Basic processes sometimes run in secret. If you look at a large river, you can see huge masses of water flowing down it at first glance. After heavy rainfall, the brownish discoloration of the water gives an idea that rivers also transport material. The dimension of this cargo becomes clearest when viewed from above: Where the rivers flow into the sea, the deposited sediments create huge delta areas – on the Amazon, Nile and Ganges or in Europe on the Rhine and Rhone, for example.
However, in many regions of the world delta areas are no longer growing – especially in Europe, North America and Asia. On the contrary, they are gradually being eroded by the sea, as are sandy beaches. The main reason: the supplies from the hinterland are missing. Because humans have drastically changed the way rivers transport sediments – especially because dams hold back the valuable freight.
First global inventory of more than 400 major rivers
After analyzing more than 400 major rivers worldwide, US researchers have now presented the first global inventory in the journal Science. Accordingly, the sediment load in the northern hemisphere – meaning the zone north of the 20th degree north latitude – has shrunk by about half since the construction of dams. In more southern regions, on the other hand, the researchers identify the opposite trend: Here, rivers – especially in South America – transport significantly more sediment.
The dimension of the study is unprecedented both spatially and temporally, write Christiane Zarfl from the University of Tübingen and Frances Dunn from the University of Utrecht in a Science commentary. Although measurements and estimates of the sediment load of some river systems were already available beforehand, global data have so far not been available, especially over longer periods of time.
Using satellite data, the team led by Evan Dethier from Dartmouth College in Hanover, New Hampshire, analyzed 414 major rivers worldwide – mainly based on coloration – in the period from 1984 to 2020. For the analysis, the researchers used a model that they had previously based on calibrated 130,000 individual measurements at 340 locations. With the sediment load, they only refer to the suspended matter floating in the water, not to the so-called bed load. Depending on the flow rate, this mostly coarser material such as gravel moves downstream on the river bed, but is difficult to determine.
Soil erosion in South America is washing much more sediment into rivers
"The global sediment transport by rivers is changing rapidly," the authors state – due to human influence. It has a different effect in the north than south of the 20th degree north latitude, which corresponds to the height of Cuba. In the north, the transport of suspended matter has fallen by an average of 49 percent since the construction of large dams, while in the south it has increased by 41 percent since the 1980s.
In Europe, it is estimated that suspended cargo has fallen by more than 80 percent since the construction of dams. On the Danube, the decline is estimated at more than 60 percent. In Asia – where half of the global sediment load once flowed into the sea – the transport of suspended matter has fallen by an average of 34 percent since 1984. According to estimates, only four percent of the original sediment load reached the mouth of the Mekong, where more than 100 dams dam the water along its well over 4,000 km long course, write Zarfl and Dunn.
While the rivers of South America once carried only a fifth of the sediments transported worldwide into the oceans, it is now more than half – also because transport in the north is decreasing. According to this, the Amazon accounts for around two thirds of South America's sediment load. But this development is unlikely to last: Deforestation in the Amazon region continues unabated, but more than 300 large dams are planned in the region, writes Dethier's team. Once completed, they should significantly reduce the flow of sediment.
The Danube Delta recedes by up to 24 meters per year
The deposition of sediments in riverbeds can have many consequences, write Zarfl and Dunn in their commentary. For example, for the shape of a river, for the habitats of plants and animals, for navigability and for the risk of flooding. And finally also for the estuary deltas: If the rivers no longer deliver any more material, deltas could be eroded again by the ocean surf.
As an example, Zarfl cites the Nile delta after the construction of the Aswan high dam in southern Egypt. Martin Pusch from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) refers to the dwindling Mississippi Delta in the Gulf of Mexico. And according to surveys, the Danube Delta is retreating by up to 24 meters per year.
Bridges, buildings and dams are at risk
But that's not all: if the bottom is no longer filled with sediment, a river digs deeper. On the Elbe, that makes up one to two centimeters a year, says Pusch. On the Rhine, with its strong current, it is several centimeters per year – that is, several meters per century without countermeasures. And if the level of the river bed drops, this endangers the stability of structures – such as bridges, buildings erected over the water and, last but not least, the dams themselves. Another consequence: with the sinking of the river bed, the groundwater level falls, says Pusch: The consequences were enough from problems in agriculture to an increased risk of fire.
"Rivers have a natural sediment balance," explains the ecologist. "If we intervene to disrupt it, we have to use technical means to compensate for the consequences. Those are tasks for eternity.” Take the Rhine, for example: to compensate for the river digging in, a sand-gravel mixture has been dumped into the river below the Iffezheim barrage near Karlsruhe since 1978 – an average of 185,000 cubic meters per year, arithmetically more than 500 cubic meters daily. There are also such references to sediment on the Elbe and Danube.
Above the dams, the opposite problem is being fought: here, for example, reservoirs are in danger of silting up due to the deposited sediments. How drastically this can reduce storage capacity is shown by the Khashm-el-Girba dam on the Atbara River in East Sudan – also known as the Black Nile because of the high suspended load. The water dammed up there was primarily intended to irrigate the fields of tens of thousands of Nubians who had fallen victim to another dam project. They had been resettled here because their home on the Nile near the Egyptian border was flooded by the Aswan High Dam.
Problems are solved by creating new problems
But just a few years after the completion of the Khashm-el-Girba Dam in 1964, the capacity of the reservoir had fallen by less than half – from 1.3 to 0.6 billion cubic meters. The regional water supply was endangered. In order to defuse the problem, the water and sediments at the bottom had to be drained regularly. Such flushing of reservoirs, in which a mush of water and fine sediment is drained down the valley, is not unusual elsewhere either, explains Pusch, and is deadly for many river dwellers.
The situation at the Khashm-el-Girba dam has been a little more relaxed for a few years: Sudan had the Upper Atbara complex, which opened in 2017, built upstream. Because the twin dams are now holding back sediment, the Khashm el Girba reservoir is silting up more slowly. Pusch considers this a typical strategy that is also common in Europe: "You solve a problem by creating another one for the next generation," he says. "That's the downside of damming rivers."