Douglas Creek Revisited - Evidence for Climate-Driven Arroyo Incision in Western Colorado

Douglas Creek, an arroyo system in Rio Blanco County, northwestern Colorado, has deeply incised (10 – 15 m) into alluvial sediments. Geologic mapping of a 3.5 km reach north of the confluence of East and West Douglas Creeks shows four mappable units consisting of three terraces (Qt1 – Qt3) and the modern floodplain. Stratigraphic analysis revealed that all three terraces were inset into an older unit characterized by extensive color mottling, providing evidence of at least four cut and fill cycles during the Holocene.

The chronology of arroyo development in this area is constrained by 14C dating and vegetation development on the terrace surfaces. 14C dates obtained indicate that the initial incision event (identifiable in the field by erosional gravel scour deposits into the mottled unit) occurred approximately 4300 – 4000 cal yr B.P. The timing of the two intermediate incision events is poorly constrained, although another 14C sample is currently undergoing analysis. However, based on the prevalence of tamarisk on the youngest (Qt1) surface, it appears that up to 4.3 m of incision have occurred since the historic period of arroyo cutting commenced in the southwestern U.S. during the late 1800s – early 1900s. This incision amount is comparable with the Little Dolores River, another arroyo system in western Colorado that has been recently studied. Tree-ring data indicate that both areas experienced a transition from drier than normal to wetter than normal conditions around the turn of the century.

Womack and Schumm's 1977 study of the Douglas Creek terraces indicated that incision was due to complex response within the system. However, mottling, tree-ring records, and regional correlation of geomorphic expression in terrace levels indicate that climate fluctuations play a major role in the onset of arroyo incision in western Colorado.