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Floods have been the most deadly natural disasters in Portugal during the last century, followed by earthquakes. The type of flood known as a ‘progressive flood’ mainly affects the larger basins, such as that of the Tagus River, and results in a large inundated area. These floods are caused by heavy rains associated with a westerly zonal circulation that may persist for weeks. The system of dams within the basin reduces the frequency of flooding, but cannot ‘tame’ the river. The dam system has even contributed to an increase in the peak flow, as in the 1979 flood. Nevertheless, these floods are not a danger for the human population. In contrast, flash floods are more dangerous and deadlier than progressive floods, as demonstrated in 1967 and 1997. They affect the small drainage basins and are caused by heavy and concentrated rainfall, created by convective depressions (active cold pools or depressions caused by the interaction between polar and tropical air masses), active in the south of the country, in the Lisbon region, Alentejo and the Algarve. Deforestation, soil impermeability, chaotic urbanization, building on floodplains, the blockage of small creeks or their canalisation, and the building of walls and transverse embankments along the small creeks all contribute to the aggravation of this kind of flood.