Floods caused through overflowing rivers result from heavy rains or from the melting of winter snow, or from both. Floods in rivers differ from flash floods in their extent and duration. Flash floods are of short duration in small streams, although floods in rivers take place in river systems whose tributaries might drain large geographic areas and encompass several independent river basins.
Floods on large river systems might continue for periods ranging from a few hours to several days. Flood flows in large river systems are influenced primarily through variations in the amount, intensity and distribution of precipitation. The condition of a ground - amount of soil moisture, seasonal variations within vegetation, depth of snow cover, and imperviousness because of urbanization - directly affects runoff. Silting, soil conditions, absorption capacity of the watershed, and the capacity of streams to carry runoff have an effect on the extent of the flooding.