LATE HOLOCENE CLIMATE VARIABILITY AND IMPLICATIONS FOR THE ONSET OF ARROYO INCISION ALONG THE LITTLE DOLORES RIVER, WESTERN COLORADO

The influence of climate fluctuations on the Holocene arroyo history of the Little Dolores River, a tributary of the Colorado River in the northern Uncompahgre Plateau of western CO, was analyzed using geologic mapping, stratigraphic sections, 14C dating, and GPS transects. Although arroyos have been studied extensively in the southwestern U.S., western CO arroyos have received little attention. As evidenced by the historic period of arroyo cutting, which occurred between ~1880 and 1920, arroyos are prone to rapid and devastating change, and can have major impacts on human activities such as agriculture. As the population of the southwest grows, an understanding of arroyo development is essential for future land-use planning.

Geologic mapping reveals a deeply incised arroyo with two Holocene fill terraces bounded upstream and downstream by Precambrian bedrock valleys. The presence of two fill terraces suggests that the arroyo was cut in two phases. Qt2, the older terrace, has a maximum thickness of 20 m and is prevalent throughout the arroyo. Qt1, which is up to 7.5 m thick, is not as widespread. Using 14C dating, initial arroyo incision (up to 10 m) began between 1430 and 1310 AD. Age estimates suggest that incision was rapid, similar to the 1880 to 1920 event. Subsequent arroyo incision of up to 9 m most likely occurred in the historic era.

Since faults do not displace Holocene fill terraces and bedrock canyons buffer the river from downstream influences such as base-level change, climate fluctuations may have triggered arroyo incision. Compilations of tree ring analyses reveal that there have been at least five transitions from drier to wetter than normal conditions over the past 1000 years. Both episodes of arroyo incision coincide with such transitions, but during other dry to wet transitions, apparently no arroyo incision occurred. Wildfires and/or large paleoflood events may also have facilitated arroyo incision. Fill terraces contain abundant charcoal. The prehistoric period of arroyo incision, which occurred between ~1400 and 1200 AD, corresponds with up to three paleofloods on the Colorado River. An historic flood estimated at approximately 300,000 cfs occurred on the main stem of the Colorado River in 1884, around the time that the historic period of arroyo cutting began.