By late May, the annual spring meltwater from the Alps meant many Bavarian rivers were already at a high water level. On May 22 the rainstorm Quartus hit the northern Alps, bringing an additional 180 litres/m², causing the Ammersee and the Amper and Isar rivers to expand and flood large areas. These flooded areas expanded quickly covering parts of the Oberallgäu on the river Iller. The flood then continued downstream to some of the more inhabited areas in Bavaria.