From Yangtze: Nature, History, and the River, by Lyman P. Van Slyke (Stanford Alumni Assn., 1988), pp. 63-64:
The Ch’ing conquest and subsequent rule illustrates another type of migration in China, one in which opposite ends of the Yangtze Valley were key locales. Thus far, we have discussed what might be called “expansive” and “consolidative” migrations, the former referring to the outward movement of large numbers of Chinese into territory that they had not previously settled, rather resembling the westward expansion of the white race across the North American continent. Consolidative migration fills in the spaces left behind expansive migration.
A third sort of movement, recurrent throughout Chinese history but particularly visible during the Ch’ing dynasty, can be termed “replacement” migration. In this case, a settled population is so decimated by famine, disease, war, or rebellion—or some combination of these—that serious underpopulation results: few are left to cultivate the fields, while cities are ravaged and largely deserted. Into this partial vacuum come new populations, replacing the dead and often surpassing earlier population levels. The history of Szechwan province demonstrates both consolidative and replacement migration.
Today, Szechwan is China’s most populous province, consisting of 110,000,000 people. If Szechwan were an independent nation, it would possess the seventh largest population in the world, exceeded only by the rest of China, India, the USSR, the USA, Brazil, and Japan. Since early times, Szechwan has always been a part of China proper, yet even late into China’s imperial history, the Szechwan Basin and the upper Yangtze region were only lightly inhabited. About A.D. 1640, with the Ming dynasty about and the Manchu conquest about to roll over China, Szechwan contained perhaps five million people.
For the next forty years, warfare became a way of life in Szechwan. First came the depredations of a murderous invading warlord, then the ruthless Manchu armies determined to crush all resistance. By about A.D. 1680, the province was finally “pacified,” but only about two million survived; one is reminded of Tacitus’s sad comment on the Roman conquest of Britain, “They make desolation, which they call peace.” The richest areas of the regional core were worst affected, and whole cities were turned into ghost towns.
During the following three centuries, Szechwan came to be populated far beyond its previous highs by steady immigration from other parts of (as well as from natural increase): 9 million in 1750, 27 million in 1850, 64 million in 1980.