INVESTIGATION INTO THE TIMING OF DOWNCUTTING, DOUGLAS CREEK ARROYO, RANGELY, RIO BLANCO COUNTY, COLORADO

Douglas Creek in northwestern Colorado is a classic example of an arroyo system seen in many arid to semiarid areas of the southwestern United States. This arroyo system has incised into the alluvial floor of the valley by as much as 15 m. Mapping of terraces below the confluence of East and West forks of Douglas Creek has identified five mappable units including four terraces, and an older mottled alluvial unit that the terrace units are inset against. These five units provide evidence of at least five cycles of incision and aggradation.

Hayden (2008) determined that arroyo cutting in Douglas Creek was initiated as late as 4000 cal yrs. B.P., whereas a study by Womack (1975) determined that the initial incision of the arroyo began in 1882. For this study new OSL age dates were processed to help clarify the timing of the erosional events that created Douglas Creek’s arroyo system.

This study found four major incision events that follow a similar pattern to both local and regional arroyo development. The incision events occurred youngest to oldest: ~1880-1940 AD, ~1500-1600 AD, ~1000-1100 AD, ~2000-4000 BCE. These four major events are responsible for creating the four mappable units in the Douglas Creek arroyo system.

These four events are contrary to conclusions previously published by Womack and Schumm (1977) that stated incision events occurred after in introduction of cattle grazing in the area. This study suggests that arroyo incision of the Douglas Creek area pre-dates prehistoric settlement and cattle grazing.