The main reason could be because China is so dam catchment is used for the benefit of agriculture. Industry and energy production to Thailand, and the country is also a customer.
The main reason is probably because the Chinese dams to store water for use for the benefit of agriculture, industry and power generation to distribution. And Thailand, as well as customers.
The main reasons may be because China dam to store water for use for the benefit of the farmer industry, industry and electric power to sell And Thailand's customers as well.