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Story Publication logo November 24, 2023

The Dark Side of Hydropower

A flat seashore with dead trees emerging from the water

Huge man-made structures are running full, but not with water.

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View the full interactive elements in this Süddeutsche Zeitung report, "The Dark Side of Hydropower," by clicking here.

Dams are supplying more renewable electricity than ever before. But an SZ investigation in cooperation with the Pulitzer Center shows how high the price is — and that the energy yield is not as secure as expected.

Hardly any longer river in the world still flows undisturbed from its source to its mouth. Most of them are now cut through by large dams: Around 10,500 of these structures currently supply electricity, making them by far the most important source of renewable energy. Our globe shows the largest dams with more than one hundred megawatts of installed capacity in important river catchment areas.

Dams are actually a blessing: They not only generate energy, but also protect entire regions from flooding. They help farmers through periods of drought and provide a habitat for fish—all at comparatively low cost. But the price is high.

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The problems often start with the tributaries, as can be seen on the Mississippi. The "Old River" has one of the largest catchment areas in the world: Together, it and its tributaries drain a third of the entire USA.

On some tributaries of the Mississippi, dam after dam is strung together like a string of pearls. These so-called cascades are a classic concept of hydropower. They are ideal for generating energy, but transform a river into an almost stagnant body of water.

This is because dams such as Gavins Point on the Missouri River are huge structures—and a massive intervention in nature.

Tons of sediment are also dammed up with the water: Stones, mud and sand get stuck to the structures. Because the reservoirs are becoming increasingly silted up, the energy yield is decreasing. Although more and more dams are being built worldwide and the built reservoir volume should therefore be increasing, the actual available reservoir volume of the lakes is decreasing.

But that's not all: At the same time, the sediment ends up missing from the river and the delta, causing erosion.

This sedimentation takes place invisibly and unnoticed by many. It is slow, its components are tiny and it happens dozens to hundreds of meters under water.

Even satellites cannot see into the depths of reservoirs. But they can make sedimentation visible elsewhere: in the river deltas, where the water of the rivers flows into the sea and forms unique natural and economic areas.

Because less and less sediment is arriving there, the surf is eroding the deltas—with devastating consequences for people and nature.

SZ has analyzed satellite images and data for eleven deltas and shows the changes to the coastlines. All but one have visibly shrunk.

One impressive example is the mouth of the Rio São Francisco in Brazil, one of the 25 longest rivers in the world. Since the middle of the 20th century, huge dams have been built in its course. This is how the Sobradinho reservoir was created, which can even be seen from space.

The delta of the Rio São Francisco on the north-east coast of Brazil has changed massively in the following decades, as the SZ analysis shows.

By the end of 2022, the delta will have lost a good eight square kilometers of land compared to 1984. That is twice the size of Munich's English Garden. Cabeço has long since sunk into the sea, its lighthouse now stands in the middle of the ocean. A lawsuit against the company operating the dams has been ongoing for decades and could set a precedent.

Find out more about the impact on the ground in this report: Brazil: Only a lighthouse remains

But the problems at the delta in Brazil are not an isolated case. The phenomenon can be observed worldwide.

The giant dams also have a massive impact on people and nature in Europe, for example on the Rio Ebro, the second longest river in Spain.

Only the Nile delta is larger than the Ebro delta in the Mediterranean. It could accommodate the whole of Munich. Instead of a metropolis, however, it is home to one of the largest rice-growing areas in Spain: 135,000 tons a year grow on the delta. In addition, one of the most important bird sanctuaries in Europe is located at the tip.

The nature reserve near the triangular Isla de Buda in particular is in danger of disappearing. If no countermeasures are taken, the delta will no longer exist in twenty years, say activists. Compared to 1984, the delta has already lost five square kilometers.

Read the report: Spain: A coastal landscape dies

The same problems are evident on the other side of the world, in Southeast Asia:

The Mekong flows through China, Myanmar, Laos, Thailand, Cambodia and Vietnam. Since the 2000s, almost 50 dams with a capacity of more than 100 megawatts each have been built on it and its many tributaries.

If they run at maximum load, they could cover more than half of Germany's electricity needs. And new mega-dams have long been under construction.

The north of Laos and the south of China are particularly heavily developed. Entire tributaries such as the Ou in Laos have been transformed into cascading dams.

The same applies to the upper Mekong in China, where some of the highest and most energy-rich hydropower plants in the world have been built. Sediments were hardly considered.

The impact on the estuary, where more than 20 million people live, is correspondingly massive. Rice cultivation in the Mekong Delta is crucial for Southeast Asia's food security.

The youngest part in the very south of the delta comprises the Vietnamese province of Cà Mau. It was still growing in the 1990s.

Today, erosion is increasing here. The coast is retreating more than 50 meters a year in some places. To the west of Cape Cà Mau, Vietnam's southernmost mainland point, the delta is still growing—but much more slowly than before.

The main delta in the north has been growing for a long time. Forty years ago, there was only one large dam on a tributary of the Mekong. The river was one of the largest free-flowing rivers in the world. The extremely fertile coastline migrated up to 80 meters a year into the sea. One hundred forty-three million tons of soil reached the estuary every year, about as much as a fifth of all the buildings in New York.

Today, the sea is gnawing away at the delta. Although many dams are still young, a good 40 of the 50 dams were opened in the last 15 years. As a result, a third of the original sediment mass still reaches the estuary. By 2040, however, it is predicted that only three percent will remain. New studies show: Erosion is increasing everywhere.

Read the report: Laos: A river starves

In this research, SZ analyzed numerous rivers, their catchment areas and deltas and calculated the extent of the impact of the dams on the coasts.

Click to explore the interactive map.

Despite all these problems, hydropower is a very important source of energy. At more than 4300 terawatt hours per year, it produces more electricity than all other renewable energy sources combined. But how can the problem of silted-up reservoirs be avoided?

There are already possibilities, say researchers. For example, the sediments can be flushed out through low-lying gates, which, however, reduce the energy yield. Or a part of the river that carries a lot of soil can be diverted. It is also possible to siphon off the sediment and divert it past the dam. However, this method is not suitable for all sediments.

However, these technologies are hardly ever installed, not even in new ones, says hydraulic engineer George Annandale. The main reason for this is that the economic benefits only become apparent after 50 to 80 years. "That's my big problem with the economic analysis of these huge projects that are supposed to last for centuries. It's short-term thinking for long-term projects."



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