Abstract

Paleohydrologic records are important for the study of past, present, and future relations among streams, climate, and humans in drylands. Alluvial deposits are often the best paleohydrologic record available. Two main approaches to studying dryland alluvial records are 1) the study of valley fills exposed along streams in broad alluvial valleys and 2) the study of slackwater paleoflood deposits in constricted bedrock canyons. Despite often being demonstrated on different reaches of the same streams, these two approaches can lead to contrasting paleohydrologic interpretations. We reconcile these two approaches and record types in Buckskin Wash, an ephemeral stream in the Paria River basin of south-central Utah that features a broad alluvial reach draining into a constricted bedrock canyon. We report a new chronostratigraphy supported by detailed sedimentology and diverse geochronology. The alluvial-reach deposits preserve at least four cycles of arroyo cutting and filling since ~3 ka. The majority of slackwater flood deposits in the slot canyon appear to be correlated to historic arroyo cutting (~ A.D 1880 to A.D. 1910) in the alluvial reach upstream. We argue that constricted reach deposits do indeed relate to arroyo cutting upstream, but that they reflect a sedimentary, not hydrologic, signal. Large-scale transfer of sediment from alluvial valleys during arroyo cutting temporarily enhanced preservation of alluvial deposits in the bedrock canyon downstream via altered stage-discharge relationships. Thus the bulk of the slackwater deposits in Buckskin Gulch are a function of upstream geomorphic changes rather than simply a record of flood frequency and magnitude. This result has important implications for those workers who rely on similar slackwater deposits to extend the flood history of a stream.