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The magnitude of flood‐runoff and the degree to which it concentrates in river‐channels with respect to time is known to vary within wide limits. To a considerable extent these variations relate to the physiographic and edaphlc features of the drainage‐basins as they have been developed by the geologic and climatic history of the particular province in which they are located. Although storms of high intensity occur throughout the United States, the runoff therefrom has been observed to have certain inherent characteristics in wet or humid areas that are different from those in arid or semiarld regions even though the laws governing the flow of water either over ground or in river‐channels are universal in application. Essential features of flood‐behavior in wet or humid areas where stream‐flow is sustained throughout the year and river‐channels and valleys are comparatively well defined and permanent, differ from those in arid or semiarld regions where stream‐flow is very erratic and flashy and the channels are formed largely by occasional intense runoff rather than the slow and orderly development by the continuous occupancy by flowing water. Moreover the flood‐problem in areas where a part of the precipitation occurs as snow differs from that in areas where all the precipitation occurs as rain. Furthermore, in areas where snow does occur there seems to be a wide range in flood‐runoff characteristics depending upon the temperature in and altitude of the particular zone. Some of the different flood‐characteristics as they relate to total runoff and the concentration thereof are described herein and possible reasons therefor are related to conditions which seem to be more or less inherent in particular physiographic and climatic provinces.

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