HOLOCENE ARROYO HISTORY OF THE LITTLE DOLORES RIVER, WESTERN COLORADO

Geologic mapping, stratigraphic sections, 14C dating, and GPS transects were used to interpret the Holocene arroyo history of the Little Dolores River, a tributary of the Colorado River in the northern Uncompahgre Plateau of western CO. Although arroyos have been studied extensively in the southwest, western CO arroyos have received little attention.

Geologic mapping reveals a deeply incised arroyo with two Holocene fill terraces bounded upstream and downstream by two Precambrian bedrock valleys. Structure plays a major role in shaping the arroyo, with fill terraces that thin upstream, and thicken downstream of isolated Precambrian knickpoints. Two knickpoints are associated with faulting. Overall, the fill terraces, which consist of bedded silty-sand with minor gravel lenses, along with widespread charcoal fragments, decrease in thickness downstream. Qt2, the older terrace, with a maximum thickness of 20 m, is prevalent throughout the arroyo. Qt1, which is up to 6 m thick, is not widespread downstream. Qt2 sediments began to accumulate around 10,000 cal yr BP and incision began at 1000 cal yr BP. Following incision of Qt2, a second fill and cut cycle formed the Qt1 fill terrace, which is inset into Qt2.

Sediment supply and alluvial sediment thicknesses increase at the confluences of the Little Dolores and its major tributaries. The formation of large alluvial fans at several of the tributary mouths has deflected the river towards bedrock canyon walls located opposite from the fans. Recent fires suggest that fans are the likely source of charcoal in the fill terraces.

The fact that faults do not displace Holocene fill terraces, and that the bedrock canyons buffer the river from downstream influences such as base-level change, points to the major factor in the formation of this arroyo being climate. Preliminary dates of Qt2 incision coincide with an increase in high magnitude and high frequency floods in the southwest U.S.