INVESTIGATING THE HILLSLOPE AND IN-CHANNEL DRIVERS OF TRANSPORT CAPACITY IN VARIABLE LITHOLOGY: A CASE STUDY IN THE OREGON COAST RANGE

In some settings, steep slopes result in strongly linked hillslope and in-stream processes. In such settings, coarse sediment and large wood directly enter streams, which may collectively influence channel geometry and the routing of sediment downstream by fluvial processes. Therefore, to predict channel geometry and sediment transport at the landscape scale, we require an improved understanding of hillslope–channel coupling. To this end, we investigate longitudinal patterns of channel geometry in two adjacent watersheds of the Oregon Coast Range with different underlying lithology (volcanic and sedimentary) that may have different hillslope and in-channel processes. We conducted field surveys in 40 reaches measuring channel geometry and slope along with surface and subsurface grain size, and in-channel large wood volume. We coupled these observations with geospatial analysis of 1-m LiDAR to quantify how transport capacity varies as a function of hillslope gradient and curvature, local relief, valley width, and in stream large wood.

Preliminary analysis of LiDAR indicated strong physiographic differences between basins. While both have the same overall range of hillslope gradients, the distribution for the basalt basin is left skewed with a large proportion of slopes above 30 degrees. In contrast, the gradient distribution in the sedimentary basin is more uniformly distributed. As yet sediment fining trends are evident in the sedimentary basin but interrupted by sudden increases of coarse material from tributaries impacted by debris flows. Indeed, the coarsest material in this basin were observed within a debris flow confined reach. Grain sizes returned to an expected level relative to the rate of downstream fining 200m downstream of the reach. These preliminary results suggest that the relationship between hillslope processes and channel adjustment could be dependent on lithology. The quantification of these feedbacks is relevant for extending current network models of sediment transport to explicitly include the influence of lithology and hillslope processes.