The Caspian Sea, the world's largest inland body of water, is in major trouble. Since the mid-1990s, this saline lake, located between Europe and Asia, has lost an area nearly the size of Sicily. But until now, it wasn't clear why.

Researchers finally provided answers in a study published June 5 in the journal Earth's Future. Using ground- and satellite-based observations, they connected falling water levels to a drastic drop in river inputs from Iran, Turkmenistan, Azerbaijan, Kazakhstan and Russia, all of which border the Caspian Sea.

The team particularly highlighted losses from Russia's Volga River, the longest river in Europe, which supplies roughly 80% of the Caspian's inflow.

To find out why the Volga is discharging less water into the Caspian Sea now than it did 30 years ago, we spoke with Amir AghaKouchak, a co-author of the study and professor of civil and environmental engineering at the University of California, Irvine.

AghaKouchak, who directs UC Irvine's Center for Hydrometeorology and Remote Sensing, explained why river inputs have dropped and how decades of water withdrawals have put the Caspian Sea on a path to collapse. He also touched on a solution to this crisis — and revealed why Russia's hunger for ammunition to attack Ukraine is making it harder to save the Caspian Sea.

Sascha Pare: Falling water levels in the Caspian Sea have been attributed to natural variability, and more recently, there's been a focus on climate change and increased evaporation due to rising temperatures. But what did the results of your study show?

Amir AghaKouchak: People quickly jump to conclusions about climate change. Of course, that is important, but it cannot explain the change that we have observed. Over the last 30 years, water levels in the Caspian Sea have shrunk by around 2 meters [6.5 feet] or so. Just to put things in perspective, this translates to about 24,000 square kilometers [9,300 square miles] of sea area loss — about 5%. When we look at drivers, we notice that rainfall hasn't changed significantly over the lake and upstream, so the decline cannot be explained simply by changes in rain.

Evaporation has increased, so it explains maybe about one-third of the change. There is less water in the lake, and that indicates there's probably some water lost along the way. We don't have a lot of human-related data ‪—‬ for example, water withdrawal for irrigation or groundwater wells. That area is very difficult; the countries are not open to sharing data. A lot of our results are based on looking at proxies. Our interpretation is that temperature caused about 30% of the change and the rest is probably because of human activities.

SP: Which human activities does your research suggest are responsible for the Caspian Sea's decline?

AA: Several of the countries on the Caspian Sea have built large dams — Russia, for example. They are all desperate for hydropower. All of the countries around the lake [have also built infrastructure to supply agriculture], and one reason is that they want to create jobs and improve the local economy. Water demand is increasing in all those countries around the Caspian Sea. It's not surprising to see lakes drying or groundwater levels falling.

It's hard to find data about how much water countries are using; they keep data very private, especially places like Russia. We don't have detailed information, but the Volga-Don Canal in Russia is one of the projects that is contributing to this crisis. [Editor's note: The Volga-Don Canal is a ship canal that connects the Volga and Don rivers at their closest points.]

But we cannot blame this whole thing on that. We have to be careful with our language; we can't say one project is responsible. Most likely, this is the collective impact of many different small withdrawals from here and there, overall contributing to a significant decline in the Volga River outflow into the lake. In the paper, we have different rivers, but the Volga dominates because it's the largest river. The Volga has a higher impact than all the other rivers combined. There are lots of demands along that river basin.

SP: Did you expect human activities to play such a big role, or were you surprised?

AA: We did not have any expectations at the beginning. In fact, when we started this project, I reached out to a lot of colleagues from neighboring countries around the Caspian Sea. We didn't want to do it on our own, sitting in California, looking at another area of the world, without having local people involved. If you look at the co-authors, you see people from Azerbaijan, Iran, Kazakhstan — so, we have local experts. We thought this was really important to have those perspectives. I didn't want to reflect the opinions of only one research group. What we have here is the collective opinion of people who have not met in person; we only exchanged emails to look at the results, and then we came up with those conclusions.

We do have a Russian involved, but not from an official Russian institution. I thought this was really important, because there's more back-and-forth [with a large group of people]. If two people have to convince themselves of an idea, it's much easier than with 20. Also, we brought in people from other countries who had projects in the area; for example, there is one group from Finland and another group from Germany. We tried to come up with something that this large group of people could agree on. It was not easy, but it worked out.

SP: What is the worst-case future scenario for the Caspian Sea if shoreline countries fail to decrease their water demands?

AA: My concern is that, in the same general area, we have the Aral Sea, which is now about 5% to 10% of what it used to be, and Lake Urmia, in northwest Iran, that has also shrunk really fast and is now very small, with low levels of ecological activity. So we have two smaller lakes in a very similar climate that, from an ecological perspective, are dead. Because we have seen those degrade, we have to consider that this is the worst-case scenario for the Caspian Sea, too.

The Caspian Sea is much, much bigger. I hope it doesn't happen, but it's on the same path. Again, it's much larger, so it would probably take 100 years [to shrink as much as the Aral Sea and Lake Urmia], even with the current trend. But because we have seen those lakes shrink really fast, it deserves attention. Due to the size of this lake, the potential impacts will be huge. Even if 20% of the Caspian Sea dries out, the consequences will be very significant. It would be a disaster.

SP: You mentioned that drying lakes can become ecologically "dead." What other impacts could the Caspian Sea's falling water levels have on the surrounding landscape and people?

AA: The lake's retreat will expose the lake bed, and because it's saline, you have all kinds of minerals that will be exposed. With the next strong wind, they will go into the air. The health impacts include respiratory diseases, and even eye diseases, that are linked to salt minerals in the atmosphere. Those are the immediate consequences. Additionally, those minerals can travel long distances. In general, salt deposition on agricultural land reduces productivity. It takes a while, but if those lands are exposed for a long time, this will have an impact on the agricultural sector in those areas, which means potential food security issues. We don't exactly know what those materials are on the lake bed, so it's hard to know precisely what the consequences will be.

There is also serious shipping happening in this region. Right now, most of the shrinking is happening in the Caspian Sea's northern part, which is shallower [than the rest of the lake]. The drop in water levels is about 2 meters, and you can imagine that if you have a port and water levels drop more, you won't be able to operate. You'll need to extend your shipping routes southward; you'll need to invest and spend a lot of money to use other ports and docking sites. We have evidence of that around the Aral Sea and also Lake Urmia. There used to be these docking sites and ports, but those are now far away from the water.

SP: What must bordering countries do to save the Caspian Sea and prevent these negative effects?

AA: Demand management is absolutely critical. Demand management doesn't get a lot of attention, because unlike big projects that make the news, management means you have to cut things and do things that people don't notice. Some people argue that politicians don't show a lot of interest in demand management, but it's something that we have control over. The technological opportunities are there.

But when you look at that area, there is a big war ‪—‬ so probably it's not the priority to invest in management. There are a lot of things that we don't talk about in the paper that are relevant. I recently learned through discussions with people from the region — this is something that came to us from co-authors and collaborators — that many countries in that area grow cotton, which uses a lot of water. It has a lot of applications, but apparently, one of them is military. Cotton is used to produce ammunition. Russia needs a lot of ammunition, so not only are they growing it themselves, but they also bought neighboring reserves from Uzbekistan and Kazakhstan. So now, even those countries have an incentive to grow more cotton. This is driving more irrigation, more groundwater extraction, more surface water depletion, and probably more impacts on the Caspian Sea.

This interview has been condensed and edited lightly for clarity.

Each year, tons of soil, silt and rocks flow into reservoirs in the U.S. and get stuck there, creating a "ticking time bomb" that threatens lives and livelihoods. According to a new study, around a fifth of the world's reservoirs are currently at high risk of filling with sediment, and by 2060, half could be inoperable as a result.

The sediment takes up space meant for water, putting additional pressure on structures that were not built to contain large quantities of sand and sludge. As the reservoir fills with the material instead of water, sediment-filled reservoirs pose a serious threat to water security and the safety of people living near or downstream of them — and as the climate changes, scientists expect the problem to worsen.

"We are losing storage space in our reservoirs faster from sediment accumulation than we gain space from new dam construction," Amy East, a geologist at the U.S. Geological Survey, told Live Science.

What is dam sedimentation?

Dam sedimentation is not a uniquely U.S. problem. Rain pulls sediment down slopes into waterways. Normally, this sediment would travel through the river system, shaping the water flow, creating habitats, and fortifying coastal wetlands and estuaries against erosion. In many parts of the world, this sediment is also a vital natural fertilizer for crops. But after a river is dammed, this sediment can collect in the reservoir.

A 2023 study published in the journal Science found that sediment buildup was the greatest threat to global water storage in existing reservoirs — even more than drought.

New research, which includes smaller reservoirs, has offered a more nuanced and concerning picture. In the study, published June 5 in the journal Nature Sustainability, researchers assessed the state of reservoir sedimentation globally. They looked at more than 550,000 reservoirs, most of which spanned less than 0.4 square miles (1 square kilometre). They found that almost 1 in 5 of reservoirs globally are at high risk of filling up with sediment.

The researchers found that sedimentation is eating into global reservoir storage capacity at about 7.3% every decade. There are 16 global hotspots that are at high risk of reservoir sedimentation, threatening water supplies to more than 2 billion people and impacting more than a quarter of global irrigated land. One of these hotspots includes the western U.S.

The authors found that sedimentation risk was especially pronounced in regions with dense small-reservoir networks supporting key crop systems.

"Without intervention, we project that, by 2060, more than half of all reservoirs could become functionally inoperable," the authors wrote. This includes more than half of all small reservoirs and more than a third of large reservoirs globally.

Where is it a problem in the U.S.?

North America has the second largest number of reservoirs, after Asia, according to the global survey. And at 7.8%, its sedimentation rate is higher than the global average, ranking it third after Oceania (9.9%) and South America (8.1%),

The U.S. is home to many small dams, said Matthias Kondolf, a geomorphologist at the University of California, Berkeley who wasn't involved in the study. "All the focus tends to be on the big dams, and these many, many small ones have been neglected," he told Live Science.

From the 1940s to 1970s, there was a big push by the U.S. government to build "farm ponds," he said. In western Oklahoma, for example, "all these dams are now filling up with sediment, and there's nothing in place to assess the safety of these features, which have now become threats."

Some of these dams have already failed, following large storms as the ageing dams were not able to cope with the volume of water. A 2018 study found that since 1980, there have been on average 24 dam failures a year in the U.S.

Reservoirs in some regions are more at risk than others. The western U.S. is a natural hotspot because it has a steeper topography, meaning more sediment naturally ends up in watersheds, East, who wasn't involved in the new research, said.

And in areas where human activities like farming or logging disrupt the landscape, rainfall is likely to drag even more sediment into reservoirs, East said. "Those types of land use can create sediment-producing hotspots almost anywhere."

In California, for example, more than half of small reservoirs in maize-growing areas are filling up with sediment at a rate of more than 10% a year, which threatens local maize production.

The threat to water security will be particularly acute in places struggling with wildfires, such as the western U.S. After fires, rain washes the debris and sediment into watersheds. A 2024 study in California found that most of this post-fire sediment erosion occurred upstream of reservoirs, rather than in places where sediment would move downstream directly to the ocean.

The dam-sedimentation situation is likely to get worse as the climate changes, researchers said. As the climate changes, there will be more extreme weather events, such as droughts and wildfires, adding more sediment to watersheds. Dams are designed to cope with the pressure of water, not sediment which exerts more force on dam walls. Moreover, in seismically active areas, the sediment vibrates at a different frequency than the concrete of the dam, which creates further impact against the structure. Additionally, the dams are just getting older.

"What do we do 50 or 100 years from now when we have all these reservoirs across the landscape filled with sediment, sitting there as ticking time bombs?" Kondolf said.

What can be done about it?

For a reservoir to be sustainable long-term, the amount of sediment entering it and leaving it should be the same.

"A big part of preventing reservoir sedimentation problems is upstream soil conservation in the watershed," East said. Interventions include ensuring that land use, such as agriculture or timber harvesting, follow best practice to reduce the amount of sediment washing off the landscapes.

If sediment is already in the dam, it can be difficult to manage and expensive to remediate, Kondolf said. Some dams, such as Fall Creek Dam in Oregon, have mechanisms that enable engineers to release sediment. But in the past, reservoirs were not designed to manage sediment and little thought was given to how to decommission them.

"This is one of those issues that falls in the cracks between different agencies and responsibilities," Kondolf said. Some larger facilities fall under the remit of the federal government. Other dams fall under state governments. California, for example, recently launched a state-sponsored program to future-proof the state's dams.

But smaller dams often get overlooked, Kondolf said.

One option is to remove the dams entirely. For example, sediment-related concerns contributed to the decision to remove the Elwha and Glines Canyon dams in Washington. The Elwha River later carried most of the sediment to its river mouth.

For smaller dams with a relatively small amount of sediment, "you just blow up the dam and let the sediment work its way downstream", Kondolf said.

There are other technological solutions, but they are expensive, such as retrofitting dams to enable them to pass sediment; creating a bypass to route sediment-heavy water around the dam; and mechanical dredging.

Another option is to empty the reservoir of water and let the river cut a channel through the sediment, Kondolf said.

However, released dam sediment brings its own risks. If not managed carefully, sediment can flood the river system and disrupt downstream ecosystems.

"The economics of managing reservoir sediment long-term were not generally incorporated into dam design and planning in the last century when many western-U.S. water projects began," East said. This has "pushed the cost burden on future generations."

But the reality is that reservoirs around the country are filling with sediment. It is reducing their ability to store water and potentially endangering the communities living downstream of them — and unless addressed, the problem will only get worse.