A Chronostratigraphic Record of Arroyo Entrenchment and Aggradation in Kanab Creek, Southern Utah

The purpose of this study is to explore the processes that lead to the formation of arroyos. Arroyos are entrenched stream channels with steep sides that form by incision into valley-fill sediment, and are common features throughout the southwest United States. Many of these systems formed during the late AD 1800s and early 1900s in one of the most significant historic geomorphic events in the region. At this time, former river floodplains were abandoned, creating terraces. This caused a decline in local water tables and associated changes in stream discharge, vegetative communities, and the ability to irrigate once fertile floodplains. Previous researchers and historical observations indicated that these streams incised during a series of large floods, suggesting that periods of arroyo cutting and filling are linked to changes in climate. Understanding what caused these events in the past will aid in better land and stream management in the sensitive landscapes of the arid southwest United States. Field and laboratory work focused on describing and dating sediments from arroyo wall exposures in two reaches of Kanab Creek, southern Utah. The alluvial history of Kanab Creek was reconstructed by combining descriptions of cross-cutting relationships with radiocarbon and luminescence dating of arroyo sediments. This record was then compared to a record from an intermediary reach of Kanab Creek, records from other regional arroyos, and records of past climate to identify similarities in the timing of arroyo cutting and filling, and identify specific climate conditions associated with each. This study finds that climate variability was likely the most important driver of arroyo events over the last ~1,500 years. However, the records are less clear for the preceding 5,000 years, suggesting the importance of geologic thresholds unique to each arroyo. This project was funded by a National Science Foundation grant (NSF-EAR 1057192), research grants from the Geological Society of America and the Society of Sedimentary Geology, and awards from the Utah State University Department of Geology and the Office of Research and Graduate Studies