Main Channel Response to Holocene Climate CHANGE and Hillslope Sedimentation

Records of episodic channel incision separated by intervals of aggradation or vertical stability are preserved in Holocene and latest Pleistocene terraces in central Idaho. Since last-glacial time, incision is primarily through glacially derived sediments, although some reaches of the modern Payette, Boise, and Salmon River channels are incising into bedrock and prominent high (~40 m) bedrock strath surfaces are present throughout the lower reaches of the Payette drainage. No Quaternary faults are mapped near alluvial (fill) terrace study locations, so intervals of aggradation and incision are likely controlled by climate-driven changes in sediment supply, stream power, and/or the periodicity of stochastic large storm events. Correlations of terrace tread heights combined with ¹⁴C and optically stimulated luminescence (OSL) dates from charcoal and fine-grained terrace sediments of alluvial terraces of the upper South Fork Payette River indicate a mid-Holocene (>6.6-6.1 ka) pervasive fill event followed by a general trend of downcutting during the mid to late Holocene. The Payette average incision rate of ~0.82-0.73 m/kyr from ~7 ka to present is similar to rates of downcutting indicated for the East Fork Salmon River (~0.55-0.65 m/kyr from ~ 7 ka to present) and the Middle Fork Salmon River (0.74 m/kyr over the last 14.5 ka). The deeply incised central Idaho river systems testify to prolonged incision despite apparently low rates of Cenozoic uplift.

Prior studies of Holocene fire-related sedimentation preserved in small tributary alluvial fans provides a measure of sediment delivery from hillslopes to the main channel of the South Fork Payette River. Episodes of vertical stability ~8000-6600, ~4000-1300, and ~1155-540 cal yr BP may correspond with increased fire-related sedimentation, punctuated by brief episodes of downcutting 1293-1155, and <540 cal yr BP. While aggradation may be related to climate-driven variations in sediment delivery, incision may be driven by episodic flood events.