Northern ecosystems are changing rapidly as permafrost thaws, glaciers retreat, and precipitation patterns change. As a result, the rhythms of river flow are shifting, too, according to a new analysis.
The research, published in Science, shows that climate change is weakening river seasonality in northern rivers, with consequences for human and ecological communities alike.
“If temperatures keep rising, this trend of river flow seasonality will not change, which has implications for the future,” said Hong Wang, the first author of the new study and a hydrologist at the University of Leeds in the United Kingdom.
Wang and her colleagues used flow data from 10,120 gauging stations around the world, most of which had collected data for at least 35 years. They found that the seasonality, or difference in flow between seasons, has weakened in many rivers above 50°N.
In northern North America, about 40% of gauge stations showed decreasing seasonality. In southern Siberia, the number was 32%, whereas in northern Europe, about 19% of gauge stations showed decreasing seasonality.
The researchers reconstructed the flow of rivers on the basis of data from the gauge stations. By excluding direct human interventions such as dams and water extraction, they showed that the changes in seasonality were primarily due to human-caused climate change.
Scientists know that climate change is raising temperatures globally. But for other Earth system variables, such as monsoon seasonality, water runoff dynamics, and river flow, it may be harder to detect human influence, Wang said. The new research “solved this problem for river flow seasonality,” she said.
Increased temperatures in the Arctic have led to earlier snowmelt, more rainfall, and less snowfall. That has raised river flow volumes in the winter, when streams had typically been slow because water had been locked up in mountain snowpack, and diminished the contributions of melting snow in the typically high flow spring months, Wang said. Earlier plant growth, spurred by warmer temperatures, also reduces runoff in the spring, the authors write.
The results confirm what other regional-scale studies have shown regarding changes in river seasonality, said LeRoy Poff, a hydroecologist at Colorado State University who was not involved in the new research.
In addition to more rainfall and earlier snowmelt, thawing permafrost may also contribute to the changes by allowing water to flow more freely through the ground, said Josh Koch, a hydrologist at the U.S. Geological Survey Alaska Science Center who was not involved in the research. Koch said the new study is consistent with the changes he’s seeing in rivers in Alaska, too: In the spring and fall, Alaskan rivers have had increased flow.
“It’s hard to predict how species will adapt.”
Koch said he wonders how the results might differ in a similar analysis of smaller rivers. Stream gauges, he said, tend to be on larger, more important rivers, so it’s harder to get long-term data on smaller streams. Those waterways are also experiencing changes, he said. “These results show [only] one side of that story.”
Gauge stations south of 50°N showed mixed results. About 25% of gauge stations in Brazil showed increasing seasonality, for example. That could be due to the increase in precipitation the country has seen during the rainy season, Poff said.
Dam Decisions
River seasonality is crucial for communities and ecosystems alike, as both are accustomed to the specific seasonal flow patterns of nearby rivers. “It’s hard to predict how species will adapt,” Koch said. For some species, increased flow could be beneficial, whereas others might suffer as a result, he said.
“We need to be smart about how we move forward.”
Awareness of changes in seasonality is important to optimize human use of water resources, Wang said. Knowing where earlier-than-normal river surges are happening can help water managers capture and store that water in time. In the Arctic, too, people rely on frozen rivers for transportation. As seasonality changes, those rivers can become unstable and unsafe, Koch said.
But as humans adapt to these changes, they should also consider the needs of wildlife, Poff said. One response to changing seasonality could be more dams and reservoirs, but dams can harm wildlife. Finding solutions that both prioritize healthy ecosystems and save water resources for people is important, he said. “We need to be smart about how we move forward.”
—Grace van Deelen (@GVD__), Staff Writer
