Channel avulsion on alluvial fans in southern Arizona

Abstract

Historical aerial photographs and field observations on five fluvially dominated alluvial fans in southern Arizona demonstrate that channel avulsion invariably occurs where bank heights are low and often at channel bends. Channel abandonment occurs through stream capture when overland flow from the main channel accelerates and directs headward erosion of smaller channels heading on the fan surface. Five distinct channel morphologies observed on the fans are related to different stages of the avulsion process and can be used to identify areas on a fan surface that are prone to avulsion. A descriptive model of channel avulsion illustrates how the morphology of a single channel reach will evolve through time as it captures the main flow path and is itself eventually abandoned. Immediately following avulsions, small preexisting channels that capture flow from the main channel will typically experience three fold or greater increases in channel width. Subsequent large floods can be stably conveyed through these high-capacity reaches. An uninterrupted sequence of sediment-charged small flows, however, will eventually begin to back-fill the wide channels as vegetation growth stabilizes the banks. The stabilized and back-filled channels are now prone to abandonment during large floods because the decrease in the channel's capacity leads to the generation of overland flow beyond the margins of the shallowed channels. The action of the small aggrading floods is critical in the avulsion process since the greatest amount of overland flow is generated where bank heights are lowest. As a result, both small and large floods are effective agents of landscape change on the fans. Channel avulsions on the five fans are not completely random events in space and time because their occurrence is controlled by the relative positioning of low banks along the main channel and smaller channels draining the fan surface. Consequently, the location and timing of future channel avulsions can potentially be anticipated in an effort to improve flood hazard assessment on fluvial fans in the rapidly urbanizing southwestern United States.