Environmental and grassroot groups opposing the government's 350-billion-baht water management scheme have called on the South Korean government to support public monitoring of the scheme.
South Korean firm Korea Water resources Cooperation, or K-Water, is taking part in the scheme, having won the bid for the most lucrative project.
Eleven organisations, gathered under the name of the River Basins Network, sent an open letter to South Korean President Park Geun-hye via the South Korean embassy in Bangkok on Sunday, asking Seoul to disclose information about K-Water.
K-Water has been selected by the government to build a flood diversion channel, or floodway, covering almost 300km across seven provinces.
The firm also won a bid to develop flood retention areas, known as monkey cheeks, covering over a million rai of land, the network said.
The South Korean government is the company's major shareholder.
"The projects will inevitably create impacts on the environment and local communities," the network said in its letter.
"Transparency, comprehensive impact assessments and mitigation measures need to be ensured at an early stage of the project.
"We would like to inform you that any company or state agency implementing such a project will be monitored [by the public] to ensure accountability."
Somkiat Khuenchiangsa, a coordinator of the Northern River Basins Network, said K-Water is likely to face opposition from locals as thousands of families would lose their land to make way for the floodway.
Mr Somkiat also warned that the project would worsen the problem of saltwater intrusion in the Chao Phraya River as a large amount of water would be diverted from the river to the new floodway.
He said his network would draft its own water management plan and submit it to Prime Minister Yingluck Shinawatra for consideration.
"Local people in provinces that will be affected by the 350-billion-baht water management scheme will work together to deliver the proposal to the government," he said.
The plan, which is expected to be completed within three months, will be based on local wisdom and needs.
The locally designed scheme would not cause negative impacts on communities and ecological systems, he said.