Over the past 50 years rice cultivation in Asian deltas has undergone impressive intensification. In the Chao Phraya Delta, Thailand, traditional deepwater and floating rice varieties have gradually been replaced by high-yield varieties (HYVs). However, until recently a floodplain of 300,000 ha was still cultivated with traditional varieties. We analyze how these varieties came to be replaced by single/double cropping of HYVs and describe the new water regime that has been established, taking the Bang Kum drainage basin as an illustration. While the shift brought a welcome increase in rural incomes, we show how the change in water management has had serious negative cross-scale consequences that have not been considered. We end by discussing the government’s plan to alter the water regime in the area again, as part of its “monkey cheek” (flood retarding) policy. We show that rather than enhancing management flexibility and the floodplain’s buffering capacity, as it claims, this policy in fact undermines both.