a change in the chemical composition of the rivers. But decoupling one process from another is difficult.
An obvious suspect is the melting permafrost in the area.
“We know that permafrost thaw is occurring in this region, and it’s something that impacts the way water moves through the subsurface. Permafrost keeps the shallow water shallow and the deep water deep. Once permafrost thaws the water can flow through longer and deeper flow-paths to get to the rivers,” Josh Koch explained.
This altered flow-path means that the water sometimes gets in contact with metals that have higher concentrations of metals, including the iron that turns the rivers orange through rust.
“When we see orange streams, it is almost always associated with higher metal loads. So the orange is actually really nice since it is an easy way to see that the river has been impacted. You can see that flying over, from the ground, or even on satellite images,” he said.
But melting permafrost is not the only way the area is changing due to climate change. Warmer summers also means that seasonal frosts stay for a shorter period of time. This makes the ground thaw faster, changes the flow of meltwater, and the times when water levels are at their highest.
Moreover, like in the Yukon River Watershed to the south, rivers in the Brooks Range are experiencing an increased erosion of its banks. This, too, leads to changing river paths, and could also be an explanation for the orange rivers.
“A lot of these processes are really interrelated so separating one from the other can be difficult to do,” Josh Koch said.
Dire consequences for ecosystems
While the changing color of the water is unusual, it is not immediately obvious why it is a cause for worry. But the color is just an indicator that the chemical composition of the rivers, and, in turn, their ecosystems are changing.
Part of the goal of Josh Koch and his team’s research is to assess what the consequences are. Already, they have observed serious differences in the orange rivers.
“In one stream we saw a significant fall in the number of invertebrates after the river turned orange, and the number of fish was also quite a bit lower,” he said.
“The question we are trying to answer now is what this means. Did the fish move? Or are they dead? The broader implications for the ecosystems are still unknown to us,” he said.
His research will continue for the next several years, and, hopefully, answer some of the many unanswered questions that the orange rivers of the Brooks Range have suddenly posed.
Ole Ellekrog, Polar Journal
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