TERRACE RECORDS OF HOLOCENE INCISION, AGGRADATION, AND RELATIONSHIPS BETWEEN HILLSLOPE EROSION AND MAIN CHANNEL PROCESSES IN CENTRAL IDAHO

Well-preserved Holocene terraces along the South Fork Payette River in central Idaho provide a record of episodic channel incision separated by intervals of aggradation or vertical stability. Correlations of terrace tread heights using ¹⁴C dating of charcoal fragments and optically stimulated luminescence (OSL) dating of fine-grained sediments in terrace deposits of the upper South Fork Payette River indicate a general trend of downcutting during the Holocene (mean incision rate ~0.82-0.73 m/kyr from ~7 ka to present), and much of the channel is presently bedrock-dominated. During the middle Holocene, however, the river incised to within at least 3 m of current bankfull, then aggraded by ~5 m over at least ~10 km of channel. This aggradation may relate to (1) increased sediment input from landslides and debris flows in steep, tributary basins in grussy weathered granite, (2) decreased peak discharge, or (3) a combination of these factors.

Channel aggradation ~7-6 ka corresponds with other evidence of widespread aridity in the western U.S. Large fire-related debris flows during Medieval droughts (~900 cal yr BP) correspond with mainstem stability at the 1.5 m terrace level, followed by a brief interval of downcutting ~ 550 cal yr BP which corresponds with a major peak in small frequent fire events and inferred cooler conditions during the Little Ice Age. While channel incision may be driven by either stochastic large storm events or climate-related increases in stream power, major fill episodes likely are more closely tied to climate-driven changes in hillslope sediment delivery and/or a decrease in stream power. Terrace deposit stratigraphy and dating improvements from multiple techniques have revealed that these Holocene terraces do not record simple stepwise incision, as terrace tread heights alone suggest, but include evidence for substantial aggradational episodes as well. This indicates that climate-driven changes in mass movement activity can have substantial impact on Holocene fluvial activity in similar forested mountain drainage basins, with accompanying large changes in short-term incision rates interrupting the postglacial trend of downcutting.