Joshua J. Roering

Geomorphic Transport Laws for Predicting Landscape form and Dynamics

A geomorphic transport law is a mathematical statement derived from a physical principle or mechanism, which expresses the mass flux or erosion caused by one or more processes in a manner that: 1) can be parameterized from field measurements, 2) can be tested in physical models, and 3) can be applied over geomorphically significant spatial and temporal scales. Such laws are a compromise between physics-based theory that requires extensive information about materials and their interactions, which may be hard to quantify across real landscapes, and rules-based approaches, which cannot be tested directly but only can be used in models to see if the model outcomes match some expected or observed state. We propose that landscape evolution modeling can be broadly categorized into detailed, apparent, statistical and essential realism models and it is the latter, concerned with explaining mechanistically the essential morphodynamic features of a landscape, in which geomorphic transport laws are most effectively applied. A limited number of studies have provided verification and parameterization of geomorphic transport laws for: linear slope-dependent transport, non-linear transport due to dilational disturbance of soil, soil production from bedrock, and river incision into bedrock. Field parameterized geomorphic transport laws, however, are lacking for many processes including landslides, debris flows, surface wash, and glacial scour. We propose the use of high- resolution topography, as initial conditions, in landscape evolution models and explore the applicability of locally parameterized geomorphic transport laws in explaining hillslope morphology in the Oregon Coast Range. This modeling reveals unexpected morphodynamics, suggesting that the use of real landscapes with geomorphic transport laws may provide new insights about the linkages between process and form.

Hillslope evolution by nonlinear creep and landsliding: An experimental study

Landscape evolution models are widely used to explore links between tectonics, climate, and hillslope morphology, yet mecha- nisms of hillslope erosion remain poorly understood. Here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion. In our experimental hillslope, disturbance-driven sediment transport rates increase nonlinearly with slope due to dilation-driven gran- ular creep, and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding. We use spectral analysis to quantify the variability of sediment flux and estimate the slope- dependent transition from creep to landsliding. The power spec- trum of sediment flux steepens with hillslope gradient, exhibiting fractal 1/f scaling just below the creep-landsliding transition. By evolving the experimental hillslope under fixed base-level boundary conditions, we demonstrate how disturbance-driven transport gen- erates hillslope convexity. The transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model, as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep-dominated transport.

How well can hillslope evolution models “explain” topography? Simulating soil transport and production with high-resolution topographic data

The morphology of hillslopes is a direct refl ection of tectonic forcing and climatic and biologic processes that drive soil production, mobilization, and transport. Soil transport on hillslopes affects river incision by providing tools for channel abrasion and controls the distribution of sediment that infl uences aquatic habitat. Although numerous hillslope transport relationships have been proposed over the past 60+ years, a comprehensive analysis of model predictions for a real landscape has not been performed. Here, we use high-resolution topographic data obtained via airborne laser swath mapping (ALSM) to simulate the long-term evolution of Oregon Coast Range hillslopes and test three published transport models and a new model that accounts for nonlinear depth- and slopedependent transport. Analysis of one-dimensional, steady-state solutions for these four models suggests that plots of gradient-curvature may be diagnostic for distinguishing model predictions. To evaluate two-dimensional model predictions for our fi eld site, we assumed local steady-state erosion for a 72,000 m 2 sequence of hillslopes and valleys. After calibrating each of the four models, we imposed constant base-level lowering for cells within the valley network, simulated 500,000 yr of soil production and transport, and determined which transport model best preserved morphologic patterns that describe the current landscape form. Models for which fl ux varies proportionally with hillslope gradient generated broadly convex hilltops inconsistent with the sharp-crested, steep-sided slopes of our study site, whereas the two nonlinear slope-dependent models produced convex-planar slopes consistent with current hillslope form. Our proposed nonlinear slope- and depth-dependent model accounts for how soil thickness controls the magnitude of biogenic disturbances that drive transport; this model best preserved the current landscape form, particularly the narrow, sharply convex hilltops characteristic of the Oregon Coast Range. According to our formulation, which provides an explicit linkage for relating the distribution of biota to hillslope processes, the degree of hilltop convexity varies nonlinearly with the ratio of erosion rate to maximum soil production rate, highlighting the profound infl uence of soil depth on hillslope evolution.

A lithospheric instability origin for Columbia River flood basalts and Wallowa Mountains uplift in northeast Oregon

Flood basalts appear to form during the initiation of hotspot magmatism. The Columbia River basalts (CRB) represent the largest volume of flood basalts associated with the Yellowstone hotspot, yet their source appears to be in the vicinity of the Wallowa Mountains, about 500 km north of the projected hotspot track. These mountains are composed of a large granitic pluton intruded into a region of oceanic lithosphere affinity. The elevation of the interface between Columbia River basalts and other geological formations indicates that mild pre-eruptive subsidence took place in the Wallowa Mountains, followed by syn-eruptive uplift of several hundred metres and a long-term uplift of about 2 km. The mapped surface uplift mimics regional topography, with the Wallowa Mountains in the centre of a ‘bulls eye’ pattern of valleys and low-elevation mountains. Here we present the seismic velocity structure of the mantle underlying this region and erosion-corrected elevation maps of lava flows, and show that an area of reduced mantle melt content coincides with the 200-km-wide topographic uplift. We conclude that convective downwelling and detachment of a compositionally dense plutonic root can explain the timing and magnitude of Columbia River basalt magmatism, as well as the surface uplift and existence of the observed melt-depleted mantle.

Climate controlled variations in scree production, Southern Alps, New Zealand

The interaction of fluvial, glacial, and hillslope processes controls the development of mountain belts and their response to tectonic and climatic forcing. Studies on the contribution of hillslope processes to mountain erosion have focused on bedrock landslides, as they have a profound and readily observed impact on sediment yield and slope morphology. Despite the ubiquity of scree (or talus) mantled slopes in mountainous terrain, the role of frequent, low-magnitude (,100 m 3 ) rockfall events is seldom addressed in the context of landscape evolution. Here we quantify the contribution of rockfall erosion across an 80 by 40 km transect in the Southern Alps, New Zealand, by analyzing the spatial extent of scree slopes mapped from aerial photographs and estimating long-term (10‐15 k.y.) rockfall erosion rates from the accumulation of slope deposits below bedrock headwalls and in debris and alluvial fans. Along the rapidly uplifting, high-rainfall western margin, where high rates of bedrock landsliding have been previously documented, scree-mantled slopes are sparse. Rainfall decreases exponentially east of the Main Divide, and the proportion of slopes mantled by scree increases monotonically, attaining a maximum value of 70%. The systematic distribution of scree deposits cannot be attributed to lithologic variation, seismicity, or the legacy of glaciation. Instead, climate may serve as a primary control on scree production, as nearly 70% of the mapped scree deposits in our transect are confined to a narrow elevation range of 1200‐1600 m above sea level (masl). Our analysis of altitudinal controls on annual temperature variations indicates that scree production via frost-cracking processes may be maximized between elevations of 1600 and 2300 masl, as higher elevations are subject to persistent permafrost which obviates the frost-cracking process. Rates of rockfall erosion near the rapidly uplifting Main Divide are low (,0.1 mm/yr), whereas rates in the scree-dominated eastern areas average 0.6 mm/yr and may approximately balance rock uplift.

Characterizing structural and lithologic controls on deep-seated landsliding: Implications for topographic relief and landscape evolution in the Oregon Coast Range, USA

In mountainous areas, landslides regulate temporal variations in sediment production and may suppress simple linkages between topographic development and tectonic forcing. Rates and mechanisms of mass wasting depend on lithology, bedrock structure, and climatic and tectonic setting. These factors tend to vary signifi cantly in active tectonic regions, thus clouding our ability to predict how landsliding modulates topographic development over human and geological time scales. Here, we use a novel DEM-based technique to document the distribution of large landslides in the Oregon Coast Range (OCR) and quantify how they affect topographic relief. We developed an automated algorithm that exploits the distinctive topographic signature (specifi cally the relationship between curvature and gradient) of large landslides to map their distribution within the gently folded Tyee Formation (Eocene deltaicsubmarine ramp sediments). In contrast to steep and highly dissected terrain frequently identifi ed as characteristic of the OCR (which exhibits steep, planar sideslopes and highly curved, low-gradient ridgetops and valleys), terrain prone to large landslides tends to have low values of both drainage density and curvature and gradient values that cluster between 0.16 and 0.44. The distribution of failure-dominated terrain in our 10,000 km2 study area is infl uenced by systematic variations in sedimentary facies and bedrock structure. The fraction of terrain altered by large landslides (>0.1 km2 ) varies from 5% in the sand-rich (delta-slope and proximal ramp facies) southern section of our study area to ~25% in the north (distal ramp facies), coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone:siltstone ratio. Local relief declines progressively northward, suggesting that deep-seated landsliding is sensitive to the thickness and frequency of low-shearstrength siltstone beds and may serve to limit topographic development in the OCR. Structural controls are superimposed on facies-related variations as deep-seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction. For 1516 strike and dip measurements in our study area, we calculated the fraction of proximal terrain (<2.5 km) altered by deep-seated landsliding. In the sand-rich southern region, the proportion of proximal slide-dominated terrain increases modestly with bedrock dip. In the silt-rich northern region, terrain altered by deep-seated landsliding is pervasive, and an increase in dip from 0° to 16° corresponds to a change in the fraction of slide-prone terrain from 10% to 28%. Our technique for mapping large landslides has utility for hazard analysis and land management. Over million-year time scales, the progradational character of the Tyee Formation suggests that continued uplift and exhumation of the OCR should result in a southward propagation of slide-prone, silt-rich distal facies. As a result, deep-seated landsliding will become increasingly prominent, and topographic relief in the central and southern OCR will progressively decline. Whereas spatial variability in climatic or tectonic forcing is often invoked to explain systematic variations in topographic development, our results emphasize the importance of structural and intraformation lithologic controls on landsliding. As such, analyses linking surface processes, climate, tectonics, and landscapes should be couched in the context of diverse geologic and topographic data.

Fire and the evolution of steep, soil-mantled landscapes

Recent burns in the western United States attest to the signif- icant geomorphic impact of fire in mountainous landscapes, yet we lack the ability to predict and interpret fire-related erosion over millennial time scales. A diverse set of geomorphic processes is often invoked following fire; the magnitude of postfire erosional processes coupled with temporal variations in fire frequency dic- tate the extent to which fires affect sediment production and land- scape evolution. In the Oregon Coast Range, several models for long-term rates of soil production and transport have been tested and calibrated, although treatment of fire-related processes has been limited. Following recent fires in the Oregon Coast Range, we observed extensive colluvial transport via dry ravel, localized bed- rock emergence due to excess transport, and talus-like accumula- tion in adjacent low-order valleys. Soils exhibited extreme but dis- continuous hydrophobicity, and no evidence for rilling or gullying was observed. Using a field-based data set for fire-induced dry rav- el transport, we calibrated a physically based transport model that indicates that soil flux varies nonlinearly with gradient. The post- fire critical gradient (1.03), which governs the slope at which flux increases rapidly, is lower than the previously estimated long-term value (1.27), reflecting the reduction of slope roughness from in- cineration of vegetation. By using a high-resolution topographic data set generated via airborne-laser swath mapping, we modeled the spatial pattern of postfire and long-term erosion rates. Postfire erosion rates exceed long-term rates (which average 0.1 mm·yr 21 ) by a factor of six, and subtle topographic variations generated local patches of rapid postfire erosion, commonly.1 mm·yr 21 . Our sim- ulations indicate that fire-related processes may account for ;50% of temporally averaged sediment production on steep hillslopes. Our analysis provides a mechanistic explanation for the coincident early Holocene timing of increased fire frequency and regional ag- gradation in Oregon Coast Range drainage basins. Given the sen- sitivity of steep hillslopes to fire-driven transport, changes in cli- mate and fire frequency may affect soil resources by perturbing the balance between soil transport and production.

Soil transport driven by biological processes over millennial time scales

Downslope soil transport in the absence of overland flow has been attributed to numerous mechanisms, including particle-by-particle creep and disturbances associated with biological activity. Process stochasticity and difficulties associated with field measurement have obscured the characterization of relevant long-term soil transport rates and mechanisms. In a series of incised fluvial terraces along the Charwell River, South Island, New Zealand, we documented vertical profiles of tephra concentration and topographic derivatives along a hillslope transect to quantify soil transport processes. Along the undissected hilltop, we observed a thin primary tephra layer (ca. 22.6 ka) within loess deposits ∼80 cm below the landscape surface. In the downslope direction, the depth to the highly concentrated tephra layer decreases, coincident with an increase in hillslope convexity (which is proportional to landscape lowering rate if soil flux varies linearly with hillslope gradient). Exhumation of the tephra layer results from landscape lowering due to disturbance-driven soil transport. Approximately 20 m downslope of the interfluve, the depth to the tephra layer declines to 40–50 cm, peak tephra concentrations decrease by a factor of 4, and tephra is distributed uniformly within the upper 40 cm of soil. The transition from a thin, highly concentrated tephra layer at depth to less concentrated, widely distributed tephra in the upper soil may result from soil mixing and transport by biological disturbances. Along our transect, the depth to this transition is ∼50 cm, coincident with the rooting depth of podocarp and Nothofagus trees that populated the region during much of the Holocene. Our observations can be used to calibrate the linear transport model, but, more important, they suggest that over geomorphic time scales, stochastic bioturbation may generate a well-mixed and mobile soil layer, the depth of which is primarily determined by flora characteristics.

Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California

In mountainous landscapes with weak, fine-grained rocks, earthflows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology. To estimate the contribution of earthflows to regional sediment budgets and identify patterns of landslide activity, earthflow movement needs to be quantified over significant spatial and temporal scales. Presently, there is a paucity of data that can be used to predict earthflow behavior beyond the seasonal scale or over spatially extensive study areas. Across 226 km^2 of rapidly eroding Franciscan Complex rocks of the Eel River catchment, northern California, we used a combination of LiDAR (light detection and ranging) and orthorectified historical aerial photographs to objectively map earthflow movement between 1944 and 2006. By tracking the displacement of trees growing on earthflow surfaces, we find that 7.3% of the study area experienced movement over this 62 yr interval, preferentially in sheared argillaceous lithology. This movement is distributed across 122 earthflow features that have intricate, elongate planform shapes, a preferred south-southwesterly aspect, and a mean longitudinal slope of 31%. The distribution of mapped earthflow areas is well-approximated by a lognormal distribution with a median size of 36,500 m^2. Approximately 6% of the study area is composed of earthflows that connect to major channels; these flows generated an average sediment yield of 19,000 t km^(−2) yr^(−1) (rock erosion rate of ∼7.6 mm/yr) over the 62 yr study period, equating to a regional yield of 1100 t km^(−2) yr^(−1) (∼0.45 mm/yr) if distributed across the study area. As such, a small fraction of the landscape can account for half of the regional denudation rate estimated from suspended sediment records (2200 t km^(−2) yr^(−1) or ∼0.9 mm/yr). We propose a conceptual model for long-term earthflow evolution wherein earthflows experience intermittent activity and long periods of dormancy when limited by the availability of readily mobilized sediment on upper slopes. Ultimately, high-order river channels and ephemeral gully networks may serve to destabilize hillslopes, controlling the evolution of earthflow-prone terrain.

Why blind thrust faults do not propagate to the Earth's surface: Numerical modeling of coseismic deformation associated with thrust‐related anticlines

High fault-tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the Earths surface. Seismic profiles of blind-thrust-related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation, such as flexural-slip or extensional fracturing. The complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems. The influence of bedding-plane slip on fault propagation is simulated with numerical models using the boundary element method. We use two parameters to estimate the tendency for thrust fault propagation, (1) the mode II stress intensity factor and (2) the maximum Coulomb stress near the fault tip. Calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the Earths surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip. Our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation.