LANDSCAPE EVOLUTION OF WESTERN COLORADO: A TALE OF TWO ARROYOS

Gibbler Gulch and Sieber Canyon – Little Dolores arroyos in Western Colorado, approximately 40 kilometers apart (straight-line distance) in areas of similar elevation, lithology, vegetation, and climate, have similar histories of Holocene incision and infilling. Carbon-14 dating of charcoal in arroyo sediment documents a period of gradual deposition from ~8,000 years BP through ~930 BP. These initial arroyo deposits comprise charcoal-rich, thin-bedded, fine-grained, clayey to silty sand and are up to 21 m thick. A relation between forest/range fires and deposition is inferred from the abundant charcoal found in these beds. First stage of incision started between ~930 and ~570 years BP. A second episode of infilling started after ~570 years BP and resulted in deposition of a few layers of coarser sediment up to 4 m in total thickness. A second episode of incision occurred during the last 100-150 years.

Climate appears to be a first-order control on the timing of incision and deposition. The two arroyos join the Colorado River system more than 100 kilometers apart (along-channel distance), and the Seiber Canyon – Little Dolores system is separated from the Colorado by a major bedrock knickpoint. These facts make simultaneous incision or infilling due to master stream base level changes unlikely. Arroyos elsewhere in the western US have histories similar to the arroyos in this study, indicating that regional climate changes probably strongly affected the timing of infill and incision.

Structure and lithology have influenced the location and geometry of the arroyos. Depth of incision and gradient in both arroyo systems has been strongly controlled by knickpoints in resistant sandstones or crystalline rocks. These knickpoints, some of which appear to be fault controlled, are also present between the arroyos and the master stream.