Analysis of Hillslope Erosion Rates Using Dated Colluvial Deposits

Hollows are sites of long-term deposition of colluvium and contain a valuable record of the timing and rates of hillslope erosion. A model is presented that uses measured depositional rates of colluvium in hollows to calculate transport rates of colluvium and bedrock lowering rates on the surrounding slopes. This model has been applied to eight dated colluvial deposits in the Clearwater River basin of western Washington. The calculated average transport rate is about 34 cm³/cm/yr, and the calculated average bedrock lowering rate is about 0.04 mm/yr, equivalent to an average sediment yield of 90 t/km²/yr for the Holocene. There is significant variability in erosion rates between sites, which may be due to local lithologic variations, although the average values are similar to an independent estimate of modern erosion rates for this area. Multiple stratigraphic levels have been dated in five deposits and document that the deposits are cumulative and that deposition has been active through the Holocene; no systematic change in bedrock lowering rates over time is apparent. Most basal dates from the hollows are early Holocene in age, a time of warmer and drier climate, suggesting a period of widespread evacuation of colluvium and increased sediment yield.