Good question, @Fluke! Since many attempts, over many decades at taming the overflowing rivers are as good to fail, rivers having only the one uncontrollable source, i.e. rain, and having only the one uncontrollable outflow, i.e. the sea, I can only see a total change of direction being necessary.
If river-levels cannot be lowered, then building and infrastructure levels must be made higher - i.e. Thailand needs raising - and the prospect of that task even has me scratching my head . . . yes, seriously!