According to the National Disaster Management Division of the Ministry of Home Affairs, a factor that can lead to floods is rivers flooding their banks because of heavy rains.
Definition of Floods: Their definition explicitly states that "Floods are a temporary inundation of large regions as the result of... rivers flooding their banks because of heavy rains."
Mechanism: When rainfall is intense or prolonged, rivers receive more water than their channels can contain. This excess water then overflows the natural banks, spreading onto adjacent dry land.