RELATIONSHIPS BETWEEN MAIN CHANNEL INCISION AND INCREASED SEDIMENT YIELDS FOLLOWING FOREST FIRES: IS CLIMATE THE DRIVER?

Well-preserved Holocene and latest Pleistocene terraces along the South Fork Payette and Salmon Rivers in central Idaho provide records of episodic channel incision separated by intervals of aggradation or vertical stability. In central Idaho, despite the deeply incised river valleys, rates of Cenozoic uplift are low (Sweetkind and Blackwell, 1989) and no Quaternary faults are mapped in areas proximal to alluvial (fill) terrace study locations. Intervals of aggradation and incision, therefore, are likely controlled by climate-driven changes in sediment supply, stream power, and/or the periodicity of stochastic large storm events. Preliminary data suggests correspondence between increased sediment yields from tributary basins and hillslopes following fire, and intervals of main channel incision.

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 (SFP) show a general trend of downcutting during the Holocene with an average incision rate of ~0.82-0.73 m/kyr from ~7 ka to present. At the confluence of the East Fork and main stem Salmon Rivers in central Idaho, OSL dates from a suite of fill terraces indicate incision rates of ~0.55-0.65 m/kyr from ~ 7 ka to present, while a ¹⁴C age of ~15280-14670 cal yr BP from a 14 m high bouldery terrace (previously mapped as a likely LGM-aged fill terrace) indicate incision rates of ~1 m/kyr.

Comparing intervals of Holocene incision of the South Fork Payette (SFP) with episodes of fire-related sedimentation in tributary channels provides a link between the timing of main channel stability and erosion and the timing and processes of hillslope erosion. Brief intervals of downcutting ~ 6600-5800, 1293-1155, and after 542 cal yr BP punctuate episodes of stability and floodplain widening ~8000-6600, ~4000-1300, and ~1155-540 cal yr BP. Incision 1293-1155 cal yr BP corresponds with a peak in fire-related sedimentation events ~1300-1150 cal yr BP, and likely increased sediment input to the main channel from hillslopes. Incision in the last ~500 yr corresponds with frequent (but likely less severe) fires ~500-300 cal yr BP and recent severe fires and large floods and debris flows in tributary basins.