The basin covers Loei, Phetchabun, Saraburi, Lop Buri and Ayutthaya provinces.
Ayutthaya Disaster Prevention and Mitigation Office chief Udomsak Kaonoona said heavy rains had forced operators of the Pasak Cholasith dam upstream in Lop Buri to accelerate the dam's release rate into the lower Pasak River.
This outflow had reached the Phra Ram VI dam in Tha Rua district of Ayutthaya, he said.
Mr Udomsak said the large water mass released from the Pasak Cholasith dam forced operators of the Phra Ram VI dam in turn to increase its drainage capacity from 250 to 500 cubic metres per second.
He said this would cause water levels to rise by one metre in the Pasak River, which passes through Tha Rua, Nakhon Luang and Muang districts and meets the Chao Phraya River at Phra Nakhon's Si Ayutthaya district.
Mr Udomsak warned residents along the Pasak to brace for higher water levels.
Tha Rua district chief Withit Pinnikorn said he had warned residents in 60 villages and shop owners in a market along the Pasak to prepare for flash floods and to move their belongings to higher ground.
Meanwhile, flooding killed one resident in Si Sa Ket Saturday.
Pramual Jarunai, 37, from Somboon village in tambon Huay Tai of Khukhan district, drowned after being washed away when floodwaters from the Phanom Dongrak mountains hit his village.
In Ang Thong province, Chao Phraya water levels had almost reached the critical level of 7.5 metres Saturday.
If the upstream Chao Phraya dam in Chai Nat province accelerates its release of water today, Ang Thong's Muang, Chaiyo and Pa Mok districts will be flooded, an official in the Irrigation Office in Ang Thong said.
Heavy rains have also caused flooding in four districts of Prachin Buri province over the past few days Parts of Muang, Kabin Buri, Na Di and Si Maha Phot were under 60cm of water Saturday.
The Prachin Buri-Sa Kaeo road was inundated and motorists were told to avoid this route.
Prachin Buri governor Chitra Promchutima said she had ordered officials in affected districts to set up a coordinating centre to help flood victims.
In Chachoengsao, flash floods hit four villages in Sanam Chai Khet district. More than 100 houses were flooded.
Authorities sent flat-bottomed boats to pick up villagers stranded in their homes.
The Bangkok Metropolitan Administration (BMA) Saturday reported water levels in Khlong Prem Prachakorn and Khlong Song canals in Don Muang, Sai Mai and Laksi districts had increased.
The BMA installed pumps at the ends of the canals to drain water into the Chao Phraya.
The Meteorological Department Saturday said rain would continue to fall in parts of the Central Plains, the East and the Northeast for a few more days.
The monsoon over the Gulf of Thailand and the Andaman Sea would generate waves of 2-4 metres, so small vessels should not leave shore, the department said.