Flash flooding. Block artwork of a mountain valley and river, showing how a sudden heavy downpour of rain from a storm can cause flash flooding. The water channelled by the mountains has caused the river further downstream to burst its banks and flood the surrounding areas. This sudden flooding can occur when a stream of moisture-laden air encounters mountains or hills, and releases its moisture as rain as it rises and cools. Flash floods occur rapidly, usually defined as occurring on a timescale of less than six hours.