THE ALLUVIAL RECORDS OF BUCKSKIN WASH, UTAH JONATHAN E. HARVEY , JOEL L. PEDERSON , AND TAMMY M. RITTENOUR Department of Geology, Utah State University, Logan, UT [email protected] [email protected] [email protected] ABSTRACT Paleohydrologic records are important for the study of past, present, and future rela- tions 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 be- ing 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. INTRODUCTION past climate changes and stream response and shed light on how future climatic changes may af- Dryland populations are often inherently fect these sensitive systems and the populations intertwined with ephemeral streams that carry they support. floodwaters from catchments to trunk drainages. One of the best ways to understand the paleo- For example, floodwater farming has sustained hydrology of a dryland stream is to study the allu- both prehistoric and historic populations in the vial deposits preserved along it. Indeed, workers southwestern U.S. (Bryan, 1929). Many mod- have utilized alluvial records in the southwest- ern settlements continue to depend on seasonal ern U.S. for nearly a century (e.g. Bryan, 1925). streamflow for crop production and the grazing of Most commonly, workers examine and interpret livestock. This important connection of civiliza- the stratigraphy of valley-fill alluvium exposed in tion to environment has motivated great interest in cutbanks along modern streams. These records the paleohydrology of these streams. Paleohydro- often record cycles of cutting and filling over dec- logic investigations can reveal linkages between ades to millennia (e.g. Bailey, 1935). However, in Geology of South-Central Utah, Stephanie M. Carney, David E. Tabet, and Cari L. Johnson, editors, Utah Geological Association Publication 39, 2010. J.E. Harvey, J.L. Pederson, and T.M. Rittenour UGA 39 recent decades, a newer approach has emerged— Paleoflood Hydrology of Bedrock Canyons paleoflood hydrology of bedrock canyons (Patton and others, 1979). There are important distinc- The second approach, paleoflood hydrology, tions between these two approaches, and they has emerged in only the last few decades. In this often lead to contrasting interpretations regard- approach, workers study sequences of slackwater ing the history of a particular stream. Here we flood deposits in order to characterize the pre-in- describe the important disparities between these strumental flood history of a stream (Patton and two approaches to studying alluvial records and others, 1979; Kochel and Baker, 1982). Workers reconcile them in a single drainage in the western often estimate paleodischarges through a mode- Colorado Plateau. ling exercise that requires estimation of the water surface profile during the flood and the cross-sec- Arroyo Cutting and Filling Cycles tional geometry at the time of emplacement (Webb and Jarrett, 2002). In order to minimize the latter In the first approach, workers study the uncertainty, paleoflood hydrologists generally fo- stratigraphy of valley-fill deposits exposed along cus their studies on slackwater deposits in bed- streams running through broad (> 100-m-wide) rock canyons where lateral channel boundaries alluvial valleys. In this setting, alluvial stratigra- are relatively stable. Importantly, it is commonly phies often record cycles of aggradation and deg- assumed in these settings that aggradation or deg- radation throughout the Holocene (Hack, 1942; radation of the channel bed is negligible over the Haynes, 1968; Hall, 1977). The magnitude of time period of interest. these changes in streambed elevation can reach up The southwestern U.S. has been the epicenter to 30 m in any particular event and can occur over of studies of this type, in part due to the abun- decadal to millennial timescales. The ages of past dance of bedrock canyons with preserved slack- arroyo cutting and filling cycles have primarily water deposits. Like with arroyo cutting-and- been constrained with radiocarbon ages within filling cycles, the ages of particular flood deposits aggradational packages and associations with cul- have been constrained mostly with radiocarbon tural material of known age. The result of these dating. In some cases, additional age constraints studies is a series of stream-specific chronologies have been provided through ring counts on buried of arroyo cutting and filling in the southwestern trees, cultural material caught in flood deposits U.S. For relatively recent reviews of arroyo cut- (e.g. post-settlement fence posts), and short-lived ting and filling cycles in the U.S. Southwest, see isotopes like post-bomb 137Cs (Ely and Webb, Cooke and Reeves (1976), Graf (1983), and Her- 1992). The first regional compilation of paleoflood eford (2002). studies was published by Ely (1997), who identi- Those particular arroyo-cutting or valley-fill- fied several episodes of ‘clustering’ of large floods ing episodes that have been detected and correlat- throughout the Holocene. These clusters were at- ed in many streams across a region are, especially tributed to centennial- to millennial-scale changes recently, interpreted as manifestations of climate in the frequency and magnitude of El Niño events. changes (Knox, 1983; Karlstrom, 1988; Hereford, Such a connection could be very important with 2002). A range of hypotheses have been suggest- regard to how streams in the Southwest might ad- ed regarding the specific mechanisms that link climate change to stream behavior. One frequent- just to changing climate in the future and perhaps ly-cited hypothesis is that arroyos are cut during what climate cycles could have driven arroyo cut- episodes of frequent, high-intensity flooding and ting and filling cycles throughout the Holocene. filled during periods of relatively infrequent and/ Disparity Between Approaches or low-magnitude flooding (Webb, 1985; Webb and others, 1991; Hereford, 2002). One way to test Both the study of arroyo cutting and filling this hypothesis would be to compare a stream’s cycles in broad alluvial valleys and the study of flood history to its cut-and-fill history. slackwater flood deposits in bedrock canyons have 20 The Alluvial Records of Buckskin Wash, Utah been demonstrated throughout the southwestern eral stream featuring a broad alluvial valley that U.S. and drylands throughout the world. Though drains into a bedrock slot canyon. The two end- they are both focused on the interpretation of member reaches feature classic examples of both Holocene alluvial deposits, these approaches dif- record types, both of which have been the sub- fer in fundamental ways (Harvey, 2009). ject of previous research efforts demonstrating One aspect of the disconnect between these the two end-member approaches. We build upon two approaches lies in those reaches that are not these previous efforts with detailed stratigraphy, clearly distinguishable as a ‘broad alluvial valley’ sedimentology, and a multi-pronged geochronol- or a ‘constricted bedrock canyon’. In these reach- ogy. This unprecedented temporal resolution and es, it may not be clear which approach is appro- sedimentological detail allows comparison of the priate. For example, O’Connor and others (1994) timing of paleoflood deposition to arroyo cutting interpreted a stack of deposits along the Colorado and filling cycles upstream, as well as the proc- River upstream of Grand Canyon as a series of esses governing sediment storage and transfer in slackwater paleoflood deposits, and used their either setting. landscape positions to reconstruct discharges of the floods that emplaced them. Just downstream, STUDY AREA Hereford and others (1996) and Tainer (2010) in- terpreted alluvial deposits in similar landscape Physiography positions as terrace remnants from an episode of Buckskin Wash is a major tributary of the aggradation that occurred over a similar times- Paria River in south-central Utah (figure