The common denominator here is that all such states have been created
near navigable water courses, but above the flood plain, where a flat, arable
plain and perennial streams made wet-rice cultivation possible. It is striking
that none of the early mainland states was located in the delta of a major river.
Such delta regions—the Irrawaddy, the Chao Phraya, and Mekong—were
settled in force and planted to wet rice only in the early twentieth century.
The reasons for their late development, apparently, are that 1) they required
extensive drainage works to be made suitable for rice cultivation, 2) they
were avoided because they were malarial (especially when newly cleared), and
3) the annual flooding was unpredictable and often devastating.25 This bold
generalization, however, needs to be clarified and qualified. First, the political,
economic, and cultural influence emanating from such centers of power, as
Braudel would have predicted, spread most easily when least impeded by the
friction of distance—along level terrain and navigable rivers and coastlines.
Nothing illustrates this process more strikingly than the gradual, intermittent
displacement of Cham and Khmer populations by the Vietnamese. This
expansion followed the thin coastal strip southward, with the coast serving
as a watery highway leading, eventually, all the way to the Mekong Delta and
the trans-Bassac