Townsend et al. 2094 Geological Society of America Bulletin, v. 131, no. 11/12 Anatomy and evolution of a dynamic arroyo system, Kanab Creek, southern Utah, USA Kirk F. Townsend 1,† , Michelle S. Nelson 1,2 , Tammy M. Rittenour 1,2 , and Joel L. Pederson 1 1 Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505, USA 2 Utah State University Luminescence Laboratory, 1770 North Research Parkway, Suite 123, North Logan, Utah 84321, USA ABSTRACT Many alluvial valleys in the American Southwest are entrenched within continuous arroyos, and stratigraphic evidence indicates that these fuvial systems experienced re- peated periods of entrenchment and aggrada- tion during the mid- to late-Holocene. Previ- ous research suggests arroyo dynamics were regionally quasi-synchronous, implying that they were driven by allogenic forcing due to hydroclimatic fuctuations. However, several of these interpretations rely on records with limited age control and include distal correla- tions across the American Southwest. While hydroclimatic variability must exert some role, autogenic mechanisms related to catch- ment-specifc geomorphic thresholds are hypothesized to partially control the timing of arroyo dynamics. If driven by autogenic processes, episodes of arroyo cutting and flling may not be regionally contemporane- ous. Recent improvements in dating methods permit more detailed reconstructions of the timing and evolution of arroyo dynamics, allowing for a more nuanced assessment of these competing hypotheses. Here we present a uniquely large and focused chronostrati- graphic data set from two alluvial reaches of Kanab Creek, located in the Grand Staircase region of southern Utah. Episodes of prehis- toric arroyo cutting and flling are recon- structed from 27 sites through recognition of soils and buttressed unconformities in the arroyo-wall stratigraphy, and age control de- rived from 54 optically stimulated lumines- cence (OSL) ages and 50 radiocarbon ages. Our chronostratigraphic data set indicates fve periods of channel aggradation occurred since ca. 6.0 ka, with each interrupted by an episode of arroyo entrenchment. Repeated aggradation to a similar channel elevation suggests attainment of a threshold profle, and comparison of the pre-entrenchment longi- tudinal profle with the modern arroyo chan- nel demonstrates that changes between end- member entrenched and aggraded states are expressed in channel concavity and slope. We propose that arroyo dynamics are partially driven by sediment supply and the rate of channel aggradation, and that these systems must approach complete re-flling before they become sensitive to incision. Entrenchment it- self appears to be associated with rapid tran- sitions from pronounced decadal-scale arid- ity to pluvial (wetter) periods. Not all such hydroclimatic fuctuations are associated with arroyo entrenchment, which highlights the importance of threshold controls on the be- havior of these systems. The collective period of “dynamic instability” characterized by epi- cycles of arroyo entrenchment and aggrada- tion did not initiate until the mid-Holocene, when a climatic shift toward warmer and drier conditions likely increased fne-grained sediment supply to the fuvial system. INTRODUCTION Alluvial valleys throughout the semi-arid American Southwest are dissected by 5–40-m- deep arroyos, which are entrenched channels characterized by near-vertical walls and fat channel bottoms (Bryan, 1925). Rapid near- synchronous historic arroyo entrenchment in the late-eighteenth century and early nineteenth cen- tury was one of the most signifcant geomorphic events in the region, causing former foodplains to become terraces and leading to a decline in local water tables, changes in riparian vegetation, and altered channel morphology (e.g., Webb and Leake, 2006). Early work and observations of stratigraphic exposures in the newly formed arroyo walls lead to speculations of driving mechanisms and indicated repeated prehistoric entrenchment and aggradation (Bryan, 1925; Bailey, 1935; Antevs, 1952; Judson, 1952). Pio- neering work by Bryan (1941, 1954) and Hack (1942) attempted to correlate fll deposits among drainages through stratigraphic and archaeologi- cal associations, and names for allostratigraphic units produced in these studies (the post-Bonito, Chaco, and Gallo of Bryan, and the Naha, Tsegi, and Jeddito alluvial flls of Hack) are still used for regional correlations and interpretations today (e.g., Hereford, 2002). From the earliest studies on arroyo dynamics, various hypotheses for entrenchment triggering mechanisms have been proposed. These hy- potheses can be grouped into three general cat- egories. (1) Hydrologic and land cover changes associated with European human settlement (Hough, 1906; Reagan, 1924; Antevs, 1952). (2) Allogenic forcings in the form of climate- related changes in vegetative cover and hydrol- ogy (Hall, 1977; Balling and Wells, 1990; Waters and Haynes, 2001; Hereford, 2002, Mann and Meltzer, 2007). (3) Autogenic controls on catch- ment- or reach-specifc geomorphic thresholds (e.g., Schumm and Hadley, 1957; Tucker et al., 2006). Generally, the timing of arroyo entrench- ment and aggradation is seen as a frst-order test of these hypotheses, with regionally syn- chronous arroyo entrenchment typically used as evidence for an allogenic forcing, and re- gionally variable entrenchment argued to be a consequence of autogenic controls. Whereas multiple reconstructions suggesting an apparent near-synchronous regional response have been used to interpret a response to climatic forcing (Hall, 1977; Balling and Wells, 1990; Waters and Haynes, 2001; Hereford, 2002, and others), many of these studies rely on limited or prob- lematic geochronology and inconclusive or po- tentially misleading interpretations due to cor- relation between distant drainages in different climatic regimes (e.g., Waters, 1985; Patton and Schumm, 1975, Hereford, 2002; Harden et al., 2010). To address competing hypotheses be- tween autogenic and allogenic forcing, detailed chronostratigraphic records with high temporal resolution are needed to refne the timing of en- trenchment and aggradation in these dynamic fuvial systems. GSA Bulletin; November/December 2019; v. 131; no. 11/12; p. 2094–2109; https://doi.org/10.1130/B35195.1; 9 fgures; 2 tables; Data Repository item 2019168; published online 2 May 2019. † Present address: Department of Earth and Envi- ronmental Sciences, University of Michigan, Room 2534, 1100 North University Avenue, Ann Arbor, Michigan 48109-1005, USA; kirkft@umich.edu. For permission to copy, contact editing@geosociety.org © 2019 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/11-12/2094/4854590/2094.pdf by Utah State University Libraries, Tammy M. Rittenour on 22 November 2019