ALLUVIAL TERRACE RECORDS OF ELEVATED SEDIMENT SUPPLY FOLLOWING VOLCANIC AND WILDFIRE DISTURBANCES IN NORTHEASTERN OREGON

Sediment supplied from small tributary channels can determine trajectories and rates of change following watershed disturbances and affect the formation and stability of valley landforms. I investigated a partial Holocene record of alluvial terrace formation in a recently burned tributary of the North Fork John Day River in Northeastern Oregon. In this region disturbance effects on sediment supply and channels are not well documented and the controls on their long-term distribution are not well constrained. This work relates estimated rapid gully erosion and mapped sediment routing patterns after a recent wildfire to long-term storage and routing patterns of mid-Holocene air fall Mazama ash ~6600 ybp. I found strong correspondence between long-term sediment routing patterns in several 2nd order channels and the erosion and sediment routing patterns following the 1996 wildfire. This observation was based on the presence and absence of pre-existing terraces, hi vs. low ash storage in upland channel initiation areas, and the mapped distribution of post-fire channel initiation processes. The observed correspondance suggests that 2nd order channel dynamics over the late Holocene and after a recent wildfire were both linked to upland topography and dominant erosion processes of 1st order channels. In this area the close proximity (~ 2 km) of distinct long-term sediment routing patterns with high and low sensitivity to disturbances suggests that strong intrinsic controls on local erosion processes determine the pattern of sediment yield after disturbances. Differences in local controls on erosion processes and long-term sediment routing are most likely related to high spatial variability in parent material and topography. It is not known how significant these differences may be for long-term aquatic habitat dynamics and larger channels.