The Central China Floods of 1931
The deadliest natural disaster in history is likely the Central China floods of 1931. In July and August of that year, the Yangtze River overtopped its banks as the spring melt mingled with heavy rains. (The Yellow River and other large waterways also reached high levels.) According to "The Nature of Disaster in China: The 1931 Yangzi River Flood" (Cambridge University Press, 2018), the flood inundated almost 70,000 square miles (180,000 square km) and turned the Yangzi into what looked like a giant lake or ocean.
Estimates of the overall death toll vary. Contemporary government numbers put the number of dead at around 2 million, but others, including NOAA, say it may have been as many as 3.7 million people.