Authorities are downplaying the prospect of further floods, though they warn that as water is discharged from the dams, the river could breach its banks in some areas. Flooding in the Northeast has claimed at least nine lives over the past week.
They are mainly worried about the Dan Prakan Chon dam in Nakhon Nayok, and the Pasak Jolasid dam in Lop Buri, whose capacities have reached up to 94% and 77% respectively.
The Royal Irrigation Department (RID) says it will have to raise the discharge rates of several dams upstream of Bangkok to ease the pressure.
The department has told the Bangkok Metropolitan Administration (BMA) that it will increase discharges from Monday, particularly from the Pasak Jolasid dam and the Chao Phraya dam in Chai Nat province.
The water will be drained into the Central Plains and into the sea, Adisak Khantee, director of the BMA's Department of Drainage and Sewerage said.
The water will have to pass through Ang Thong, Sing Buri, Ayutthaya, Pathum Thani, Nonthaburi and Bangkok on its way there.
The Thai Meteorological Department Tuesday issued a warning that a strong high pressure front from China will cause a strong monsoon trough across the country until Sunday. (Story continues below)
Heavy rain is likely to fall in the Central Plains and the lower Northeast on Saturday and Sunday, lower North and Northeast on Thursday and on Friday, and the North and the upper Northeast over the next day or so.
Royol Chitradon, director of the Hydro and Agro Informatics Institute, said waterflow in the river this year would exceed last year's total, but would still be less than that in 2011 when severe flooding hit the country.
Mr Adisak, of the BMA, said the department will increase flow rates from the dams until early next month and try to keep the supply rate into the Chao Phraya River at 2,200-2,400 cubic metres per second (cm/s).
The river's water-handling capacity is 2,500-3,000 cm/s.
Maitree Pitinanont, irrigation director in Ayutthaya province, said the discharge rate of the Chao Phraya dam could rise to 2,500 cm/s, the critical level at which flooding would occur in high-risk areas, and consequently the level of the Chao Phraya River below the dam would rise by 75-125cm.
Still, RID spokesman Thanar Suwattana insisted heavy flooding will not hit the lower central provinces and Bangkok unless the amount of water released from the Chao Phraya dam is more than 3,000 cm/s.
As of Tuesday afternoon, waterflow at the dam had reached 2,195 cm/s, while the waterflow through Nakhon Sawan province was 1,717 cm/s.
The waterflow at Nakhon Sawan is expected to increase to between 1,800 cm/s and 2,300 cm/s as the coming monsoons will force the Chao Phraya dam to release more water.
Mr Thanar said the department has prepared water retention zones in the northern provinces of Phitsanulok, Sukhothai and Phichit, comprising more than 30,000 rai of paddy fields, to hold water overflow.
He said the department also plans to divert water from the Chao Phraya River to other waterways to the west and east, aimed at reducing the waterflow which will pass through Ayutthaya and lowland areas in northern Bangkok.
On the eastern side, the overflow from the Chao Phraya River will be diverted into the southern Raphiphat canal to Nakhon Nayok province and travel further to the Bang Pakong River and to the sea in Chachoengsao.
In the west, waterflow will be diverted into the Makam Thao-Uthong canal in Suphan Buri before pushing out to the Tha Chin River.
Hanarong Yaowalert, chairman of the Thai Water Partnership, said he was concerned about floodwalls built to protect properties including industrial estates since the big floods hit in 2011.
The barriers can enhance the strength of floodwater passing into nearby unprotected areas as the water has nowhere to go.
"The water will come faster and rise higher, as it cannot flow through to low-lying areas. That means disaster," he said.
Meanwhile, floods triggered by heavy rain have killed at least nine people - six in Surin and three in Si Sa Ket. One person has been reported missing in Surin since Sept 17, while up to 1.5 million people have been affected, the Disaster Prevention and Relief Department says. Flooding has hit 23 of the country's 77 provinces, it said Tuesday.