Coupling the New with the Old: High-resolution Elevation Data and Radiocarbon Dating Yield Holocene Erosion Rates for Steep Mountainous Catchments

Abstract:

We present methods for normalizing fan aggradation time-series by coupling radiocarbon dating of alluvial fan deposits with analysis of high resolution elevation data, which yield catchment-averaged denudation rates for small, steep catchments within the semi-arid montane Dry Creek Experimental Watershed, located within the Idaho Batholith of the Northern Rockies. Alluvial fan deposits are useful for understanding past landscape response to fire, and relationships among erosional processes and climate variability. However, although radiocarbon dating of alluvial stratigraphy provides a time-series of fan aggradation, which reflect catchment erosion rates, fan deposition is also influenced by catchment size and depositional confinement, which complicates comparisons of aggradation rates among catchments. Modern high resolution elevation data facilitate measurement of fan and catchment geometries, which we use to convert fan aggradation rates into catchment erosion rates. Airborne LiDAR point clouds were iteratively processed to produce optimal 1 and 0.5 m resolution elevation data for delineation of landforms. Alluvial fan 3-D surface areas were reconstructed by fitting conical surfaces through fan surface remnants. Aggradation time-series were converted to volumetric deposition by extrapolating surface-normal sample depths across reconstructed fan surfaces. Normalizing volumetric deposition by the 3-D surface areas of respective source catchments yielded catchment-averaged denudation time-series, with rates ranging from 0.001 to 0.016 mm/yr during the Holocene for a range of catchments of various size, aspect, and vegetation cover. These erosion rates span time periods between 500-8,000 cal. yr. BP, and are intermediary to short-term (10-84 yr) rates of 0.001 to 0.01 mm/yr and long-term (5,000-27,000 yr, and 10 m.y.) rates of 0.02 to 0.15 mm/yr previously established in the region. This suggests the method is viable for measuring erosional time-series for catchments in steep mountainous terrain, where the dominant erosional processes are related to wildfire and produce, and are recorded in, alluvial fan deposits. This work will facilitate ongoing comparisons of fire and erosional time-series among catchments of varying size, relief, elevation, aspect, and vegetation cover.