LINKING THE HOLOCENE FLUVIAL HISTORY OF THE UPPER ESCALANTE RIVER, S. UTAH TO REGIONAL RECORDS USING OSL AND RADIOCARBON

In 1909, the upper Escalante River in south-central Utah began to incise during a series of large floods to form the ~35 km long and <5 - 15 m deep arroyo present today. Around the same time (late 1800s – early 1900s), other semi-arid catchments in the region also experienced arroyo entrenchment. This nearly synchronous and economically devastating event prompted early intense study of the “arroyo problem” and the development of several hypotheses related to the causes of past entrenchment events.

The current research project along the upper Escalante River was designed to test existing hypotheses regarding the factors driving degradation and aggradation in these systems by examining the alluvial stratigraphy and developing a chronology of Holocene arroyo cycles. Alluvial packages were dated using both accelerator mass spectrometry (AMS) radiocarbon and optically stimulated luminescence (OSL) dating in order to constrain the timing of arroyo cycles. A total of 18 radiocarbon samples and 21 OSL samples were collected. Nine of the OSL samples were analyzed using small aliquots of sand (1-mm diameter), and the other 12 were analyzed using single-grain dating given their young age and characteristics indicative of incomplete bleaching. OSL ages obtained show good agreement with independent age control.

The chronostratigraphic record developed here suggests that arroyo cut and fill cycles became an important agent of landscape evolution after 5 ka. Since that time, at least five (possibly six) cycles have occurred, with evidence for 5-6 distinct aggradational packages, and incision occurring at approximately 4 ka, ~ 2.7 – 2.4 ka, ~ 1.9 – 1.5 ka, ~ 1.0 – 0.8 ka, possibly ~ 0.4 ka, and the historic period of incision which began in 1909.

Similarities and dissimilarities between this newly developed record and other regional fluvial chronologies will be discussed. Additionally, existing regional paleoclimate records will be discussed in regard to potential forcing mechanisms for arroyo entrenchment in the region.