Late Holocene behavior of small drainage basins on the Colorado Plateau: Influences of lithology, basin form, and climate change
— Anne Tillery, Peter Fawcett, Les McFadden, Louis Scuderi, and Joseph McAuliffe

In the semiarid landscape of the Colorado Plateau in northeastern Arizona, small basins associated with the “Blue Gap” escarpment have been rapidly aggrading over the last millennium. The mode of operation of the fl uvial systems in a few sub-basins, however, has changed since the late 19th century from net aggradation to net erosion as shown by the formation of deeply incised, discontinuous arroyos. Detailed study of stratigraphy and soils in the upper reaches of some of the basins, along with radiocarbon dates and dendrochronology, shows that sediment in these portions of the basins, sometimes as thick as 3-4 m, is roughly 1000 years old. Sediment that is older than a millennium is not stored in the upper reaches of the basins. Additionally, the soil-stratigraphic record shows that during this period, aggradation has generally been the consistent mode of channel behavior in this area. The lack of paleochannels shows that until the 19th century arroyos formed, there were no previous episodes of deep channel incision. The observed rapid aggradation is partly attributable to the highly erodible Jurassic sandstones, siltstones, and mudstones that make up the cliffs and slopes of the escarpment, including the Bluff Sandstone of the San Rafael Group, and the Salt Wash Member of the Morrison Formation. The recent change from aggradation to incision in some of the basins could be attributed to the effects of late Holocene climate change. The timing of this change (late 19th century) suggests that the change could be associated with the end of the Little Ice Age (LIA ca.1200-1850AD). The modern geomorphic processes acting in adjacent basins today are inconsistent across the fi eld area. This inconsistency is attributed to the lag time between climate change and basin response, and to subtle variations in basin characteristics such as slope, aspect, and vegetation that would affect surfi cial processes. This study reinforces previous studies (Bull, 1991; McFadden and McAuliffe, 1997) recognizing lithology as a key factor in dictating rates and processes of hillslope sediment production and sediment deposition. In this case, the weakly cemented sandstones of the Blue Gap fi eld area are likely more sensitive to minor climatic changes of the Holocene than are more resistant lithologies.