<p>Riverbank filtration is a common technique used in water supply systems, but flooding of the well field and the associated deterioration of water quality are a constant concern. However, well-documented examples of flood effects are scarce. Therefore, several data sets of such a case from the beginning of the 20th century from the city of Breslau (Wrocław), Poland, were brought together and evaluated with modern methods, including quantitative reaction modeling. Overpumping and a drought had led to an extensive aeration of the aquifer and the subsequent oxidation of pyrite present in the fluvial sediments. In March 1906, the acidic reaction products were washed out by a flood, leading to a sudden increase of sulfate, iron, and manganese, the latter released by the reduction of manganese oxide by ferrous iron. The infiltrating cold floodwater decreased groundwater temperatures and elevated microbial counts; however, these effects were quickly and effectively buffered during the underground passage. The events spurred the development of countermeasures, which were widely adapted. Climate extremes such as the described cycle of drought and flood are likely to occur more often in the future in Central Europe. In this regard, the historical data from Breslau serve as a useful example for their potential effects on riverbank filtration systems.</p>