Medieval flood
© British Library G70076-48"In many places in Poland a great flood, caused by incessant rain in August, has drowned the crops, herds and draught animals." - the year 1368 in the Annals of Jan Dlugosz
Polish researchers examining medieval sources have discovered that the country was hit by flooding 166 times between the 11th and 15th centuries, revealing details on the causes of these disasters.

Their study, published in the Journal of Hydrology, examined flooding within modern-day Poland's borders, focusing on the Oder and Vistula rivers and their basins. They looked for mentions of floods in 164 different sources, including chronicles, administrative records and even private letters. One example would come from the Annals of Jan Dlugosz, who recorded for the year 1475:
Rivers are everywhere low, except in Cracow, where days and nights of rain have caused unprecedented flooding of the Vistula on July 24 and the following three days, when the water rises to the level of the altars in the churches of St. Bernard and St Agnes. The great bridge joining Kazimierz and Cracow is swept away, and the orchards are all destroyed, yet food remains cheap all the rest of the year.

Comment: The above sounds like a report of the Earth Changes seen in the last decade.

The researchers were able to discover several interesting aspects of flooding in medieval Poland, including that the Vistula River was more prone to these disasters compared to the Oder River. Heavy rainfall was the most often-mentioned cause of floods, with others taking place because of snow melt and ice jams.

flood poland
Flood near Toruń - ice jam at the city walls. After a drawing by H. Penner made in 1879.
When comparing periods within the Middle Ages, over 60% of the floods recorded took place in the 15th century, with some years having up to three different floods taking place. The researchers note that the main reason for this is that there are far more sources from the later Middle Ages than earlier centuries. "However," the researchers write, "other important reasons for floods in Poland being most frequently reported in this time are the observed increase in number of settlements and increased deforestation near rivers. From the mid-14th century, the construction of the first, but poorly-built, dikes in the Lower Vistula paradoxically also caused an increase in the number of floods. The reason was the increase in population density in the river valley behind the dikes due to the false sense of security. In reality, however, they were easily and often broken by waters."

The study also tries to evaluate the intensity of these floods as well as whether they were part of events that could be seen in other areas in Eastern Europe.

Even today, floods remain the main form of natural disaster in Poland and can cause billions of dollars in damage. The researchers believe this study can also help deal with this contemporary issue. They write:

These impacts show the vulnerability of different regions worldwide to floods, emphasizing the need to improve the knowledge upon which we assess the frequencies, intensities, and occurrences of floods. Therefore, the long-term study of floods is necessary to comprehend them as natural phenomena and associated with anthropogenic activities.
The article, "The frequency, intensity, and origin of floods in Poland in the 11th-15th centuries based on documentary evidence," by Babak Ghazi, Rajmund Przybylak, Piotr Oliński, Katarzyna Bogdańska and Aleksandra Pospieszyńska, appears in Journal of Hydrology. Click here to read it.