Major flood warnings have been issued after heavy rains experienced in most of the upstream catchment areas of the Maha Oya basin.
The Department of Irrigation said minor flooding has already been reported in low-lying areas.
Director of Hydrology of the Department SPC Sugeeshwara said major floods can be experienced in Giriulla, Mirigama, Pannala, Wennappuwa, Negombo and Katana during the next few hours.
He urged the public to remain vigilant of rising water levels and noted that the water level of the Maha Oya has already risen extensively following overnight showers.
SPC Sugeeshwara also noted that minor flooding has been reported from high-risk areas in Baddegama with Gin River spilling over.
The Department of Irrigation also said minor flooding is expected in low-lying areas adjacent to the Kelani River during the next six to 12 hours.
Accordingly, flood warnings have been issued to residents of Ruwanwella, Dehiowita, Wattala, Dompe, Biyagama, Kaduwela, Kolonnawa, Seethawaka and Colombo.
The Department said low lying areas of the Attanaglu Oya, Kalu River and Kuda Oya could also experience minor flooding owing to incessant rains.
Accordingly, those residing in low lying areas and along river banks have been urged to remain vigilant of rising water levels.
Director of Hydrology of the Department SPC Sugeeshwara also noted that spill gates of several reservoirs have also been opened following heavy rains.
He said all 20 spill gates of the Thabbowa reservoir have been opened while 10 spill gates of the Rajanganaya reservoir and several spill gates of Angamuwa reservoir have also been opened.
SPC Sugeeshwara noted that therefore several low-lying areas adjacent to Kala Oya and Deduru Oya and a number of areas in the Puttalam districts have been inundated.