Reconciling arroyo cycle and paleoflood approaches to late Holocene alluvial records in dryland streams

Abstract

A century of research into the environmental records of alluvial-valley fills in drylands has led to new theories about landscape response to climate change and cultural evolution over the Holocene. In a largely separate line of inquiry, paleoflood hydrologists have scoured bedrock canyons for slackwater deposits, extending the flood record and exploring their climatic significance. Both approaches rely on the analysis and dating of late Holocene alluvium, sometimes along the same drainages, yet they differ in the geomorphic setting in which they should be applied. Studies of arroyo cut-and-fill cycles are focused on broad alluvial valleys, whereas paleoflood hydrology has been focused mainly in narrow bedrock canyons. With a focus on the southwestern U.S., we review and compare the fundamentals of these two approaches and their paradigmatic disconnect, and then discuss potential linkages that could lead to a more complete understanding of how dryland streams adjust to external stimuli. Recent regional compilations provide insight into the broader relation between the two record types over entire physiographic provinces. Meanwhile, new tools such as OSL dating, short-lived isotopes, and improved hydraulic modeling are paving the way for refinement and reconciliation of the two approaches within individual drainages. The relation between past arroyo cut-and-fill cycles and paleofloods must be thoroughly explored if we are to fully understand how drainages will respond to a changing climate over the coming decades to centuries.