Floodwater does not follow the boundaries imposed by roads, buildings or other human interventions. It follows the landscape, often returning to former riverbeds, rushing down hillsides and collecting in low-lying areas. Even when concrete and urban development have transformed the terrain, water can find its way through hidden channels beneath streets and buildings.
Recent flooding in the Sporades islands, Evia, Rhodes and the Attica region has renewed questions about which parts of Greece face the greatest risk as autumn rainfall intensifies. The answer depends on a combination of factors, including terrain, drainage basins, riverbeds and the way settlements and infrastructure have developed.
Greece has a diverse range of flood risks, from Crete’s mountainous catchments to the plains of Thessaly and the streams running beneath the urban landscape of Attica. Although previous disasters cannot predict exactly where the next flood will strike, historical records and geographical data can help identify vulnerable areas and conditions that accelerate runoff.
How geography shapes flood risk
Heavy rainfall sets the process in motion, but its effects vary considerably depending on the characteristics of each region, according to Niki Evelpidou, a professor in the Faculty of Geology and Geoenvironment at the National and Kapodistrian University of Athens.
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In Crete, where flooding was recorded in early October, steep slopes and mountainous terrain contributed to the rapid concentration of water.
During intense or prolonged rainfall, the ground gradually becomes saturated and loses its ability to absorb additional water. More rainfall then flows across the surface and gathers in streams and riverbeds, rapidly increasing the volume of water carried by torrents.
The water can also sweep along sediment and other debris, placing additional pressure on river channels and drainage infrastructure. When it reaches roads, settlements, farmland and other infrastructure, impermeable surfaces and interventions that have altered natural waterways can further increase the danger.
Thessaly faces a different set of challenges. Water flowing down from the surrounding mountains accumulates across a large, flat basin containing extensive rivers and drainage networks.
The consequences of the extreme weather event known as Storm Daniel highlighted the need to assess flood-control infrastructure and improve the overall management of the region’s drainage basin.
In Attica, urbanization is a major factor. Asphalt and buildings prevent water from soaking into the ground and accelerate surface runoff. When rainfall exceeds the capacity of drainage systems, flash floods can occur. The deadly flooding in Mandra in 2017 remains a prominent example of the consequences that can arise when natural waterways intersect with human development.
Western Greece, Epirus and the Peloponnese face risks from steep mountainous catchments that produce rapid runoff, while low-lying areas and river mouths are vulnerable to water accumulation.
In Central and Eastern Macedonia and Thrace, extensive river basins and low-lying terrain create different flood-management requirements. The course of each event depends on rainfall, water flowing in from upstream areas, soil saturation and drainage capacity.
The Greek islands in both the Aegean and Ionian seas face particular challenges. Steep slopes, small torrents, narrow valleys and settlements close to the coast can cause sudden flooding, leaving little time to respond.
When these conditions coincide with a powerful storm system, the consequences can be devastating, as recent flooding in Rhodes demonstrated. Evelpidou said island regions require systematic flood-risk assessments.
What nearly 2,000 historical floods reveal
A research team at the University of Athens analyzed 1,994 flood events recorded in Greece between 1886 and 2022. The findings confirmed a strong seasonal pattern: 51.5% of the events occurred in autumn and 27.3% in winter, with October and November emerging as the main months for flooding.
Historical records can help researchers identify recurring patterns in where and when floods occur, supporting more targeted prevention measures. Their value increases when they are combined with current data, flood-hazard maps and information about changes in land use.
Similar amounts of rainfall can produce very different outcomes depending on the landscape.
“In a mountainous catchment, water can accumulate quickly and create a flood wave. In a lowland area, it can build up gradually and remain for a long time. In an urban environment, it can inundate roads and underground spaces within a short period,” Evelpidou explained.
Heavy rain does not inevitably cause flooding. The outcome depends on the terrain, geology, soil moisture and the ability of rivers to carry the water away.
Risk increases when river channels cannot accommodate the volume of water, natural flow paths have been restricted, or settlements and infrastructure have been built in vulnerable locations.
How Greece can reduce flood damage
“We cannot prevent rain, but we can significantly reduce the chances of it turning into a disaster,” Evelpidou said.
She emphasized that effective flood prevention requires a coordinated approach to managing entire drainage basins, updating flood-hazard maps, maintaining drainage networks, protecting natural waterways and providing early warnings.
Restoring wetlands and riverbank areas can also help, while cities can reduce runoff by creating permeable surfaces and spaces where water can be temporarily stored.
According to Evelpidou, nature-based solutions should complement conventional engineering projects as part of a broader prevention strategy. They also underline the importance of preserving enough space for water to move naturally through the landscape.
Planning for the next extreme rainfall event
Recent floods across Greece have demonstrated how natural processes and human activity combine to shape the scale of a disaster.
The country has the scientific knowledge and tools needed to identify flood risks, but they must be put to use before another extreme weather event occurs.
The central question, Evelpidou said, is not which area will be next to flood, but whether cities, settlements and infrastructure can be planned around the way the natural environment works, rather than repeatedly addressing the consequences after disaster strikes.