A new chronostratigraphy for a suite of Holocene alluvial deposits along Buckskin Wash, Utah, shows that dynamic upstream alluvial systems have an important and as-yet-unrecognized influence on the origin and preservation of downstream paleoflood records. An ephemeral stream featuring an upstream alluvial reach that flows into a narrow slot canyon, Buckskin Wash has previously been the subject of important arroyo cut-and-fill and paleoflood studies. We expand upon and reconcile these earlier efforts through detailed sedimentology and a diverse geochronology (radiocarbon, optically stimulated luminescence, tree-ring, and 137Cs).
The alluvial-reach deposits preserve at least four arroyo cut-and-fill cycles since ca. 3 ka. In contrast, the majority of the constricted-reach deposits date to ca. A.D. 1850–1950, coincident with the most recent arroyo-cutting event upstream. These new data show that transfer of sediment from alluvial valleys during historic arroyo cutting temporarily enhanced preservation of alluvial deposits in the bedrock canyon downstream. Thus, we argue that slack-water deposition in canyons like Buckskin Gulch can be as much a function of upstream geomorphic changes as of flood frequency and magnitude. This suggests that some paleoflood records may be seriously incomplete, and that paleoflood chronologies could be as much related to the phase of the arroyo cycle as to temporal variability in storm frequency. This finding has important implications for studies that utilize paleoflood records and for our broader understanding of the ways in which dryland fluvial systems respond to climate change.