What happens when the water in our rivers and lakes reaches record lows?

On April 22, chemist Andreas Fath jumped into the Danube River in Ulm, Germany. The water was just 11 °C, and Fath was wearing two full-body wet suits to protect himself from the cold. It was the official start of a journey that would take the next 8 weeks as Fath swam over 2,700 km along the river before arriving at the Black Sea.

Fath, a professor at Furtwangen University and an experienced long-distance swimmer, wasn’t just looking for a physical challenge. Every day he stuck passive-​sampling membranes to his legs to absorb persistent organic pollutants in the water. On a boat that traveled with him, scientists sampled the water to gather temperature, pH, electrical conductivity, and other measures of water chemistry and quality.

Fath hoped his swim, and the outreach activities he conducted along the way, would draw people to learn more about the waterways running through their cities and encourage better protection of an important water source in Europe. He also hoped to measure just how many pollutants were in the river’s waters and at what levels.

Fath’s swim ended on June 17, the beginning of Europe’s summer, which was to become the hottest on record. While his swim brought attention to the Danube, European heat waves soon brought more focus to Europe’s rivers. Across the continent, water levels dropped, stalling shipping, and hundreds of metric tons of fish died.

Scientists working for the World Weather Attribution initiative have found that climate change made droughts in Europe, North America, and China more likely. Examples of how this played out during this year’s hot summer are microcosms of what’s happening on a global scale.

Climate change isn’t changing just the water levels in rivers and lakes across the world but also the water’s chemistry. That has knock-on effects on ecology, industry, and human health. Complex ecosystems are under threat, river traffic is disrupted, and toxic dust is being blown aloft from dried lake beds. The combination of climate change and pollution is changing the waters that thread through our continents.

Toxic combination

Although Fath and his crew were monitoring water quality as he swam the Danube, the chemist was also a form of monitoring equipment himself. Fath says that when he was swimming in the water, he could feel the pollution at times. In 2014, Fath swam the length of the Rhine River, and his measurements found 128 pollutants in the river, including antibiotics, hormones, and pesticides. The analysis of this year’s Danube data isn’t complete, but the river is definitely more polluted than the Rhine, Fath says. “I could see it. I could feel it. I could smell it. I could taste it.”

At every town Fath stopped at along his journey, he would get out, dry off, and help run workshops about the environmental risks to the river. One risk Fath particularly cares about is plastic pollution, especially microplastics created when plastic waste in the water becomes degraded. During the workshops, he would point out how these particles can soak up organic pollutants—and how the plastics are attractive to fish, which eat them and concentrate the pollutants in their bodies. He encouraged plastic cleanups along the route.

Then he arrived in Serbia. Near Belgrade, Fath decided to suspend the swim for around 15 km. The city of 1.7 million people releases its wastewater directly into the river without treatment. Fath decided that swimming was too big a risk to his health. He mentioned this during an interview on Serbian national television, and suddenly the project became a much bigger news story. Journalists were contacting him daily to ask how polluted the river was, whether it was safe to swim there, and whether to eat fish caught in the river. The biggest response to a press event, he says, was “not because of swimming but instead because of not swimming.”

Fath ended his swim in the middle of June, and by August, temperatures on the continent had reached record highs. News headlines reported that water levels on the Danube were so low that fully laden boats couldn’t pass. But then another river hit the headlines. Hundreds of thousands of fish died in the Oder River, prompting outrage in Poland and Germany. The river travels between the two countries.

Residents and politicans originally assumed there had been a chemical spill. Polish prime minister Mateusz Morawiecki fired the heads of the Polish water management agency and the Chief Inspectorate of Environmental Protection and offered a reward of 1 million zloty ($200,000) for information about who might have dumped chemicals into the river. But the real story that emerged in the weeks that followed was a tale of the combined stresses of climate change and human impact.

“We basically created the setting for such a disaster to occur,” says Tobias Goldhammer, who runs the chemical analysis lab at the Leibniz Institute of Freshwater Ecology and Inland Fisheries. The direct cause of the fish die-off was a toxic algal bloom so big that it could be detected using remote-sensing satellites. But Goldhammer explains that human action had made the river ecosystem susceptible to the summer’s heat, creating ideal conditions for the bloom.

Lower water levels caused by a climate change–fueled lack of rainfall, along with human-made structures upstream that reduced the river’s flow, increased the concentration of pollutants, pesticides, and salts in the river. High water temperatures then helped the toxic algae flourish and changed the amount of dissolved oxygen in the river, stressing the fish even before the algae bloomed. The combination of stresses had deadly consequences. “In a healthy river, high water temperatures could be sustained for a while. Low runoff could be sustained for a while. Salt concentration could probably also be tolerated,” Goldhammer says. “But all together, it’s kind of unusual.”

Goldhammer says the disaster should be a warning for other systems as well. With climate change and ongoing pollution, he says, “this can also happen in other rivers.” Much of Goldhammer’s research is analyzing the water quality in various rivers in Germany, especially in historical mining regions in the north.

The Spree River that runs through Berlin, for example, provides the city’s drinking water. Goldhammer says that while compounds like sulfates can be anticipated to decrease in river runoff once opencast mining is shut down, the team’s analysis shows that they remain a persistent environmental problem that is exacerbated by lower water levels concentrating the pollution. Goldhammer hopes the Oder disaster has shown why looking after the health of rivers is so important. “I really wonder if this summer has sort of triggered a change in thinking in people, but I’m not really sure,” he says. “I want to be optimistic, but I’m not sure.”

The hundreds of metric tons of dead fish, snails, and shellfish now cleared from the Oder have left a hole in the ecosystem. The region may take years to recover. Goldhammer and his colleagues have published recommendations to restore the Oder and build its resilience in the face of climate change. These recommendations include getting water moving more quickly by discontinuing engineering works that slow the flow and allow silt to build up in the river, and reconnecting the river to its backwaters to help dilute pollutants and provide refuge for aquatic life.

They also argue that government agencies should improve international cooperation and monitoring to make water management more transparent and that monitoring stations should make comparable data freely available. In Europe, where rivers run through multiple counties and jurisdictions, a patchwork of over 100,000 agencies is in charge of the continent’s water quality and reducing chemical pollution.

Dustup

In the Oder and elsewhere, climate-​driven drought affects water quality. But exposing the soil below the water has chemical consequences too.

In Utah, for instance, the Great Salt Lake is disappearing. According to the US Geological Survey (USGS), the lake hit a record low this July. And with every drop in water level, more of the lake bed is exposed. USGS data show that the lake has lost nearly half its surface area.

“I completed an entire survey of the lake bed in 2018. And since that time period, the lake has dropped another 5 ft, which is just mind-bogglingly fast,” says Kevin Perry, an atmospheric scientist at the University of Utah. “It’s exposed another 100 square miles of lake bed just in the last 5 years. And so things are changing very rapidly.”

The lake bed, he says, is composed of sand, silt, and clay. The silt and clay particles are small enough that they can become airborne. “The smaller particles can stay in the atmosphere for several weeks,” Perry says.

Particulates are harmful by themselves—​​for example, they cause respiratory problems—but Perry has found an additional, longer-term danger. The lake bed dust contains arsenic that is spread uniformly across the surface. “If the arsenic were dominated by industrial sources, you would have an uneven distribution across the lake,” Perry says. “My hypothesis at this point is that the arsenic is mostly naturally occurring, being deposited slowly over thousands of years.” Long-term, cumulative arsenic exposure for the area’s 1.3 million residents could lead to health issues such as cancers and developmental problems, he says.

The lake’s water levels are dropping for multiple reasons. First, much of the water that feeds into the lake from snowpack in the adjacent Wasatch Range is diverted for homes, businesses, and farms. Climate change then decreases water levels faster by increasing evaporation and reducing precipitation. “We’re in this place where the water coming to the system is not enough to keep the system going right now because of the pressures of climate change,” says Bonnie Baxter, director of the Great Salt Lake Institute and a professor of biology at Westminster College. “Diversions were first,” she says, “and they set us up to fail as we meet the pressures of climate change.”

With less water, the Great Salt Lake is getting saltier. Normally, Baxter says, the main part of the lake is between 12 and 15% salinity. This summer, readings have edged over 18%. This is driving a cascade of ecological changes that affect different creatures that call the lake home. The brine shrimp industry on the lake, for example, helps feed farmed fish around the world. It’s a $67 million-a-year industry that is endangered by decreasing water levels and increasing salinity, Baxter says. While the shrimp can survive now, Baxter expects that the shrimp will struggle if the lake’s salinity level stays the same or worsens. For brine flies, however, the lake has already reached a tipping point because of the falling water levels.

Baxter studies life forms near the bottom of the lake’s ecosystem: the algae that live in the lake and that also need the salinity’s level to remain below a certain threshold. In addition to being food for brine shrimp and flies, the algae combine to build rocklike structures called microbialites that are key to the flies’ life cycle. These microbialites form a reef-like system that hosts mats of photosynthesizing blue-green algae on the top. These mats provide a home for the pupal stage of brine flies—which are food for some 10 million migratory birds that rely on the Great Salt Lake. But the mats are now either drying out or dying from the high salinity.

“The mats are gone,” Baxter says, “and it’s devastating.”

“There are usually millions of brine flies on the shore, and the birds are out there eating,” Perry says. This year, he says, the birds are in the water trying to eat the brine shrimp. “And that is to me just a very stark example that the end is near for the ecosystem in terms of the food chain.”

Yet if you ask Perry or Baxter about their long-term feelings about the Great Salt Lake, both are perhaps more optimistic than they were a few years ago, in part because they are now seeing others take their concerns seriously. And legislation at the state and local level is beginning to try to solve the issues of maintaining the water levels in the Great Salt Lake. “This is a multifaceted, multilayered, multibillion-dollar problem. That’s not going to be solved overnight,” Perry says. “The question is, you know, ‘How quickly can we make the cultural changes and the legal changes and all that sort of stuff to try and reverse this trend?’ ” He admits that the lake is closer to an ecological tipping point than it was 3 years ago, but he is still more optimistic about its future than he was before.

Water is infrastructure

The Yangtze is the world’s third-largest river. It provides drinking water to more than 400 million people in China, and its flow drives hydroelectric power stations along its length, including the massive Three Gorges Dam. But in August, high temperatures and a lack of rainfall simultaneously led to an increased demand for energy generation and low water levels. The power-generation system came under increasing strain.

In Sichuan Province, a major manufacturing hub where hydropower provides around 80% of energy supplies, authorities had to take drastic steps. They chose to suspend or limit the power delivered to thousands of factories and homes to ensure that critical services could be maintained. At the same time, supplies of drinking water became limited. As crops failed and forest fires broke out, provinces around the river turned to cloud seeding to try to trigger rainfall.

While hydropower will remain important as China’s population continues to grow, the government may need to adapt to deal with climate change, says Faith Chan of the School of Geographical Sciences at the University of Nottingham Ningbo China. Grid improvements and additional hydropower plants closer to sources of meltwater could be part of the solution, but the government also needs to find some backup plans. These droughts will happen again, he says: 2022 “raised the alarm bell to the Chinese government” for both water and energy.

The Three Gorges Dam, of course, isn’t designed just to generate power but also to manage water levels downstream on the Yangtze. Chan says the Chinese government is committed to building new water infrastructure projects such as water storage facilities and “sponge cities,” new urban developments designed to retain rainwater where it lands. These can both capture water and help with flood management. Being able to manage droughts and floods is important. While southern China had no rain for weeks on end, heavy rain in the north led to flooding.

Managing water for human use requires infrastructure, but water is also infrastructure itself, transporting heavy goods and people on the waterways that cross continents. Along the Yangtze, low water levels disrupted cargo transport. Parts of the river became impassable to freight transport, and many other ships had to travel with only partial loads.

The same was true in Europe this summer. Inland waterways are used to transport a large amount of coal, crude oil, and natural gas within Germany. That mode of transportation has become more important since the war in Ukraine led to reduced Russian gas imports. The low waters and increased traffic have created snarls in the system and increased the price of transporting goods across the rivers of Europe.

Along the Rhine, barges had to lighten their loads, which disrupted the delivery of raw materials to German and Swiss industry. The economic impact can’t yet be fully calculated, but after 2018, when a drought hit the Rhine, chemical giant BASF suffered a €250 million ($242 million) hit. The Kiel Institute for the World Economy calculates that in a month with 30 days of low water, Germany’s industrial output is reduced by 1%.

Back home in Germany’s Black Forest, Fath is happy to be taking some well-earned rest. But his team members are busy analyzing the water samples they took. They are also looking for collaborators that have the sensitive equipment and compound libraries necessary to analyze the samplers he wore on his wet suit.

Fath has now swum the lengths of the Rhine, Tennessee, and Danube Rivers. On each journey he has monitored the river water but also tried to show people why it matters. He uses the swimming as a tool to make people aware of their impact and to bring them down to rivers to see the water as he sees it, he says. People protect what they love with everything they have, he says. And “you cannot love what you do not know.”