The 1967 Portugal floods (the "great floods") were flash floods that took place in 25 and 26 November 1967, Portugal, in the Lisbon metropolitan area, affecting a total of 14 municipalities. It was the deadliest flood recorded in Portugal, accounting for more than half of all recorded deaths by floods since 1865. It was also the deadliest natural hazard in Portugal since the 1755 Lisbon earthquake. Background The network of river basins affected by the flood were small (17 with basin areas smaller than 40km²) or medium-sized (Trancão River, Alenquer River, and Grande da Pipa River, all with basin areas ranging from 100 to 300km²). Yet, their natural characteristics (slope, low permeable formations, etc.) enable them to generate flash floods, a risk that some of the affected areas still had in 2005. Extreme poverty and high birth rates had led to a rural flight towards the main cities of Portugal, particularly the Lisbon metropolitan area. Some of these newcomers, lacking money, would build their houses illegally, occupying plains prone to flooding and river banks. Flood details During the night of 25 November 1967, heavy rain poured down. The rain was most intense (>120 mm) in the metropolitan area of Lisbon, particularly in a 60km line between Estoril and Alenquer, but intense rain (>75mm) affected a large area in an axis of South West to North East orientation from Lisbon to the border with Spain. Most of the rain poured in a five-hour interval, between 7 p.m. and midnight. Another factor that increased the impact of the flood that followed was the high tide, which was highest between 6 p.m. and 1 am. The rain led the water levels of the Tagus river near Lisbon and its tributaries to rise 3 to 4 meters, flooding many houses up to level of their first floor. The flood dragged a lot of debris, namely stones and mud, but also wood, tiles and metal from houses and people's belongings. The flood took most people by surprise as they were home or already sleeping and no flood alert system existed at the time. Its negative effects were worse in Odivelas riverside and in the Trancão River margins. The mud dragged away many bodies, so it was difficult to estimate casualties. The official death toll was 495 deaths, unofficial estimates are of at least 700 deaths. Based on available sources (such as newspapers), there were 2045