If we discount pandemics and famines and focus instead on sudden environmental disasters, one of the worst, if not the worst, of all time was the China Floods of 1931.
These were a series of floods that occurred in the Republic of China after a period of long drought. Sudden abnormal weather occurred over central china with heavy winter snowstorms, a sudden spring thaw and heavy rains. This had the effect of causing river levels to rise dramatically. The rainfall increased over July and August and was accompanied by extreme cyclone activity. There were seven cyclones in July alone, compared to a previous average of 2 per year. The widespread destruction and flood damage rocketed the death toll to between145000 to four million, with many dying from waterborne diseases such as typhoid and cholera. The situation became so desperate that wives and daughters were sold, and there were even cases of infanticide and cannibalism. The highest water level was measured as exceeding 53ft (16 m) above normal.