Swelled by two days of heavy rain, the Mae Lao River overflowed into Wiang Pa Pao district, cutting its road link between Chiang Mai and Chiang Rai.
Highway 118 running south from Chiang Rai to Chiang Mai was also reduced to one lane by flooding.
Homes and farms in Wiang Pa Pao’s Ban Hong were inundated by the river runoff.
Wiang Pa Pao district chief Banchong Khunphet said 40 communities were hit by the flash floods, though there were no reports of injuries or deaths so far.
The local administrative organisation was assisting flood victims and cooperating with police to facilitate traffic, he said.
Elsewhere, the Chan River burst its banks and flooded markets and communities in Chiang Rai's Mae Chan district. More than 300 homes were flooded, with retailers and residents forced to move their belongings to higher ground.
Pheu Thai MP Laong Tiyapairat led a team to survey the flood damage and assist victims, while locals reinforced the riverbank in a bid to prevent more overflows. The police have closed the area to traffic until the rain eases and water levels recede.
Heavy rain also caused a landslide in Chiang Rai's Mae Saruay district, burying a home in Ban Huai Nam Khun village. Many roads in the area have also been cut. Heavy equipment is being deployed to clear traffic and assist victims.