Geomorphologic analyses and surficial geologic mapping provide information about the age and type of alluvial deposits on piedmonts that is critical in assessing the character of piedmont landforms and the nature and extent of piedmont flood hazards. Piedmonts in Maricopa County are covered by complex mosaics of surficial deposits with different physical characteristics related to the ages of the deposits. Surficial geologic maps differentiate alluvial deposits based on physical characteristics of the deposits (sediment size and character) and geomorphic surface characteristics associated with the deposits. Differences in the primary physical characteristics of surficial deposits result from differences in rock types in drainage basins, distance from uplands, and differences in the size and character of the stream system that transported the sediment. Surficial deposits are subsequently altered by processes of weathering, inputs of fine dust from the atmosphere, soil development, and local erosion, so the character of the surface and nearsurface portion of the deposits is related to the length of time that the deposits have been exposed at the surface. Geologically young deposits on piedmonts record relatively recent activity of piedmont fluvial systems; laterally extensive young deposits are indicative of widespread flood inundation, shifts in flooding patterns, or both, in the past few thousand years. Thus, surficial geologic maps are very useful in defining the physical framework of active fluvial systems on piedmonts, and in particular are critical in evaluating the potential for alluvial fan flooding.