The river Tisza is the longest river in Hungary, it has its source in the Eastern Carpathians and it meets the Danube flowing through Serbia. The regulation of rivers was the biggest undertaking in the 19th century. It started in the 1840s on István Széchenyi’s initiative following the plans of Pál Vásárhelyi, a water-engineer. The river became navigable, its dried flooding territory was transformed into agricultural areas. Besides advantages there appeared disadvantages, as well. The water of a river flows at a greater speed in a straight bed, so floods are more frequent. The biggest tragedy in the history of Szeged is the Great Flood in 1879. It almost completely destroyed the medieval city. The actual structure of the town with boulevards and avenues is a result of the reconstruction following the flood.