Grain size signature of divide migration is restricted to local hillslope scale

Recent work has shown both that drainage divides shift location over geologic timescales in response to contrasts in erosion rates and that fluvial and hillslope grain size is correlated with erosion rate, with faster eroding basins tending to have coarser grain size distributions. However, it remains unclear whether (and how) divide migration might impact grain size distributions in eroding watersheds. Here, we investigate hillslope grain size and soil production in two adjacent watersheds with a twofold difference in erosion rates in the Oregon Coast Range, a humid upland landscape in the western United States. This erosion rate contrast is set by the exposure of a resistant dike at the outlet of one watershed, and is likely to have persisted on the order of 0.5 Myr, based on regional erosion rates and the height of the knickpoint. We collected grain size data from soil pits along transects of the migrating divide itself and from sites further downstream in sub-watersheds that drain the migrating divide. We also measured soil depth and soil production rate across the divide to gain insight into the nature of the subsurface divide. Grain size measurements from the migrating divide show that for sediment < 2 mm, the grain size distribution is nearly indistinguishable on each side, while the faster side of the divide has a larger coarse fraction. This difference becomes more pronounced further from the divide. Grain size data further downstream show that the faster eroding basin has systematically coarser grain size distributions, as expected, but the size-dependent patterns seen at the divide are not evident. Soil depth across the divide reveals that the subsurface divide is offset by a few meters towards the slower-eroding side, such that there are deeper than expected soils near the divide. While our data indicate that divide migration does impact hillslope grain size and soil production, the lack of a clear signal of divide migration further downstream suggests that these effects are localized to the scale of the divide itself