The main streams in the eastern portion of the State flow through canyons or very narrow, confined mountain valleys and finally into desert canyons. Some meadows, usually in native grass, and only a few small local areas of farmland are subject to overflow. Nearly all the main highways and railroads, as well as residential areas, are above flood levels. Highest flow occurs in the steams in this region in May and June during spring runoff from melting snow.
The most serious floods in Utah have occurred in the Great Lake Basin, particularly in the Weber River drainage on the western slopes of the Wasatch Mountains. During the past 100 years approximately 300 flask floods, resulting from high intensity rainfall accompanying thunderstorms, and 135 snowmelt floods, have been recorded. Some have been very limited in area and extent of damage, while others have been highly destructive in cities, towns and agricultural areas. However, severe floods are not likely to occur in any given locality more than once in several years, or even several decades.