ARROYO CUT-FILL DYNAMICS IN SOUTHERN UTAH, CHRONOLOGY AND CAUSAL LINKAGES

In the late 1800’s to early 1900’s AD many streams in the semi-arid southwestern US abruptly entrenched into their alluvial floodplains and formed 5-30m deep steep-walled arroyo channels. Historic arroyo entrenchment is considered one of the most significant geomorphic changes in the region and led to lowering of alluvial aquifers, loss of original wet-meadow and riparian habitat, loss of agricultural and municipal lands and infrastructures, and in some cases, the abandonment of early settlements. While initial speculations suggested that channel entrenchment was due to overgrazing and channel diversions, stratigraphic relationships exposed in arroyo walls indicated natural causes for past episodic entrenchment. Despite over a century of investigation into alluvial records of past arroyo entrenchment, a clear linkage to a causal mechanism has not been determined.

This research examines the arroyo cut-fill chronostratigraphy of five adjoining catchments in the Grand Staircase region of southern Utah to help better understand the processes of arroyo aggradation and the timing of entrenchment in relationship to regional hydroclimatic forcing, erosion rates and autogenic drivers. Age control for the alluvial deposits come from AMS radiocarbon dating of charcoal (n=120) and single-grain OSL (optically stimulated luminescence) dating (n=110). Results indicate four to five periods of aggradation followed by entrenchment during the past 5-6 ka. High bedrock erosion and flashy discharge are proposed to have fueled mid-Holocene aggradation and instability that lead to episodic arroyo entrenchment. Individual entrenchment events occurred during high-discharge events once aggradation brought the systems to a threshold in slope related to increase convexity.