The municipality of Nakhon Nayok and resort areas suffered rapid flooding in the early hours of Monday, and residents accused Khun Dan Prakarnchon Dam of releasing water without prior notice.
The Irrigation Department denied the flooding was caused by an announced discharge of water, blaming it on heavy rain upstream.
Several areas along the Nakhon Nayok River in Muang district were inundated, the provincial public relations office reported on Monday, while media outlets showed flooding at resorts near the dam and on the river banks.
Volunteers at Ruam Katanyu Foundation's Nakhon Nakhon branch told Thai PBS and Thairath online they began receiving calls about rising water flooding houses and resorts about 2am.
Resort employee Singha Butamkha was reported as saying that water discharged from the dam started flooding the area after midnight. There had been no advance warning, he said.
About 20 guests had to quickly pack their bags and move to a safe area. Some cars were damaged by the flood, he added.
Royal Irrigation Department director-general Chuchart Rakjit denied any abrupt increase in the discharge rate from the dam. He said the flooding was due to rain in the area behind the dam.
Khun Dan Prakarnchon Dam had been releasing the same amount of water, posing no danger to areas downstream, since Thursday, he said. The department had decided to stop the discharge from the dam on Monday, to alleviate the flood situation, he said.
The Disaster Prevention and Mitigation Department's Nakhon Nayok office on Monday warned people living downstream from the dam that a massive volume of water was on the way and they should move valuable belongings to higher ground. The water level of the Nakhon Nayok River had reached a critical stage, it added.
Khun Dan Prakarnchon Dam releases water into the Nakhon Nayok River, which flows through Nakhon Nayok, Prachin Buri and Chachoengsao provinces before merging with the Bang Pakong River.