Civilization and Capitalism 15th-18th Century, Volume I, The Structures of Everyday Life, Fernand Braudel, translated by Sian Reynolds. 1981:-
The water [in paddies] is muddy—which is a good thing, as mud restores the fertility of the soil and does not suit the malaria-carrying mosquito. The clear water in hills and mountains, on the other hand, favours mosquitoes; so the ladangs and ray are regions of endemic malaria and therefore of limited demographic growth. In the fifteenth century Angkor Wat was a thriving capital, with rice-fields irrigated by muddy water. Siamese attacks were not themselves responsible for its destruction; but they threw daily life and agriculture into confusion. The water of the canals cleared and malaria triumphed, and, with it, the invading forest. Similar dramas seem to have occurred in seventeenth-century Bengal. If the rice-field was too small and flooded by adjacent clear water, the destructive onslaught of malaria was unleashed.