The Disaster Prevention and Mitigation Department (DDPM) said the river level began rising on Friday, reaching a depth of more than 6 metres under bridges.
On Saturday morning the river burst its banks and flooded Sailom Joy Market at the Mae Sai-Tachilek border checkpoint. More than 2,000 households were inundated, according to local reports. Some residential areas are under 1 metre of water.
Local officials and military personnel have deployed boats and six-wheel trucks to evacuate people living on the riverbank or stuck in the market. The riverbank has also been reinforced with sandbags to prevent further overflows.
Tachilek border market on the Myanmar side has also been inundated by the overflow.
The DDPM said tropical storm Mulan has triggered flooding in 11 northern provinces since Friday, with 2,312 households in 410 villages inundated. So far, there are no reports of deaths or injuries.
In Chiang Rai alone, floods have hit 1,081 households in 206 villages across eight districts – Khun Tan, Wiang Kaen, Mueang Chiang Rai, Chiang Saen, Chiang Khong, Mae Chan, Thoeng, and Mae Sai.