Wikipedia has a page on this but its "factual accuracy is disputed":
This Chinese famine was directly caused by the 1906 China floods (April–October 1906) ...
Two modern commentaries estimate the total famine deaths in the range of 20–25 million, implying that most of the population of northern Anhui and northern Jiangsu population were wiped out, but offering no explanation on how the calculation is made.
In the "Reports and Accounts, 1910-1911" by the Central China Famine Relief Fund (1912) there is a chapter on "Honan" and another on "Anhui" two of the provinces affected by flood referred to by the OP. The flood seems to be the major focus of the entire report. The report on Honan reads as follows in part:
Right Rev. Wm. C. White, Anglican Bishop in Honan, wrote from Kaifeng, under date of May 2, 1911, as follows: I have just returned from the famine region in the east of the Province... Many a tragic tale had these people to tell of parties dwindled by starvation to one half and even one third the original number. Most of them were still living on roots and bark, the blades of new wheat, and 'kaoliang' straw, and the mortality must be even greater than in the winter, for we occasionally saw bodies in the fields usually being eaten by dogs, and heard gruesome tales of survivors eating the dead bodies of their comrades... A great deal of seed grain is being taken east, but all under armed escort, and we daily heard stories of grain being looted...But Kweiteh is practically only on the fringe of the famine district, the famine centre in Monan being the districts of Yungcheng and Siayi. The Chinese say that one-fifth of the population of Yungcheng have succumbed to the famine. If this is so, and if the census of this district taken last January is approximately correct, it means that the appalling number of over 200,000 have died of starvation.
Throughout the report there are similar eyewitness testimonies to the effects of the flood and the resulting starvation. The estimates of the death toll tend to be specific to various cities or regions (such as the one above) and in the report on Anhui the author writes:
It is impossible to make even an estimate of the number who have lost their lives in this famine. No one, so far as I know, has the data available for such an estimate. Nothing but a careful census, covering large sections of the country, can show how many have perished of famine and of the resulting famine fevers. The death rate has undoubtedly been a large one, notwithstanding the relief that has been given. It would undoubtedly have been very greatly increased had not so much help been sent in. There is not a village without a considerable list of the dead. Many have left home to try and exist by begging in more fortunate centres who have perished by the roadside or have been lost to sight. Families have been scattered, wives and children sold in large numbers, and many have been reduced to a condition of permanent beggary.
