The Royal Irrigation Department (RID) will increase the discharge rate from the Chao Phraya Dam to make room for the increasing volume of runoff from the North.
A warning of possible flooding is issued for communities downstream.
The department on Tuesday said heavy rain in the upper part of the country caused a huge volume of water to flow into the Chao Phraya River in Nakhon Sawan. The water level in the river rapidly rose.
The department will increase the water discharge rate at the Chao Phaya Dam to accommodate this runoff in the reservoir.
People living downstream should keep a close watch on river levels and news reports, the RID News announcement said.
RID director-general Praphit Chanma said the water station in Nakhon Sawan on Tuesday measured the flow in the Chao Phraya River at 2,521 cubic metres of water per second. It was 1.88 metres below the high mark and rising.
It would be joined by runoff from the Sakae Krang River before flowing into the Chao Phraya Dam reservoir.
The department will take 306 cubic metres of water per second into its irrigation system and it will adjust the discharge rate at the dam to 2,631 cubic metres of water per second.
This will affect riverside communities and water levels in areas outside flood embankments will increase.
Areas likely to be affected are: Ban Tha Sai village and tambon Pho Nang Dam in Sappaya district of Chai Nat province; Wat Sue Kham and Wat Sing In Buri district, Phrom Buri and Muang districts of Sing Buri province; Khlong Phong Pheng canal and Wat Chai Yo in tambon Thewarat of Chai Yo district, and Pa Mok district of Ang Thong province; and Khlong Bang Ban in tambon Hua Wiang of Sena district, tambon Lat Chid and tambon Tha Din Daeng of Phak Hai district in Ayutthaya.
If the water level in upstream areas continues to increase, it will result in a huge volume of water flowing into the Chao Phraya River, the department said.
The department has adjusted its water management plan by releasing water and turning low-lying areas into water retention areas. Water discharge from the Chao Phraya Dam will be increased to accomodate the approaching surge from the North, the department said.
It said updates will be issued.