Climate change is altering seasonal river levels

Alpine river

The water levels of the Alpine rivers, here the Lech, fluctuate less over the course of the year than before. © PK Photos/ iStock

Depending on the season, rivers carry different amounts of water. These seasonal fluctuations are important for the river ecosystem and human water supplies. A study now shows that seasonal changes in water levels in many rivers have weakened since 1965 - probably due to climate change. Rivers in North America, Northern Europe and Russia are particularly affected. In the Alps, too, the rivers carry less water in spring than before because less snow accumulates in winter.

When the temperatures get warmer in spring and the plants sprout again, the snow at higher altitudes also begins to melt. The rivers swell due to the meltwater and provide the reawakening nature with plenty of moisture. The fish also sense that the warm season is approaching. The rising river levels are a signal for them to start migrating to their spawning areas. “River ecosystems have adapted to natural flow fluctuations across seasons,” explains a team led by Hong Wang from the Southern University of Science and Technology in Shenzhen, China. “But there is increasing evidence that climate change is already changing how much water rivers carry seasonally.”

Changed levels

So far, this evidence has been limited to local observations. To get a global picture, Wang and her team evaluated data from more than 10,000 measuring stations worldwide, covering the period from 1965 to 2014. “Our research shows that rising air temperatures are fundamentally changing natural flow patterns,” reports Wang. The team factored out human interventions such as reservoirs and water withdrawals. “The observed weakening of seasonal fluctuations is a direct consequence of human-caused emissions,” says Wang. “This suggests that the seasonality of river flows will continue to deteriorate sustainably and significantly as air temperatures increase.”

Especially in rivers at latitudes above 50 degrees north latitude, i.e. in North America, Northern Europe and Russia, it can be seen that the water levels fluctuate significantly less seasonally than before. In North America this affected 40 percent of the measuring stations examined and in Europe 19 percent. In the Alps, an important reason for this is the decreasing amount of snow in winter. Since a lot of precipitation comes down in the form of rain instead of snow even in the cold season, the rivers carry more water in winter than before. This water is missing in spring, when the sprouting nature could use it better than in winter.

Problematic for river ecosystems

This change in water levels has ecological consequences. “Changes in the amount of water are important clues for the species that live in water,” explains co-author Megan Klaar from the University of Leeds. “For example, many fish use a certain increase in water volume as a signal to move to their breeding grounds upstream or to the sea. If they don't have these signals, they can't spawn.” The weakening of the peaks and troughs of runoff water can therefore impact the ecosystems in and around the river in a variety of ways.

The team recorded the opposite trend in South America. In southeastern Brazil, every fourth measuring station showed that seasonal fluctuations have increased significantly over the decades. Again, climate change is likely to be the explanation, as rising temperatures have led to changes in seasonal rainfall. In order to protect freshwater ecosystems and continue to guarantee human water supplies, the researchers believe it is important to find suitable adaptation strategies. The study also underlines the urgency of limiting climate change as much as possible.

“Many fears relate to the effects of climate change in the future, but our research shows that there are already serious effects today,” says Klaar’s colleague Joseph Holden. “We should start thinking about mitigation strategies and adaptation plans to mitigate future weakening of seasonal level fluctuations, particularly in areas such as western Russia, Scandinavia and Canada.”

Source: Hong Wang (Southern University of Science and Technology, Shenzhen, China) et al., Science, doi: 10.1126/science.adi9501

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