Kristin E. Sweeney

Experimental evidence for hillslope control of landscape scale

Landscape evolution in a sandbox The long-term response of hills and valleys to changes in climate depends on a variety of physical factors. Sweeney et al. performed tabletop erosion experiments as a function of rainfall and uplift: variables that are impossible to precisely control in nature (see the Perspective by McCoy). Ridge and valley spacing are set by the balance of sediment moving down hillslopes or being washed out of valleys by rivers. Landscapes therefore evolve as a response to climate change altering erosion rates. Science, this issue p. 51; see also p. 32 A carefully controlled analog experiment probes the underlying theory of landscape evolution. [Also see Perspective by McCoy] Landscape evolution theory suggests that climate sets the scale of landscape dissection by modulating the competition between diffusive processes that sculpt convex hillslopes and advective processes that carve concave valleys. However, the link between the relative dominance of hillslope and valley transport processes and landscape scale is difficult to demonstrate in natural landscapes due to the episodic nature of erosion. Here, we report results from laboratory experiments combining diffusive and advective processes in an eroding landscape. We demonstrate that rainsplash-driven disturbances in our experiments are a robust proxy for hillslope transport, such that increasing hillslope transport efficiency decreases drainage density. Our experimental results demonstrate how the coupling of climate-driven hillslope- and valley-forming processes, such as bioturbation and runoff, dictates the scale of eroding landscapes.

How steady are steady-state landscapes? Using visible–near-infrared soil spectroscopy to quantify erosional variability

Although topographic steady state is often used as a simplifying assumption in sediment yield studies and landscape evolution models, the temporal and spatial scales over which this assumption applies in natural landscapes are poorly defined. We used visible–near-infrared (visNIR) spectroscopy to measure the weathering of hilltop soils and quantify local erosional variability in two watersheds in the Oregon Coast Range (United States). One watershed appears adjusted to base-level lowering driven by rock uplift in the Cascadia forearc, while the other is pinned by a gabbroic dike that locally slows river incision and hillslope erosion. Models for uniformly eroding hillslopes imply uniform soil residence times; instead, we observe significant variability around the mean value of 18.8 k.y. (+31.2/–11.8 k.y.) for our adjusted watershed. The magnitude of erosional variability likely reflects the time scales associated with stochastic processes that drive bedrock weathering, soil production, and soil transport (e.g., tree turnover). The residence time distribution for our pinned watershed has a mean value of 72.9 k.y. (+165.6/–50.6 k.y.) and is highly skewed with a substantial fraction of long residence time soils. We speculate that this pattern results from the lithologic control of base level and lateral divide migration driven by erosional contrasts with neighboring catchments. Our novel and inexpensive methodology enables us to quantify for the first time the magnitude of erosional variability in a natural landscape, and thus provides important geomorphic context for studies characterizing regolith development. More generally, we demonstrate that soils can record catchment-scale landscape dynamics that may arise from lithologic controls or forcing due to climate or tectonics.

Controls on the geometry of potholes in bedrock channels

Potholes (circular depressions carved into bedrock) are the dominant roughness elements in many bedrock channels. Here we show, using data from previous studies and new data from the Smith River, Oregon, that pothole depths increase in proportion to both the mean pothole radius (such that the most common pothole depth-to-radius ratio is 2) and the diameter of the largest clasts episodically stored in potholes. We present a theory for these observations based on computational fluid dynamics and sediment transport modeling of vortices in cylindrical cavities of different shapes and sizes. We show that the shear stress at the bottom of a pothole (which controls the rate of pothole growth) is maximized for potholes with a depth-to-radius ratio of approximately 1 and decreases nonlinearly with increasing depth-to-radius ratio such that potholes with depth-to-radius ratios larger than 3 are uncommon. Our model provides a mechanistic explanation for pothole shapes and sizes.