The deadliest natural disaster in history occurred in the summer of 1931 when the Yellow River, the Yangtze River and the Huai River all flooded past their banks in central China resulting in the deaths of millions directly or indirectly. China had suffered through an extended drought 1928 to 1930 which was followed by an extreme winter of heavy snowstorms and blizzards starting in late 1930. Intense rains that were far above average started in the spring of 1931 and increased into the summer as the Yangtze River experienced 24 inches in July alone. Adding to the turmoil were nine cyclones hitting the region over those months when two was the average in a year. The combination of the snow thawing and the heavy rains contributed to the devastating flooding.
The Yangtze River officially peaked on August 18. The next day the city of Hankou recorded water levels passing 53 feet or 16 meters above normal. The Yellow and Huai were also breaking their banks at the same time. The Yangtze and Huai Rivers were connected by the Grand Canal and the flood waters rushed into the canal and on August 25 the dikes on Gaoyou Lake were washed away and over 200,000 people drowned in their sleep.
Once the Huai River broke its banks the flood waters raced toward Nanjing which was 230 miles or 370 km away. Nanjing was the capital of China at the time. The floods drowned between several hundred thousand to over one million. Millions more died in the aftermath due to famine and disease.
The flooding from the three rivers had destroyed the rice fields. That led to famine causing many people to starve to death. There were reports that some people had resorted to cannibalism. The polluted rivers also contributed to many others dying from cholera, dysentery and typhoid. The death toll from all factors was between 3.5 million to 4 million.