Breach at Cucca
The so-called breach at Cucca (Italian: rotta della Cucca) traditionally refers to a flood in the Veneto region of Italy that should have happened on October 17, 589 according to the chronicles of Paul the Deacon. The Adige river overflowed after a "deluge of water that is believed not to have happened after the time of Noah"; the flood caused great loss of lives, and destroyed part of the city walls of Verona as well as paths, roads and large part of the country in lower Veneto.
Contemporary historians think that the breach never really happened, and the tradition simply refers to the disasters due to the lack of maintainment of the streams that followed the fall of the Roman Empire. The Lombards did not repair the banks, and the waters of the Adige had been let free to flow through the lower Veneto for centuries, in order to set a swamp on the borders with the Exarchate of Ravenna.
This point of view should be balanced against the worldwide disastrous climate changes of 535-536. Even though the dates do not exactly align, it is a fact that in that century there was at least "one year without summer", it is conceivable that the exceptionally bad weather conditions reported worldwide for that unknown year, whose consequences included skipped harvests and famine in places as far apart as Ireland, Scandinavia and China, constitute the real background also for this reported climate disaster.
The hydrography of the lower Veneto had a dramatic change after the breach at Cucca: the river Adige did no more pass through Montagnana and Este and diverted to the south passing through Legnago; centuries later, as the land dried up, it started following what had been the former course of the Chirola canal, passing through Badia Polesine and Cavarzere.
The Tartaro river contributed to the swamp; as the land dried up, some villages started to be set around its course: they were the first hamlets of Lendinara, Villanova del Ghebbo, Rovigo and Villadose.
The Mincio river diverted to the south and has become a tributary of the Po river since then; it had been a waterway from the Adriatic Sea to the lake Garda until then. The loss of this last significance contributed to the definitive decline of Adria and its port.