One of the richest biodiversity hotspots in the world is Peru’s Madre de Dios, a region of the Amazon located at the base of the Andes Mountains. When biogeochemist Jacqueline Gerson first traveled there in 2017, she found herself on a boat heading downstream through the forest. As she passed the banks of the river, she noticed a change of scenery.

At first, “it was beautiful, old-growth forest, lots of birds, lots of different wildlife,” says Gerson, a Ph.D. student at Duke University at the time. “Then as I continued downstream … you see these rocks first,” she adds. “As you go along, you see clump after clump after clump and then you start to see some deforestation.”

She witnessed signs of small-scale and artisanal gold mining. Unlike large-scale industrial operations with fleets of dump trucks and excavators, workers here use basic tools or their hands to extract ore. These informal gold mining efforts are so prolific in Madre de Dios that they support at least half of the region’s economy.

But there is a price for this gain. Small-scale miners mix mercury into river bank sediments containing gold particles. This produces a gold-mercury amalgam that can be easily separated from the trash and then burned to isolate the gold. But this combustion also emits mercury fumes into the open air.

For Gerson, now at Cornell University, shedding light on how toxic pollutants flow through the environment is a calling. It studies how human activities contribute to these pollutants and change their pathways.

Globally, humans release more than 2,000 metric tons of mercury into the air each year from coal-burning plants, waste incineration facilities, cement production sites, mining operations, and other sources. Artisanal and small-scale gold mining generates more than 35 percent of these mercury emissions, making it the main anthropogenic source.

In the environment, bacteria convert the element into the more toxic methylmercury, which bioaccumulates more easily in wildlife and humans.SN: 19.6.14). Exposure to large amounts of mercury can wreak havoc on the central nervous system, digestive tract and kidneys, leading to seizures, blindness, sleep loss, memory loss, headaches, muscle weakness or even death.

Much of Gerson’s work focuses on mercury, but she has studied the movements of other dangerous pollutants, such as selenium released from coal mining and sulfur released from agriculture. In most cases, Gerson already has a good idea of ​​where the substances are coming from when she begins her investigation. It’s the rest of the story – where they go, where they end up – that she’s looking for.

Before we can better manage and reduce the risks these pollutants pose to humans, she says, “we first need to understand their fate.”

An Amazon hotspot

Even before her first trip to Madre de Dios, Gerson was aware that signs of mercury exposure had been reported in communities upriver from mining areas. Perhaps people were eating mercury-laden fish that had swum upstream, but Gerson wondered if there might be other routes of exposure. So she and colleagues returned to the region collecting samples during the summer of 2018 and the following winter.

Surprisingly, mercury levels in the air correlate with proximity to mines. But the water shed by leaves in the forest canopy, known as runoff, offered a more complicated picture. The denser the canopy, the more concentrated the mercury is in the stream, with the highest levels occurring in a conservation area called the Los Amigos Biological Concession, Gerson and colleagues reported in 2022 in Nature Communications. Falling mercury levels in Los Amigos are “the highest burdens of any place on the globe,” says Gerson. “That was really surprising [to find] in this area … which we think of as one of the most remote areas in the world.”

What set Los Amigos apart was its pristine, old-growth forest. The large leaves in the mature forest canopy act as mercury collectors, providing ample surfaces for airborne mercury to collect, accumulate and later be washed to the ground by rain, Gerson says. “If you have a mining community surrounded by old-growth forests, you’re going to see really high mercury loads here.”

And it wasn’t just the leaves. Mining also contaminated wildlife. Mercury levels in the feathers of three species of songbirds with different diets were on average two to three times higher at Los Amigos than in another old-growth forest located away from the mines. Runoff and shed leaves send mercury to the soil, where the pollutant is methylated by bacteria and consumed by plants and animals, Gerson explains.

“It’s important to get this information out,” says biogeochemist Mae Gustin of the University of Nevada, Reno, who was not involved in the research. The impact is more widespread than people realize, she says. “Everyone [eco]the system is getting contaminated.”

A spark in Senegal

Gerson’s fascination with mercury did not begin in the Amazon, but rather dates back to a trip to central Senegal. After receiving her undergraduate degree at Colgate University in Hamilton, NY, she went to Senegal as a Peace Corps volunteer.

There, Gerson met another Peace Corps volunteer who had been living in the Kédougou region, a full day’s drive away in southeastern Senegal. “I remember her talking about the need to change communities because [the] The Peace Corps was concerned about her personal exposure to mercury from mining operations there, Gerson says. “It really piqued my interest.”

With Mercury on her mind, Gerson returned to New York and began a master’s program at Syracuse University in 2014. Her master’s advisor, Charles Driscoll, gave her “tremendous flexibility to choose what I wanted to do for my thesis,” Gerson recalls. Over the next several years, she would publish some of her earliest work on how mercury and methylmercury patterns evolved over a decade in a remote area of ​​the Adirondack Mountains, launching a career in search of quicksilver. .

Research eventually drew Gerson back to Senegal. For a study reported in 2023 in Papers of purest productionshe and colleagues worked with local community members—more than 80 percent of whom identified as miners—to spread awareness of the dangers of mercury. The team also distributed locally made devices called retorts that mitigate miners’ exposure to mercury fumes. Surveys showed that the work helped: The share of respondents who reported that they believed mercury was dangerous increased from 83 to 94 percent, and the percentage of individuals who used replicas at least sometimes increased from 3 to 64 percent.

In the United States, polluter movements aren’t the only avenues Gerson is drawing attention to. It is also dedicated to enlightening science entrants. As a Ph.D. student, she co-founded GALS, a free summer science program that organizes overnight camps and backpacking trips for high school girls and gender non-conforming students. And she wrote a 2023 article on Bulletin of the Ecological Society of America entitled “Demystifying the Graduate School Application Process”.

“There’s a lot of hidden agendas going into high school and going into the sciences in general,” Gerson says, that aren’t easily accessible to those who don’t already know the process. “I’m really passionate about trying to make STEM much more inclusive and helping people find their way as well.”

Dust pollution is known to contribute to asthma and heart and lung disease. But dust blowing from the Great Salt Lake in Utah can cause an additional unwanted shock.

Metals in dust and sediments around the Great Salt Lake are more reactive than dust from nearby lake beds, researchers report in November. Atmospheric Environment. When inhaled, the dust has the potential to cause inflammation, although the actual impacts on humans in the area will require further study.

The Great Salt Lake is steadily shrinking as drought, climate change and consumption drain water faster than it can be replenished, leaving over 1,900 square kilometers of the lake bed exposed (SN: 17.4.23). As the lake dries up, it leaves behind dust laden with metals, minerals and sediments that were carried into the lake from upstream.

To better understand the composition of the dust, chemical engineer Kerry Kelly and colleagues aerosolized samples collected from around the lake. They then filtered out any dust particles larger than 10 micrometers, leaving only dust particles small enough to inhale.

Analysis of respirable particles revealed several metals — including manganese, copper, iron and lead — in higher concentrations than dust from other nearby playas. Lithium and arsenic were also present at levels exceeding US Environmental Protection Agency regional control levels, a benchmark for further risk assessment.

The team also found that the oxidative potential of Great Salt Lake dust, which indicates how likely the dust is to generate reactive oxygen species, is generally higher than that of dust from other nearby lakes. Reactive oxygen species are unstable oxygen-containing molecules that interact with—and sometimes damage—molecules in living cells.

“Our body has all kinds of antioxidants,” says Kelly, of the University of Utah in Salt Lake City. These compounds allow us to breathe and handle reactive oxygen species—up to a point. “However, if we get too many of these reactive particles or reactive species entering our lungs, it can cause an imbalance. Then that can lead to inflammation, and then inflammation leads to a number of negative health effects.”

But experts advise not to draw quick conclusions. “I think it’s good to look at environmental components and look at their potential to have this or that effect,” says David Lo, a biomedical scientist at the University of California, Riverside. “But then you want to ask on the same side, is there any evidence that people are actually being harmed?” Linking exposure to highly oxidative dust to specific public health outcomes would require more data on the extent of exposure and studies linking oxidative potential to specific health concerns, he says.

Kelly agrees. “I don’t mean, ‘the sky is falling, we’re all going to die.'” Rather, she says, the study “shows that dust from the Salt Lake is potentially a significant health concern, so we need to do more work.” Utah has funding for equipment to measure the rate at which dust from the Great Salt Lake blows into nearby cities, she says, but it hasn’t been deployed.

“We also need to get more water into the Great Salt Lake,” Kelly says, “because that’s really the solution.”

“I remember standing there just looking at it,” says Zeng, a climate scientist at the University of Maryland. He recalls thinking, “Wow, do we really need to continue our experiment? The evidence is already here, and better than we could do.”

That trunk was once part of an eastern red cedar that pulled carbon dioxide from the air and turned it into wood about 3,775 years ago, researchers report Sept. 24. Science. Buried under two meters of clay soil for millennia, the log retained at least 95 percent of that carbon, the study estimates.

“Scientists and entrepreneurs have long considered wood burial as a climate solution. This new work shows that it is possible,” says Daniel Sanchez, an environmental scientist at the University of California, Berkeley, who was not involved in the study. “High-sustainability, low-cost climate solutions like these hold tremendous promise for combating climate change.”

New solutions are much needed. Reducing greenhouse gas emissions is not enough to meet global climate targets, according to the Intergovernmental Panel on Climate Change (SN: 1/9/16). In addition, about 10 gigatons of atmospheric carbon must be captured and stored each year by 2060. Plants store about 220 gigatons of carbon dioxide each year just by growing, but most of this is released back into the atmosphere through decomposition. Preventing just a portion of that decomposition by burying the wood can help achieve this goal. But that potential relies on finding conditions that would prevent air, water and microbes from breaking down that carbon long enough to make a difference.

The ancient record gives researchers a clue. Zeng suspects that the largely impermeable clay soil covering the region helped prevent oxygen from reaching the trunk, even at relatively shallow depths. “This type of land is relatively widespread. You just have to dig a hole a few meters down, bury the wood and it can be stored,” he says.

Burying wood can cost $30 to $100 per ton of CO₂researchers estimate. That simplicity and cost, Zeng says, makes wood vaults more practical than developing direct air capture technology, which costs $100 to $300 per ton of CO2. If the conditions that preserved the Canadian log can be replicated—which is still unclear—buried biomass from discarded wood and sustainable harvesting could sequester up to 10 gigatons of carbon per year, the researchers estimate.

Despite finding the ancient trunk, Zeng’s team carried out their planned experiment and is now closing the analysis, in part to figure out best practices. But the trunk itself exemplifies the arched promise of wood, he says. “Now we have the evidence to say ‘yes, it’s ready to be implemented’.”

In just 60 hours, the NHC predicted that PTC9 would intensify at a record pace, going from winds of less than 35 knots (about 65 kilometers per hour) to hurricane-force winds of at least 100 knots (185 kilometers per hour).

It was the fastest predicted transition from concern to major hurricane in NHC history.

And these predictions were right. Driven by the deep, super-heated waters of the Gulf of Mexico and unhindered by any shearing winds that could stutter the storm’s growth, Helene began to surge.

Here are three things to keep in mind as Helene continues to barrel into the southeastern United States.

Rapid intensification is becoming the new normal for hurricanes.

The NHC defines rapid intensification as when a storm’s maximum sustained winds jump by at least 56 km/h (35 mph) in less than a day (SN: 9/13/23).

Against the background of persistent temperatures, record tropical water, numerous storms in recent years have met and even exceeded this definition (SN: 15.6.23). In 2023, for example, Atlantic hurricanes Idalia and Lee increased their intensity by about 58 km/h within 24 hours.

Helene is not just a text case of such rapid intensification – she is the star student.

Scientists have been gnashing their teeth, predicting just such an event, given the super-hot waters of 2024. Helena’s fury was fueled by record hot temperatures in the Gulf of Mexico. Sea surface temperatures in the Gulf are high, in some places 2 degrees Celsius higher than the September average of around 29°C. But even more importantly, the excess heat of the Gulf’s ocean isn’t just deep: the waters stay very warm. deep in the water column, increasing the overall heat content of the ocean and providing even more fuel for a rotating storm (SN: 7/2/24).

Another favorable factor for hurricane formation in the Gulf was the absence of wind shear, changes in wind direction or higher speed in the atmosphere. Faster upper-atmosphere winds can eat into a tightly rotating cyclone, removing the heat and moisture they need from their centers.

Smaller cyclones may be even more prone to rapid intensification.

Even as Helene took center stage, forecasters were reeling from the sudden intensification of another tropical cyclone, Hurricane John, which bore down on Mexico’s southern Pacific coast on September 23.

That was two full days earlier than researchers had predicted.

Also fueled by warm ocean waters, the storm became a Category 3 storm just hours after being classified as a tropical storm. This dramatic, sudden increase in power and speed caught both scientists and officials off guard as they scrambled to sound the alarm before it hit the ground.

As Helena and John show, both large and small storms can be subject to rapid intensification. But recent research suggests there may be a reason why forecasters were caught off guard by John’s sudden rise. And that may have something to do with the size of the storm.

A 2014 analysis of tropical cyclone size and intensification from 1990 to 2010 suggested that smaller, compact storms like John — only a fraction of Helena’s width — may be particularly prone to intensification so suddenly that they can confound predictions.

In particular, how large the inner core—the eye of the hurricane—is at first may come into play. This may be because storms with larger inner cores may be more resistant to structural changes by external forces. Such forces may include heat transfer from ocean waters.

New projections of inland impacts show how intensification is not just a coastal problem.

In August, the NHC debuted an experimental hurricane forecast cone that includes not only a storm’s projected path toward land, but also regions where its powerful winds can be felt far inland. The purpose of this new type of projection, the center said in February, is to raise public awareness of storm hazards that may exist far from the eye of the storm, or long after landfall (SN: 29.2.24).

That’s especially important for Hurricane Helene, which was forecast to bring catastrophic storm surges of up to six feet as it made landfall in Florida’s Big Bend region — one of the largest surge forecasts the center has ever made. This is equivalent to a two-story high wall of water jutting out onto the shore.

Helena also had a large field of tropical storm-force winds that could extend as much as 500 kilometers from the storm’s center — essentially covering the entire state. It is estimated to end up as one of the five largest Gulf of Mexico storms on record in terms of wind field size.

The experimental forecast suggested that Helene’s dangers would extend across the southeastern United States. Hours after landfall on September 27, Helene was downgraded to a tropical storm as it continued to barrel northward, bringing strong winds and power outages, as well as torrential rain and flash flooding across Georgia. South Carolina and North Carolina.

Powerful eruptions pushed the sea ashore, generating record floods that inundated coastal communities in meters of seawater. Near Keaton Beach, Fla., the storm was estimated to have reached at least 4.5 meters (15 feet) in height.

Preliminary Post-Landfall Modeling of Hurricane Storm Surge #Helen shows areas within Florida’s Big Bend region near Keaton Beach, Steinhatchee and Horseshoe Beach where water levels reached more than 15 ft above ground level.

— NHC Storm Surge (@NHC_Surge) September 27, 2024

And that was just the beginning. After making landfall, Helene moved north through Georgia, giving Atlanta a record 28 centimeters (11 in) of rain in 48 hours, beating the previous record of 24 centimeters (9.6 in) set in 1886. While Helene moved into the Appalachian Mountains, its rainfall causing widespread flooding and rapid landslides called debris flows, deadly and unstoppable slush of water, soil and rock that can creep downhill for miles.

Mountainous parts of western North Carolina were hit particularly hard, with some places like Jeter Mountain and Busick reporting more than 76 centimeters (30 inches) of rain. Washed-out roads and downed power lines caused outages that isolated the city of Asheville, home to nearly 100,000 residents.

Since Oct. 1, the death toll from Hurricane Helene has surpassed 130 people in six states, and that number could rise over the coming days as hundreds are still reported missing. Additionally, the associated economic damages are estimated to be somewhere around $150 billion.

To find out how Helena was able to leave such a devastating trail of damage far in the mountains, Scientific news spoke to four experts. Charles Konrad is a climatologist at the University of North Carolina at Chapel Hill. Coastal oceanographer Rick Luettich and aquatic ecologist Hans Paerl are both with UNC, based in Morehead City. And geologist Brad Johnson of Davidson College in North Carolina studies landslides, erosion, and landscape evolution in the southeastern United States. Their responses have been edited for clarity and brevity.

SN: Why was this hurricane’s storm surge so devastating?

Luettich: The thing about Helene was that it was really big, and that meant it could push a lot of water along with it. [Tropical storm force winds reached over 480 kilometers (300 miles) from its center.] Our models predicted that almost all of the barrier islands, from Estero Island south of Fort Myers to Tampa Bay, would go under water. In our present understanding, this was quite accurate. The second thing was that as Helena moved over the Gulf [of Mexico]and especially when it began to fall to land, it was on very warm water. This helped him quickly develop a very strong core.

The West Florida shelf is also quite wide and shallow, making it suitable for hurricanes. Deep water is hard to catch. And of course, Florida’s Big Bend is C-shaped, and as you push water up into that area, water tends to pool in the hook.

SN: Are there ongoing effects or risks to coastal areas from this storm?

Luettich: Our barrier islands, which usually consist of sand dunes, are the main defense against flooding. When a storm like Helene comes through and damages or covers them, then a later, smaller storm can flood areas that would otherwise be protected.

There is no doubt that Helene has made the west coast of Florida more susceptible to flooding from smaller events, should they occur over the next month. There’s some kind of storm brewing in the Gulf right now. We’re not quite sure what it will look like. But something is likely to happen there.

Paerl: All that rain that has fallen becomes runoff and carries all kinds of pollutants. You can just imagine a gas station flooding and all the pollutants coming out of it. Or a sewage treatment plant. There are pesticides, herbicides, PFAS, a whole soup of chemicals in those flood waters.

And then there are the nutrients that leach from manure into farmland. When a storm comes, it can spread these nutrients into our estuary and coastal areas and can lead to algal blooms. These blooms can sometimes produce toxins that can be harmful to fish, invertebrates, pets, and humans, and they can last anywhere from days to months.

SN: Why did Helena hit the Appalachian Mountains so hard?

Konrad: In the mountains, there was what meteorologists call an antecedent event, which happened just before the hurricane came in. I think the Asheville airport got six or seven inches of rain before Helena’s rainfall got there.

You can think of it as the beginning of rain. There was already considerable flooding. The soils were saturated and the streams were already in minor to moderate flood stage.

To make matters worse, the winds were blowing from the southeast and east, and that air must rise over a large, steep landform in the mountains called the Blue Ridge Escarpment. When air rises to higher altitudes, it encounters lower pressure, causing it to expand, cool, and release moisture as precipitation. As Helene began to push air over the escarpment, it caused massive increases in rainfall in the area.

Johnson: It is not surprising that there are landslides and debris in these situations.

The threshold set for landslides in North Carolina is five inches of rain. If you look at any set of landslides that have occurred, it’s basically always in an event where at least that much rain falls.

When the storm began to hit, every rain gauge I had access to in the mountains was over eight inches of rain, some were 10 inches, and the hurricane was still 100 miles out in the Gulf. I just thought, I can’t see a way out of this that doesn’t have dozens and hundreds of landslides.

SN: Is there any ongoing danger in the mountains from this hurricane?

Johnson: The maximum risk of flooding, landslides and debris flows is during the rainfall event. In my experience, once that precipitation event is over, you’re pretty much in the clear. But there are other dangers moving around, with people going out in the rain with downed power lines, and inevitably there is flooding at the bottom of the valley.

Konrad: Hopefully it will dry out, but the grounds are really wet. I’m sure there are many places where rainfall has set the stage for landslides and debris flows, so it wouldn’t take that much rainfall to cause one now. Rock slide too.

Many people in these communities will not be able to access medication or health care because of road damage, and so I think there will be a lot of what we call indirect deaths. It is a public health disaster that is still unfolding.

Some tadpoles do not poop in the first weeks of their lives. At least, this is the case for Eiffinger’s tree frogs (Kurixalus eiffingeri), scientists report on September 22 at Ecology.

Eiffinger tree frogs are small frogs that live in Taiwan and two Japanese islands: Ishigaki and Iriomote. Tree-dwelling amphibians lay their eggs in small pools, which are often found in plant stems, tree hollows, and bamboo trunks.

After the chicks hatch, they spend their early life in these ponds. However, in pools as small as these, there isn’t much water to dilute the ammonia – a toxic chemical that animals release when they urinate or defecate.

Bun Ito and Yasukazu Okada, biologists from Nagoya University in Japan, have now discovered chickens’ secret hygiene strategy – self-induced constipation. Tadpoles store their excrement in a gut pouch until they begin to metamorphose into full-fledged frogs.

Ito and Okada raised tadpoles from four different species of frogs in makeshift nurseries. After the experiment began, they moved the chickens to smaller cribs, plastic boxes with little more than a tablespoon of water. The team measured and compared how much ammonia each species emitted. They also measured the amount of ammonia stored in the guts of each species.

Eiffinger tree frog tadpoles released on average less than half the amount of ammonia as the species that released the most. And compared to two of the other species, chickens carried more ammonia in their guts. The researchers note that unlike Eiffinger’s tree frogs, other species typically lay their eggs in open ponds where ammonia easily dilutes.

“The behavior likely serves to prevent pollution of small bodies of water,” says Ito. Some ammonia still seeped into the tree frogs’ water, potentially through their urine.

It turns out that Eiffinger tree frog tadpoles have another superpower: Experiments showed that they can survive higher concentrations of ammonia than any of the other species included in the study. Dryophytes japonicus, better known as the Japanese tree frog. While this may seem counterintuitive, given the no-lunch period of shrews, Ito notes that chickens sometimes share their beds with other animals, such as mosquito larvae, which also release ammonia.

“We hypothesize that chickens have developed a tolerance to ammonia as a dual defense mechanism,” says Ito, “both against ammonia produced by other organisms and ammonia they produce themselves.”

As dust from the Sahara blows thousands of kilometers across the Atlantic Ocean, it becomes progressively more nutritious for marine microbes, a new study suggests.

Chemical reactions in the atmosphere grind iron minerals into dust, making them more soluble in water and creating an essential nutrient source for iron-starved seas, researchers report Sept. 20. Frontiers in Marine Science.

Dust clouds settling in the Atlantic can create phytoplankton blooms that support marine ecosystems, says Timothy Lyons, a biogeochemist at the University of California, Riverside. “Iron is incredibly important for life,” he says. Phytoplankton requires it to convert carbon dioxide into sugars during photosynthesis.

By further studying dust transport and chemical reactions in the atmosphere, scientists can better understand why parts of the ocean are biological hotspots for phytoplankton and fish.

Over 240 million metric tons of Saharan dust blows over the Atlantic Ocean each year. In Bermuda, the Bahamas and other islands, the earth turns red. But most of it settles in the ocean, providing a major source of iron in areas too far from land to get it from rivers.

Lyons and marine geologist Jeremy Owens, then also at UC Riverside, tried to answer another dust question: Have the types of dust settling in the Atlantic changed over the past 120,000 years? They analyzed the minerals that flowed from the dust in four cores ripped from the muddy sea floor – two in the eastern Atlantic near Africa and two from farther west near North America.

What they found prompted another line of inquiry.

In dust and soils around the world, approximately 40 percent of the iron is typically present within “reactive” minerals such as pyrite or carbonates. This type of iron can be decomposed by weak acids and can be used by life. In core samples from the bottom of the Atlantic, only about 9 percent of the iron in dust ores sampled farther west consisted of reactive iron ores, compared with about 18 percent in dust ores sampled from closer to Africa. That, Lyons says, was “the big surprise.”

He and Owens, now at Florida State University in Tallahassee, concluded that during the dust’s several-day flight across the Atlantic, more and more of its reactive iron was altered—attacked by acids and ultraviolet radiation, which separated the minerals.

“There are photochemical transformations that tend to make iron more soluble” in water, Lyons says. As the modified iron later settles into the ocean, it is dissolved—and devoured—by phytoplankton. The only reactive iron that reaches the bottom of the sea is the stuff that wasn’t altered during air transport and wasn’t ingested later. Their results suggest that the farther the desert dust flies, the less iron remains.

By spawning phytoplankton blooms, iron leaching from the dust can also feed small fish and other animals that graze on the plankton, as well as predators that eat the grazers. A recent study suggested that Atlantic tuna, an important commercial fish, is attracted to areas where the Saharan dust has settled.

The new results are plausible because previous studies have shown that iron minerals react in the atmosphere, says Natalie Mahowald, an atmospheric scientist who studies dust at Cornell University. Their conclusion “matches what I thought was going on,” she says.

But she points out that Saharan dust isn’t the only possible source of that iron: The samples came from far enough north in the Atlantic that some of their iron could have come from smoke from fires in North America over 120,000 years. the last one. she says.

Determining a dust source buried deep on the sea floor can be challenging. But Owens and Lyons tried to identify dust fingerprints by measuring the ratios of iron to aluminum and the ratio of light iron atoms to heavy iron atoms in their samples. Both measurements were roughly consistent with the type of dust coming from the Sahara, they found. It may be possible, in the future, to analyze sediment from more places in the Atlantic, providing a clearer picture of how dust has blown across the ocean and changed chemically.

Displacement is in the nature of a river. But when a river breaks from its channel and carves a new path across the landscape, devastating floods can descend on communities with little or no warning.

For decades, researchers have struggled to explain exactly how river channels prepare for such sudden diversions or avulsions. A study published on September 18 in Nature may have finally settled the debate, showing how two factors work together to orchestrate a river’s course change. Based on their findings, the researchers also developed a promising algorithm that can predict the new path of a fallen river.

“These are monumental floods, civilization-changing floods in some cases,” says sedimentologist Douglas Edmonds of Indiana University in Bloomington. In 2010, dams on the Indus River in Pakistan contributed to floods that forced approximately 20 million people from their homes. However, flood risk models remain unable to predict where rivers will be redirected, Edmonds says. “It’s really an invisible flood hazard.”

Avulsions require a setup and a trigger – an overburdened camel’s back and a last straw (SN: 28.6.24). “The trigger could be a flood, an earthquake, it could be a blockage in a river,” says Edmonds. Configuration refers to how sediment deposition has prepared a river to divert—and is the root cause of diversion, Edmonds says. “Rivers flood all the time, but they don’t burst all the time.”

The new study focused on determining the structure, for which there were two competing hypotheses. Avulsions were thought to occur when a river rises too high, or sediment deposition raises the water level of a river above the surrounding land. The other asserted that avulsions occur when there is a slope advantage, or when the slope of a new, potential path becomes steeper than that of the current river path.

Edmonds and his colleagues began by using satellite data to investigate approximately 170 avulsions, noting how far downstream rivers tend to divert. They found that avulsions were roughly three times more common near river mouths and mountain fronts than in the middle.

Focusing on 58 river channels for which high-resolution topographic data were available, the researchers measured the advantage of elevation and slope before subsidence. They found that elevation best explained avulsions near mountains, while slope advantage best explained those near estuaries and deltas.

There’s so much sediment coming down from the mountains that the rivers just collect it until they get too high and overflow, Edmonds says. Meanwhile in deltas, there is a lot of cohesive mud that forms very steep natural sheets around deep channels, and avulsions need a big slope advantage to start cutting the slope, he adds.

These two factors – slope advantage and overhang – work together in an opposite way, the researchers found. The higher a river becomes, the less slope advantage it has to give up and vice versa. “It’s the first time anyone has been able to show this with data,” says Penn State geologist Elizabeth Hajek, who was not involved in the study.

Avulsions occurred when the mathematical product of the two factors exceeded a threshold value, the researchers found. As long as accurate topographic measurements of a river’s channel are available, which is more likely for larger rivers and in clear-sky locations, you can probably use that threshold metric to identify where avulsions are likely to occur, says geomorphologist Vamsi Ganti of the University. of California, Santa Barbara, who was also not involved in the study.

The researchers developed a computer algorithm that outlined where on a map an inverted river might go, taking into account the slope of the terrain and the speed of the river. When tasked with predicting the paths of 10 past avulsions, the algorithm correctly captured the path of each one. “It’s a really good tool,” says Hajek. “It can be really, really useful for identifying areas of concern.”

The plan is to develop avulsion risk maps for the globe or vulnerable regions, Edmonds says. “Now that we have this metric, we can measure it in rivers all over the world.”