INFLUENCE OF ARROYO CYCLES ON DOWNSTREAM PALEOFLOOD RECORDS - AN EXAMPLE FROM BUCKSKIN WASH, UT/AZ

Valley-fill terraces run alongside many southwestern streams. When found in broad, alluvial reaches, these terrace deposits are usually interpreted as cycles of streambed aggradation and degradation (i.e. arroyo cutting and filling). However, terrace deposits with similar landscape positions can also be found in constricted bedrock canyons of the same streams, where they are instead interpreted as paleoflood deposits along a channel with stable grade and geometry. Our goal is to reconcile these two contrasting approaches to interpreting dryland alluvial records via focused chronostratigraphic study in a single drainage.

Kitchen Corral Wash flows through a broad, alluvial valley in a continuous arroyo before draining into Buckskin Gulch, a classic slot canyon that eventually meets the Paria River in south-central Utah. Previous research in the catchment includes study of the arroyo dynamics in the alluvial reach as well as a paleoflood study in the narrows of Buckskin Gulch. We employ detailed sedimentology, stratigraphy, and geochronology to build upon these records and evaluate the relation between the two deposit types.

The alluvial reach deposits include at least four aggradational "packages" composed of interfingering tributary and mainstem channel-bottom, channel-margin, and valley-surface deposits. While each package represents several centuries of deposition, they are bound by erosional surfaces representing much shorter episodes of arroyo cutting. In contrast, the constricted reach deposits downstream are strictly slackwater deposits emplaced during two clusters of rapid deposition. Notably, there are no deposits in the constricted reach correlated to the aggradation of the youngest package upstream (~1300 to ~1850 AD). Instead, the bulk of the deposits in the constricted reach span ~1850 to ~1950 AD, broadly coincident with the incision of the alluvial reach during historic arroyo cutting. We argue that upstream incision during arroyo cutting may overwhelm the constricted reach with sediment, leading to temporary sediment storage and altered stage-discharge relations. Thus the preservation of floods is episodically enhanced, leading to a paleoflood record that is not simply a function of paleoflood frequency and magnitude, but is also a function of geomorphic changes upstream.