View of the former Kakhovka reservoir bottom near the village of Novovorontsovka after the drainage. The sign says: "Swimming is forbidden!" And "Attention! Mines!". | Photo: Ivan Antipenko
"In modern warfare, rivers continue to be used not only as the frontlines, but increasingly as weapons", said IGB researcher Dr Oleksandra Shumilova, lead author of the study. In Ukraine, dams on the Irpen, Oskil and Inhulets rivers have been deliberately destroyed since 2022, and the power plants of several other large reservoirs have been damaged. The destruction of the Kakhovka dam was the most dramatic event, attracting the attention of the international community and the scientific community. Its reservoir, located at the downstream end of the Dnipro reservoir cascade, stored 18 cubic kilometres of water used for multiple purposes, including water supply, agriculture and industry.
The destruction of the Kakhovka dam led to the draining of the reservoir, causing flooding downstream and polluting the freshwater and marine environment. An international research team investigated the ecological impact of the dam's destruction. "The task was not trivial, however, as the scale of the impact exceeded all known dam failures and targeted dam removal projects by several orders of magnitude. This was further complicated by restrictions on field observations and measurements due to the ongoing combat", said IGB researcher Dr Alexander Sukhodolov, co-lead author of the study.
Catastrophic flood
Damage of the dam led to the release of 16.4 cubic kilometres of water into the Dnipro-Bug estuary and then into the Black Sea, which continued for 2 weeks. Overall, 110,000 people and 60,000 buildings were affected. The researchers performed numerical modelling of flow hydrodynamics for the area downstream of the dam at the maximum of flood: Strong turbulence developed between the floodplain and the main channel, leading to the washing out of reed-belt vegetation, with debris scattered 250 kilometres along the coastline. The flood created a freshwater plume with high concentrations of pollutants which, as it spread across the Black Sea shelf, reduced salinity and affected benthic organisms.
Toxic time bomb: around 83.3 thousand tonnes of heavy metals in the exposed sediment
Upstream of the dam, an area of 1,944 square kilometres of the reservoir bottom was suddenly exposed. To put this in perspective, this is about 80 percent of the area of Luxembourg. The results of the study highlight an overlooked problem – the exposed sediment will act as a long-term source of heavy metal pollution. Since the 1950s, the Kakhovka reservoir has accumulated about 1.3 - 1.7 cubic kilometres of fine silt eroded from the banks. This giant sponge has accumulated pollutants from industrial and agricultural areas in the Dnipro catchment. According to estimates based on pre-disaster field sediment sampling combined with post-disaster remote sensing results, the exposed sediments are contaminated with approximately 83.3 thousand tonnes of highly toxic heavy metals such as lead, cadmium and nickel. "Our analysis suggests that less than one per cent of the reservoir’s sediments are likely to have been released during the drainage process. Surface runoff and seasonal flooding can lead to erosion of contaminated soils and increase the concentration of pollutants in river waters and in temporarily flooded areas", said co-author Dr Natalia Osadcha from the Hydrometeorological Institute of the National Academy of Sciences of Ukraine.
Nature returns quickly: Restoration of floodplain vegetation equivalent to 80% of an undammed ecosystem expected within five years
Within the former reservoir, the river has returned to its historic course, although some large ponds have formed. Scientists were concerned about what would happen over such a large area? "Understanding how the ecosystem recovers after extreme events is a task for predictive ecology and can be understood from the basic principles of self-organisation", explained Oleksandra Shumilova. The period when the reservoir was drained in June coincided with the period of seed dispersal by typical riparian plants such as willows and poplars. Prior to drainage, seeds settled on the surface of the water and were then transported long distances by the ebbing water and trapped in cracks in the sediment. Within 3 months, about 18 percent of the former reservoir bed was covered by pioneer vegetation, which also showed extremely high growth rates. The results suggest that a floodplain vegetation equivalent to 80 percent of an undammed ecosystem can be expected within five years.
What can we learn from the Kakhovka reservoir case and what's the future?
"The lessons learned from the case of the Kakhovka reservoir in Ukraine are valuable for understanding ecosystem processes in rivers with planned dam removals, which are now being addressed in Europe and the USA. In addition, the framework developed provides insights into how unexpected dam failures can be assessed and what tools and approaches are available. This is important to consider in view of the ageing of dams and related water infrastructure", commented IGB researcher Prof. Hans-Peter Grossart, a senior author of the study.
While the war continues, discussions have begun among decision-makers, scientists and water experts about the future of the Kakhovka dam. Opinions differ on whether the dam should be rebuilt: without it, the river's ecosystem would quickly return to its pre-dam state, but heavy metals could spread through food webs. The authors suggest that the release of pollutants could be controlled by building two temporary barriers, 15 kilometres long, separating the main channel from the two largest wetlands.
Any plans to restore Ukraine's conflict-damaged water ecosystems depend on an end to the war, but there remains a significant risk of new missile attacks on dams in the Dnipro and Dniester cascades. "If more dams are attacked, the consequences for people and the environment could be catastrophic, as the collapse of the Kakhovka dam showed. The protection of dams in areas affected by military activities should be a priority of international law, as conflict-related dam failures can have far-reaching and long-term environmental consequences", concluded Dr Oleksandra Shumilova.
The genesis of the paper:
An early warning of the potential dangers associated with the destruction of large dams and reservoirs in Ukraine was given by IGB scientists in the article published in the journal Nature Sustainability, which assessed the military impact and threats after the first three months of the Russian-Ukrainian armed conflict. Not surprisingly, when the Kakhovka dam collapsed, the research team was asked to comment on the ecological consequences of this disaster. To address these challenges, the IGB initiative group organized a workshop that brought together leading scientists from Ukrainian research institutions and scientists from Germany, the USA, the Netherlands, Turkey and Argentina. Having a team with different expertise and perspectives allowed us to develop a methodological framework to link field observations, remote sensing, numerical and analytical modelling to gain a better understanding of the spatio-temporal scales of this disaster and to outline trends in the recovery of damaged ecosystems.
