What Causes the Floods in September

Monsoon Season + Heavy Rainfall
September falls in the middle of the monsoon season for northern Thailand, so Chiang Mai often gets very heavy rain. The persistent rainfall raises river levels, especially in the Ping River, which runs through the city

River Overflow & Dam Releases
When upstream dams are full, they may release water, contributing to the rising water levels downstream.

Watershed Saturation
Some experts point to saturated watersheds upstream (due to earlier rains) as a factor, meaning the land can't absorb much more water, so more of it runs off into rivers. 

Flash Floods from Hills and Mountains
Areas near Doi Suthep and other hills can contribute to flash flooding when rainwater rushes down into lower-lying urban districts.