Exploring relations between arroyo cycles and canyon paleoflood records in Buckskin Wash, Utah: Reconciling scientific paradigms

Abstract

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 infl uence on the origin and preservation of downstream paleofl ood records. An ephemeral stream featuring an upstream alluvial reach that fl ows into a narrow slot canyon, Buckskin Wash has previously been the subject of important arroyo cut-and-fi ll and paleofl ood studies. We expand upon and reconcile these earlier efforts through detailed sedimentology and a diverse geochronology (radiocarbon, optically stimulated luminescence, treering, and 137 Cs). The alluvial-reach deposits preserve at least four arroyo cut-and-fi ll 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 allu vial valleys during historic arroyo cutting temporarily enhanced preservation of alluvial deposits in the bedrock canyon downstream. Thus, we argue that slackwater deposition in canyons like Buckskin Gulch can be as much a function of upstream geomorphic changes as of fl ood frequency and magnitude. This suggests that some paleofl ood records may be seriously incomplete, and that paleofl ood chronologies could be as much related to the phase of the arroyo cycle as to temporal variability in storm frequency. This fi nding has important implications for studies that utilize paleofl ood records and for our broader under standing of the ways in which dryland fl uvial systems respond to climate change.