The legacy of initial conditions in landscape evolution

Landscapes subject to constant forcing tend to evolve toward equilibrium states in which individual landforms have similar characteristics. Yet, even in landscapes at or near equilibrium, there can be significant variability among individual landforms. Furthermore, sites subject to similar processes and conditions can have different mean landform characteristics. This variability is often ascribed to on‐going transient evolution, or to heterogeneity in processes, material properties, forcing, or boundary conditions. Three surprising outcomes of landform evolution models suggest, however, that such variability could arise in equilibrium landscapes without any heterogeneity in the physical processes shaping the topography. First, homogeneous systems subjected to constant forcing can generate a heterogeneous distribution of equilibrium landforms. Second, even simple non‐linear systems can have multiple stable equilibrium states. Third, evolving landscapes can exhibit path dependence and hysteresis. We show how these three mechanisms can produce variability in landforms that arises from the characteristics of the initial topographic surface rather than from heterogeneity in geomorphic processes. Numerical experiments on the formation of low‐order fluvial valleys and transportational cyclic steps in erodible streambeds illustrate why it is important to consider the influence of initial conditions when comparing models with natural topography, estimating the uncertainty of model predictions, and studying how landscapes respond to disturbances. Copyright © 2011 John Wiley & Sons, Ltd.     

The headward extent of fluvial landforms and associated vegetation on massanutten mountain,Virginia

Variation in fluvial landforms and associated vegetation in the headward (upstream) direction has received little study and the controlling factors are not well understood. The relations among channel gradient, basin area, stream order, and the headward extent of fluvial landforms and vegetation was studied in 18 small basins and larger nearby stream reaches in the Massanutten Mountain area, northern Shenandoah Valley, Virginia. Low-order streams were traversed to their basin heads. Notice was made of the point or region of the disappearance of fluvial landforms. Indicator species were used to confirm landform identification. The studied landforms include the channel bar, channel shelf, floodplain, and terraces. Basin geomorphic characteristics were determined from topographic and geologic maps and ground surveys. Results suggest that gradient is the most important factor controlling the development of fluvial landforms. Floodplains have not developed along stream reaches where average channel gradients exceed 0.15. Channel shelves and associated vegetation occur farther upstream than floodplains.     

Erosion of an active fault scarp leads to drainage capture in the Amazon region,Brazil

Far from the continental margin, drainage basins in Central Amazonia should be in topographic steady state; but they are not. Abandoned remnant fluvial valleys up to hundreds of square kilometers in size are observed throughout Amazonia, and are evidence of significant landscape reorganization. While major Late Miocene drainage shifts occurred due to initiation of the transcontinental Amazon River, local landscape change has remained active until today. Driven either by dynamic topography, tectonism, and/or climatic fluctuations, drainage captures in Amazonia provide a natural experiment for assessing the geomorphic response of low‐slope basins to sudden, capture related base‐level falls. This paper evaluates the timing of geomorphic change by examining a drainage capture event across the Baependi fault scarp involving the Cuieiras and Tarumã‐Mirim River basins northwest of the city of Manaus in Brazil. A system of capture‐related knickpoints was generated by base‐level fall following drainage capture; through numerical modeling of their initiation and propagation, the capture event is inferred to have occurred between the middle and late Pleistocene, consistent with other studies of landscape change in surrounding areas. In low‐slope settings like the Amazon River basin, base‐level fall can increase erosion rates by more than an order of magnitude, and moderate to large river basins can respond to episodes of base‐level fall over timescales of tens to hundreds of thousands of years. Copyright © 2013 John Wiley & Sons, Ltd.     

Evolutionary creativity in landscapes

In biological evolution, creativity occurs in the appearance of new entities by evolutionary dynamics. This is linked to mutations and genetic drift, which cannot occur in geophysical phenomena. Biota can exhibit evolutionary creativity that influences landforms, but how does creativity (defined here as the capacity for emergence of new entities that increase the adjustedness of the landscape to environmental conditions) occur in landforms and landscapes as entities independent of biota? Creativity in geomorphic evolution does not require any sort of goal functions or purposeful innovation – just that geomorphic development is capable of producing novelties that may be better adapted (more efficient or durable) than predecessors. Independently of biota, evidence exists that landforms may develop to become more or less ‘fit' in terms of efficiency and/or durability. Thus, emergence of novel features may lead to their persistence. Emergence of novel forms is illustrated for the case of karst sinkholes (dolines), which indicates increasing geomorphic diversity over Ma and Ga timescales. A case study of fluviokarst chronosequences in Kentucky demonstrates emergence and elimination of landforms as landscapes evolve. Some of these may represent generally (as opposed to locally) novel landforms. While this article is more suggestive than demonstrative, results strongly suggest evolutionary creativity in geomorphology both tied to, and independent of, biological evolution. This occurs due to emergence of geomorphic entities that are subject to selection that tends to increase efficiency and durability. © 2019 John Wiley & Sons, Ltd.     

Hydrologic response to valley‐scale structure in alpine headwaters

Few systematic studies of valley‐scale geomorphic drivers of streamflow regimes in complex alpine headwaters have compared response between catchments. As a result, little guidance is available for regional‐scale hydrological research and monitoring efforts that include assessments of ecosystem function. Physical parameters such as slope, elevation range, drainage area and bedrock geology are often used to stratify differences in streamflow response between sampling sites within an ecoregion. However, these metrics do not take into account geomorphic controls on streamflow specific to glaciated mountain headwaters. The coarse‐grained nature of depositional features in alpine catchments suggests that these landforms have little water storage capacity because hillslope runoff moves rapidly just beneath the rock mantle before emerging in fluvial networks. However, recent studies show that a range of depositional features, including talus slopes, protalus ramparts and ‘rock‐ice’ features may have more storage capacity than previously thought. To better evaluate potential differences in streamflow response among basins with extensive coarse depositional features and those without, we examined the relationships between streamflow discharge, stable isotopes, water temperature and the amplitude of the diurnal signal at five basin outlets. We also quantified the percentages of colluvial channel length measured along the stepped longitudinal profile. Colluvial channels, characterized by the presence of surficial, coarse‐grained depositional features, presented sediment‐rich, transport‐limited morphologies that appeared to have a cumulative effect on the timing and volume of flow downstream. Measurements taken from colluvial channels flowing through depositional landforms showed median recession constants (K_r) of 0.9–0.95, δ¹⁸O values of ≥?14.5 and summer diurnal amplitudes ≤0.8 as compared with more typical surface water recession constant values of 0.7, δ¹⁸O ≤ ?13.5 and diurnal amplitudes >2.0. Our results demonstrated strong associations between the percentage of colluvial channel length within a catchment and moderated streamflow regimes, water temperatures, diurnal signals and depleted δ¹⁸O related to groundwater influx. Copyright © 2014 John Wiley & Sons, Ltd.     

How long is a hillslope?

Hillslope length is a fundamental attribute of landscapes, intrinsically linked to drainage density, landslide hazard, biogeochemical cycling and hillslope sediment transport. Existing methods to estimate catchment average hillslope lengths include inversion of drainage density or identification of a break in slope–area scaling, where the hillslope domain transitions into the fluvial domain. Here we implement a technique which models flow from point sources on hilltops across pixels in a digital elevation model (DEM), based on flow directions calculated using pixel aspect, until reaching the channel network, defined using recently developed channel extraction algorithms. Through comparisons between these measurement techniques, we show that estimating hillslope length from plots of topographic slope versus drainage area, or by inverting measures of drainage density, systematically underestimates hillslope length. In addition, hillslope lengths estimated by slope–area scaling breaks show large variations between catchments of similar morphology and area. We then use hillslope length–relief structure of landscapes to explore nature of sediment flux operating on a landscape. Distinct topographic forms are predicted for end‐member sediment flux laws which constrain sediment transport on hillslopes as being linearly or nonlinearly dependent on hillslope gradient. Because our method extracts hillslope profiles originating from every ridgetop pixel in a DEM, we show that the resulting population of hillslope length–relief measurements can be used to differentiate between linear and nonlinear sediment transport laws in soil mantled landscapes. We find that across a broad range of sites across the continental United States, topography is consistent with a sediment flux law in which transport is nonlinearly proportional to topographic gradient. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Badass geomorphology

The archetypal badass is individualistic, non‐conformist, and able to produce disproportionate results. The badass concept is applied here to geomorphology. The individualistic concept of landscape evolution (ICLE) is introduced, based on three propositions: excess evolution space, capacity of all landforms to change, and variable selection pressure from environmental factors within and encompassing landscapes. ICLE indicates that geomorphic systems are idiosyncratic to some extent, and that even where two systems are similar, this is a happenstance of similar environmental selection, not an attractor state. As geomorphic systems are all individualistic, those that are also non‐conformist with respect to conventional wisdoms and have amplifier effects are considered badass. Development of meander bends on a section of the Kentucky River illustrates these ideas. The divergence of karst and fluvial forms on the inner and outer bends represents unstable amplifying effects. The divergence is also individualistic, as it can be explained only by combining general laws governing surface and subsurface flow partitioning with a specific geographical and environmental setting and the history of Quaternary downcutting of the Kentucky River. Landscape evolution there does not conform to any conventional theories or conceptual frameworks of geomorphology. The badass traits of many geomorphic systems have implications for the systems themselves, attitudes toward geomorphic practice, and appreciation of landforms. Badass geomorphology and the ICLE reflect a view, and approach to the study of, landforms as the outcome of the interplay of general laws, place‐specific controls, and history. Badass geomorphology also implies a research style receptive to contraventional wisdoms. Aesthetically, amplifier effects and individualism guarantee an essentially infinite variety of landforms and landscapes that geoscientists can appreciate both artistically and scientifically. Non‐conformity makes the interpretation and understanding of this variety more challenging – and while that increases the degree of difficulty, it also makes for more interesting and compelling professional challenges. Copyright © 2014 John Wiley & Sons, Ltd.     

The use of a landscape simulator in the validation of the SIBERIA landscape evolution model: transient landforms

SIBERIA is a physically based model for the geomorphic evolution of landforms. It is essential that the SIBERIA model be tested or validated against controlled landform development. Previous studies have demonstrated that SIBERIA is able to simulate declining equilibrium landforms and in this paper we examine SIBERIA's ability to simulate landforms as they evolve to their declining equilibrium form. These landscapes are termed transient landforms. Landscapes generated by SIBERIA were compared to those produced by a physical model (experimental model landforms) at stages of evolution. Comparison of the experimental landscapes with the simulated landscapes using total mass, hypsometric curve, width function, cumulative area distribution and area–slope demonstrate that SIBERIA can simulate the experimental model landscape during development (i.e. transient landscapes). Copyright © 2002 John Wiley & Sons, Ltd.     

Unraveling the roles of asymmetric uplift,normal faulting and groundwater flow to drainage rearrangement in an emerging karstic landscape

Landscape adjustment to tectonic, lithologic and climatic forcing leads to drainage reorganization and migration of divides. The respective contribution of these forcings, especially on carbonate landscapes is not well defined. Here, we have addressed this issue by combining field observations, satellite image interpretation and digital elevation model (DEM) quantitative analysis to assess drainage response to spatially heterogeneous rainfall, asymmetric uplift, and normal faulting on an emerging carbonated platform (Sumba Island, Indonesia). We map geomorphic markers of fluvial dynamics and drainage rearrangement and compute a χ parameter that incorporates the contributions of unevenly distributed precipitation and asymmetric uplift to estimate erosional disequilibrium across drainage divides. We find that asymmetric emergence of Sumba Island created an initial parallel drainage, asymmetric across a divide that propagates landwards. Soon after establishing itself on the emerging slopes this drainage was disturbed by normal faulting, which has become the main force driving drainage rearrangement. Vertical offsets across normal fault scarps first triggered aggradation within valleys over the hanging walls, and then disconnected upstream reaches from downstream reaches, leading to the formation of wind gaps atop the fault scarps and upstream perched sedimentary basins. The defeat of rivers by growing fault scarps was catalysed by the possibility for surface water to be rerouted near the fault scarps into underground water networks inside the underlying carbonates. At the end of the process, the opposite drainage across the main water divide captured the struggling drainage. Capture mechanisms include initial groundwater capture of the perched alluvial aquifers, followed by ground sapping at the head of the opposite drainage and surface stream diversion by avulsion. Finally, normal faulting is the main driving force of drainage rearrangement allowing avulsion and karstic rerouting whereas asymmetric uplift and climate forcings have shown a low efficiency. The role of karstification is more ambiguous, catalysing or inhibiting drainage rearrangement. Copyright © 2018 John Wiley & Sons, Ltd.     

Coupled numerical–analytical approach to landscape evolution modeling

The Earth's topography is shaped by surface processes that operate on various scales. In particular, river processes control landscape dynamics over large length scales, whereas hillslope processes control the dynamics over smaller length scales. This scale separation challenges numerical treatments of landscape evolution that use space discretization. Large grid spacing cannot account for the dynamics of water divides that control drainage area competition, and erosion rate and slope distribution. Small grid spacing that properly accounts for divide dynamics is computationally inefficient when studying large domains. Here we propose a new approach for landscape evolution modeling that couples irregular grid‐based numerical solutions for the large‐scale fluvial dynamics and continuum‐based analytical solutions for the small‐scale fluvial and hillslope dynamics. The new approach is implemented in the landscape evolution model DAC (divide and capture). The geometrical and topological characteristics of DAC's landscapes show compatibility with those of natural landscapes. A comparative study shows that, even with large grid spacing, DAC predictions fit well an analytical solution for divide migration in the presence of horizontal advection of topography. In addition, DAC is used to study some outstanding problems in landscape evolution. (i) The time to steady‐state is investigated and simulations show that steady‐state requires much more time to achieve than predicted by fixed area calculations, due to divides migration and persistent reorganization of low‐order streams. (ii) Large‐scale stream captures in a strike‐slip environment are studied and show a distinct pattern of erosion rates that can be used to identify recent capture events. (iii) Three tectono‐climatic mechanisms that can lead to asymmetric mountains are studied. Each of the mechanisms produces a distinct morphology and erosion rate distribution. Application to the Southern Alps of New Zealand suggests that tectonic advection, precipitation gradients and non‐uniform tectonic uplift act together to shape the first‐order topography of this mountain range. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of bedrock structures on the spatial pattern of erosional landforms in small alpine catchments

Structural settings and lithological characteristics are traditionally assumed to influence the development of erosional landforms, such as gully networks and rock couloirs, in steep mountain rock basins. The structural control of erosion of two small alpine catchments of distinctive rock types is evaluated by comparing the correspondences between the orientations of their gullies and rock couloirs with (1) the sliding orientations of potential slope failures mechanisms, and (2) the orientation of the maximum joint frequency, this latter being considered as the direction exploited primarily by erosion and mass wasting processes. These characteristic orientations can be interpreted as structural weaknesses contributing to the initiation and propagation of erosion. The morphostructural analysis was performed using digital elevation models and field observations. The catchment comprised of magmatic intrusive rocks shows a clear structural control, mostly expressed through potential wedges failure. Such joint configurations have a particular geometry that encourages the development of gullies in hard rock, e.g. through enhanced gravitational and hydrological erosional processes. In the catchment underlain by sedimentary rocks, penetrative joints that act as structural weaknesses seem to be exploited by gullies and rock couloirs. However, the lithological setting and bedding configuration prominently control the development of erosional landforms, and influence not only the local pattern of geomorphic features, but the general morphology of the catchment. The orientations of the maximum joint frequency are clearly associated with the gully network, suggesting that its development is governed by anisotropy in rock strength. These two catchments are typical of bedrock‐dominated basins prone to intense processes of debris supply. This study suggests a quantitative approach for describing the relationship between bedrock jointing and geomorphic features geometry. Incorporation of bedrock structure can be relevant when studying processes governing the transfer of clastic material, for the assessment of sediment yields and in landforms evolution models. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of globally available,coarse-resolution digital elevation models for delineating valley bottom segments of varying length across a catchment

The worldwide availability of digital elevation models (DEMs) has enabled rapid (semi-)automated mapping of earth surface landforms. In this paper, we first present an approach for delineating valley bottom extent across a large catchment using only publicly available, coarse-resolution DEM input. We assess the sensitivity of our results to variable DEM resolution and find that coarse-resolution datasets (90 m resolution) provide superior results. We also find that LiDAR-derived DEMs produce more realistic results than satellite-derived DEMs across the full range of topographic settings tested. Satellite-derived DEMs perform more effectively in moderate topographic settings, but fail to capture the subtleties of valley bottom extent in mild gradient, low-lying topography and in narrow headwater reaches. Second, we present a semi-automated technique within ArcGIS for delineating valley bottom segments using DEM-derived network scale metrics of valley bottom width and slope. We use an unsupervised machine-learning technique based on the k-means clustering algorithm to solve a conundrum in GIS-based geomorphic analysis of rivers: the delineation of valley bottom segments of variable length. The delineation of valley bottom segments provides a coarse-scale entry point into automated geomorphic analysis and characterization of river systems. © 2020 John Wiley & Sons, Ltd.     

Preserved landscapes underneath the Antarctic Ice Sheet reveal the geomorphological history of Jutulstraumen Basin

The landscape of Antarctica, hidden beneath kilometre-thick ice in most places, has been shaped by the interactions between tectonic and erosional processes. The flow dynamics of the thick ice cover deepened pre-formed topographic depressions by glacial erosion, but also preserved the subglacial landscapes in regions with moderate to slow ice flow. Mapping the spatial variability of these structures provides the basis for reconstruction of the evolution of subglacial morphology. This study focuses on the Jutulstraumen Glacier drainage system in Dronning Maud Land, East Antarctica. The Jutulstraumen Glacier reaches the ocean via the Jutulstraumen Graben, which is the only significant passage for draining the East Antarctic Ice Sheet through the western part of the Dronning Maud Land mountain chain. We acquired new bed topography data during an airborne radar campaign in the region upstream of the Jutulstraumen Graben to characterise the source area of the glacier. The new data show a deep relief to be generally under-represented in available bed topography compilations. Our analysis of the bed topography, valley characteristics and bed roughness leads to the conclusion that much more of the alpine landscape that would have formed prior to the Antarctic Ice Sheet is preserved than previously anticipated. We identify an active and deeply eroded U-shaped valley network next to largely preserved passive fluvial and glacial modified landscapes. Based on the landscape classification, we reconstruct the temporal sequence by which ice flow modified the topography since the beginning of the glaciation of Antarctica.     

Delayed delivery from the sediment factory: modeling the impact of catchment response time to tectonics on sediment flux and fluvio‐deltaic stratigraphy

The sediment flux from a catchment is driven by tectonics and climate but is moderated by the geomorphic response of the landscape system to changes in these two boundary conditions. Consequently, catchment response time and the non‐linear behavior of landscapes in response to boundary condition change control the downstream propagation of climatic or tectonic perturbations from catchments to neighboring basins. In order to investigate the impact of catchment response time on sediment flux, we integrated a spatially‐lumped numerical model PaCMod, with new routines simulating the evolution of landscape morphology and erosion rates under tectonic and climatic forcing. We subsequently applied the model to reconstruct the sediment flux from a tectonically perturbed catchment in central Italy. Finally, we coupled our model to DeltaSim, a process‐response model simulating fluvio‐deltaic stratigraphy, and investigated the impact of catchment response time on stratigraphy, using both synthetic scenarios and a real world system (Fucino Basin, central Italy). Our results demonstrate that the differential response of geomorphic elements to tectonic and climatic changes induces a complex sediment flux signal, and produces characteristic stratigraphic architectures and shoreline trajectories. Copyright © 2014 John Wiley & Sons, Ltd.     

Hillslope response to tectonic forcing in threshold landscapes

Hillslopes are thought to poorly record tectonic signals in threshold landscapes. Numerous previous studies of steep landscapes suggest that large changes in long‐term erosion rate lead to little change in mean hillslope angle, measured at coarse resolution. New LiDAR‐derived topography data enables a finer examination of threshold hillslopes. Here we quantify hillslope response to tectonic forcing in a threshold landscape. To do so, we use an extensive cosmogenic beryllium‐10 (¹⁰Be)‐based dataset of catchment‐averaged erosion rates combined with a 500 km² LiDAR‐derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the San Gabriel Mountains, California. We also calibrate a new method of quantifying rock exposure from LiDAR‐derived slope measurements using high‐resolution panoramic photographs. Two distinct trends in hillslope behavior emerge: below catchment‐mean slopes of 30°, modal slopes increase with mean slopes, slope distribution skewness decreases with increasing mean slope, and bedrock exposure is limited; above mean slopes of 30°, our rock exposure index increases strongly with mean slope, and the prevalence of angle‐of‐repose debris wedges keeps modal slopes near 37°, resulting in a positive relationship between slope distribution skewness and mean slope. We find that both mean slopes and rock exposure increase with erosion rate up to 1 mm/a, in contrast to previous work based on coarser topographic data. We also find that as erosion rates increase, the extent of the fluvial network decreases, while colluvial channels extend downstream, keeping the total drainage density similar across the range. Our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes, and highlight the need for process‐based studies of threshold hillslopes and colluvial channels. Copyright © 2012 John Wiley & Sons, Ltd.     

Gully position,characteristics and geomorphic thresholds in an undisturbed catchment in Northern Australia

Gullying is a significant process in the long‐term dynamics and evolution of both natural and rehabilitated (i.e. post‐mining) landscapes. From a landscape management perspective it is important that we understand gully initiation and development, as it is well recognized that catchment disturbance can result in the development of gullies that can be very difficult to rehabilitate. This study examines gully position using geomorphic statistics relating to features such as depth, width and length in a catchment undisturbed by European activity in the Northern Territory, Australia. The results demonstrate that gullying occurs throughout the catchment and that a slope–area threshold does not exist and that gully position broadly follows the catchment area–slope relationship. Simple relationships relating catchment area and slope to gully depth, width and length provide poor results, despite these relationships having been found to apply for ephemeral gullies in cropland. The results suggest that gully initiation thresholds are low as a result of an enhanced fire regime. A threshold model for gully position that uses catchment area and slope to switch between gully and hillslope was evaluated and found broadly to capture gully position. Copyright © 2006 John Wiley & Sons, Ltd.     

Channel movement and erosion response to rainfall variability in southeast Australia

Intrinsic and extrinsic forces on the catchment and stream channel network drive morphological change. Separating individual forcings is difficult given the complexity of such nonlinear systems. Here a modelling approach is used to investigate the sensitivity of channel position and movement under a series of realistic rainfall scenarios for a catchment in southeastern Australia. The results demonstrate the sensitivity of the catchment to different rainfall patterns and how relatively small changes in rainfall can lead to much larger sediment outputs revealing sensitivity to subtle changes in climate. Channel movement occurs as an avulsion. This is the first time such a process has been observed and modelled in an ephemeral stream environment and demonstrates fluvial geomorphic change at human time scales. Human intervention by rock lining channels was demonstrated to prevent the movement of the main channel. Overall the CAESAR landscape evolution and erosion model used in this study is able to replicate both erosion rates and the variation in past channel movement. The modelling suggests that any landscape change is based on both internal and external forcing and that landscape history also plays a significant role. Here, we demonstrate the potential to quantify many of the nonlinearities and thresholds in soil‐mantled catchments using a landscape evolution model. Copyright © 2011 John Wiley & Sons, Ltd.     

Estimating restorable wetland water storage at landscape scales

Globally, hydrologic modifications such as ditching and subsurface drainage have significantly reduced wetland water storage capacity (i.e., volume of surface water a wetland can retain) and consequent wetland functions. While wetland area has been well documented across many landscapes and used to guide restoration efforts, few studies have directly quantified the associated wetland storage capacity. Here, we present a novel raster‐based approach to quantify both contemporary and potential (i.e., restorable) storage capacities of individual depressional basins across landscapes. We demonstrate the utility of this method by applying it to the Delmarva Peninsula, a region punctuated by both depressional wetlands and drainage ditches. Across the entire peninsula, we estimated that restoration (i.e., plugging ditches) could increase storage capacity by 80%. Focusing on an individual watershed, we found that over 59% of restorable storage capacity occurs within 20 m of the drainage network, and that 93% occurs within 1 m elevation of the drainage network. Our demonstration highlights widespread ditching in this landscape, spatial patterns of both contemporary and potential storage capacities, and clear opportunities for hydrologic restoration. In Delmarva and more broadly, our novel approach can inform targeted landscape‐scale conservation and restoration efforts to optimize hydrologically mediated wetland functions.