Property:CSDMS meeting abstract
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Arctic coasts have been impacted by rapid environmental change over the last 30 years. Warming air and water temperatures and the increased duration of the open water season, correlate with increases in the rate of already rapid erosion of ice-rich bluffs along the Beaufort Sea coast. To investigate longer-term changes in near-shore wave dynamics and storm surge set up as a result of sea-ice retreat, we coupled two simple modules.
Following Dean and Dalrymple (1991), we model wind-driven setup as a function of wind speed and direction, azimuth relative to the shore-normal, fetch and bathymetry. The wave module calculates the wave field for fetch-limited waves in shallow water based on the Shore Protection Manual (1984). For a given wind speed, dynamic water depth and fetch, we predict the significant wave height and wave period. Both modules require fetch as a controlling parameter. Sea-ice influenced coasts, are unique in that fetch is spatially variable due to the geometry of the shoreline and temporally variable as the location of the sea ice edge moves through the sea ice free season. We determine the distance to the sea ice edge using daily Nimbus 7-SMMR/SSM/I and DMSP SSMI Passive Microwave Sea Ice Concentration data. The sea ice edge is defined at a threshold sea ice concentration of 15%. We find a good match between the model predictions and our observed records of meteorological conditions and nearshore water level and waves along the Beaufort Coast in the summers of 2009 and 2010.
Over the period 1979-2012, fetch has increased significantly. In our study area near Drew Point, Alaska, the open water season itself lengthened from ~45 days to ~90 days. In the 1980’s and early 1990’s wave dynamics were fetch-limited during a significant period of the open water season. More recently, the distance from the coast to the sea ice edge shifts extremely rapidly (often 100’s of km over 1-2 weeks); fetch therefore only minimally influences wave dynamics as offshore distance exceeds the 140 km threshold over most of the open water season. Wave heights and surge set-up events on average have not changed in magnitude significantly, but storm surge set up events have increased in frequency.
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Arctic hydrological processes impose an important feedback on permafrost thermal conditions. Changes in permafrost hydrology could accelerate its thawing, resulting in a positive effect on permafrost carbon decomposition rates. Therefore, it is important to understand how geomorphic and other landscape processes control permafrost distribution and its properties such as soil saturation, ice content, active layer thickness (ALT) and temperature. The Advanced Terrestrial Simulator (ATS) is a collection of hydro-thermal processes designed to work within a flexibly configured modeling framework. ATS includes the soil physics needed to capture permafrost dynamics, including ice, gas, and liquid water content, multi-layered soil physics, and flow of unfrozen water in the presence of phase change. In this study, we directly address one of the tasks of the NGEE-Arctic project by modeling the effect of climate and environmental drivers on ALT and permafrost thickness and its distribution along the subarctic hillslope. Model runs demonstrate the likely role of vegetation-snow-permafrost-hydrology interactions by exploring snow depth and organic layer influence on horizontal and vertical patterns of permafrost. Understanding changes in hydrologic flow paths and soil moisture is important to predict evolution of ecosystem and biogeochemical processes that control climate feedbacks. In addition, hillslope flowpaths, vegetation, soil organic matter distribution, variation in soil depth and mineralogy are important components of the subgrid spatial extent of permafrost. This study explores the ways to improve the quality of the permafrost predictions at the subgrid scale and contribute to the better modeling of the permafrost related processes at the pan-Arctic scale. +
Arctic rivers play a crucial role in transporting sediment and nutrients from permafrost landscapes to the Arctic Ocean, influencing both landscape evolution and biogeochemical cycles. These river systems are undergoing significant transformations due to decreasing snow, intensified summer precipitation, altering vegetation, and permafrost thaw. Over a seasonal cycle the thermal state of Arctic rivers changes as their beds and banks thaw. Long-term observations indicate a rise in Arctic river discharge. However, our understanding of the complex mechanisms governing sediment transport in these rivers remains limited.
To address this gap, we focus on the Canning River, a gravel-bed river situated in continuous permafrost in Alaska. Previous studies on small nearby rivers during the 1970s suggested that sediment transport is hindered during the ice break-up flood because the channel bed remains frozen while cold river water starts running, slowing the sediment bed from thawing. This would imply a decoupling of sediment transport from water discharge, at least seasonally, in Arctic rivers. To investigate this hypothesis, we conducted fieldwork during the summers of 2022 and the spring of 2023, representing periods of high river discharge with differing thermal states. Our data collection included measurements of discharge, temperature, suspended sediment fluxes, grain size distributions, seismic signals, ground temperature, and river ice thickness, which we compared to a historical 5-year river discharge record.
We model how the river freezes to its bed over extended stretches during winter, and how it forms aufeis up to 2 meters thick despite limited water flow. Observed water temperature around the ice break-up period hovers around 0°C, potentially requiring several days to thaw the matrix sands and prohibiting pebble movement, according to our thermal model. Conversely, by mid-summer, water temperature rises to approximately 12°C. Although mid-summer river discharge peaks are lower, suspended sediment increases substantially during intense rainfall events, indicating a strong coupling with river discharge. These initial findings suggest that annual sediment transport might amplify with warming conditions, as the river water may no longer freeze to the channel bed and as summer flows intensify.
As a foundation of many ecological systems, vegetation is often a central component of ecological models used for forecasting and management. Many models are narrowly constrained by the system, species, and/or processes of interest and lack the ability to simulate specific management actions. This specificity limits their applicability to new, nonstationary, or actively-managed systems. The objective of this work is to create a Landlab component that combines an individual-based model design with grid-based model components to describe vegetation dynamics within and between grid cells. GenVeg is process-based, incorporating polymorphic plant-scale processes such as photosynthesis, dispersal, and seasonal allocation of biomass resources. Plant taxonomic principles are used to adapt the model methods based on the species (or representative species) of interest. Feedbacks between plants, plant communities, and the local physical environment utilize existing Landlab components and grid geometry to represent vegetation dynamics across the landscape. GenVeg is designed to be applied at a scale on the order of 10s to 1000s of meters over years to decades, which are scales relevant to ecosystem management and engineering planning. While the component is still under development, we will demonstrate its use within a dune environment utilizing coastal water levels and soil moisture to drive vegetation distribution across an idealized foredune system. +
As a rift evolves from its initiation until continental breakup it goes through a number of different phases that can be associated with distinct rifted-margin domains and major sedimentary basins. Seismic and geophysical data around the globe can give us glimpses into the progression through these domains, however, it is not well understood how the fault network evolves to produce them. Additionally, sedimentation and erosion are known factors that influence the longevity of an evolving fault and may affect the overall rift evolution. Previous work has qualitatively investigated the effect surface processes have on an evolving rift, however, there has not been a quantitative approach to analyze changes to the fault network through time.
To investigate the quantitative effect of surface processes on an evolving rift fault network, we utilized the two-way coupling between the geodynamics code ASPECT and the landscape evolution code FastScape to run 12 high-resolution 2D rift models. Using FastScape, we vary the erosional efficiency of the stream power law by changing the bedrock erodibility (Kf) from no surface processes to low (Kf= 10-6 m0.2/yr), medium (10-5 m0.2/yr), and high (10-4 m0.2/yr) efficiency. We then apply this to three different model setups that represent a wide, asymmetric, and symmetric rift. We analyze the models using the fault analysis toolbox (fatbox), which can track and correlate individual faults and their properties through time. Specifically, we utilize this toolbox to track the evolution of the number of faults and the cumulative fault system length and displacement through time and investigate how they change depending on the efficiency of surface processes and the rift type.
Through this analysis, we find that regardless of the rift type or the efficiency of surface processes the rift fault network evolves through up to five distinct phases: 1) distributed deformation and coalescence, 2) fault system growth, 3) fault system decline and basin-ward localization, 4) rift migration, and 5) continental breakup. While we find that surface processes do not exert a strong control on the phase progression or final rifted margin architecture, they do affect the temporal evolution of the fault network by increasing fault longevity. As faults live longer with greater surface processes, the fault network phases are prolonged and continental breakup is delayed. Additionally, greater surface process efficiency leads to fewer faults forming which causes a less complex fault network.
As climate change and environmental variability increase pressure on vulnerable communities, migration is one possible adaptation strategy. However, the decision to migrate is complex, and environmental factors are rarely the sole drivers of that decision. Rather, the decision to migrate is often influenced by a combination of economic, social, political, and environmental pressures. This is especially true in coastal communities in Bangladesh, where temporary migration has long been a method of livelihood diversification, and researchers are trying to understand how environmental factors influence existing migration flows. This work addresses a gap in current research by beginning to investigate how different “push” and “pull” drivers of migration might have distinctive variables that contribute to the ultimate decision to move or stay. In this study, random forest classification models are applied to a dataset consisting of household surveys from more than 1,200 households in southwestern Bangladesh to directly assess key variables that influence five types of migration in coastal communities: temporary migration within a village due to environmental stress, migration for education, migration for healthcare, migration for trade or commerce, and migration to visit relatives. This work demonstrates that these types of migration do have different drivers, which yields insights into the complex motivations that impact the decision to migrate. However, livelihood variables and individual aspirations were key for all investigated forms of migration. In the process, this work demonstrates that random forest models could be a powerful method for improving predictive accuracy of migration models to better inform migration policy and planning. +
As coastal regions become more developed, many communities are considering costly engineering solutions to address coastal change, including "soft" approaches, such as beach replenishments or dune constructions, and hard structures, such as seawalls, revetments, bulkheads, or groins. Given current rates of sea level rise and the associated shoreline losses that coastal communities face, however, it is unclear whether the benefits generated by these protection measures justify the costs. We are building a set of integrated geologic and economic models to better understand the coupled evolution of developed shorelines under alternative protection policies. The first model incorporates dune construction and sediment overwash relocation into a morphodynamic model for dune evolution. We use this model to assess the costs of constructing an optimal cross-sectional area for a long-term dune system, and we explore the “geo-economic” effects on ocean views that may be diminished by constructing a dune system of particular size seaward of protected properties. A second model simulates beach width dynamics for two adjacent communities, each with their own groin structure. We use the model to analyze both coordinated and uncoordinated strategies between the two communities, reflecting individual community decisions to protect or retreat. A third model incorporates beach nourishment practices into a morphodynamic model for barrier evolution that accounts for shoreface dynamics. Results show that the efficiency of beach nourishment can be affected by the dynamic state of the shoreface during each nourishment episode. In general, these models reinforce the need to refine numerical coastal management tools to incorporate bi-directional interactions between natural processes and human responses to shoreline change. +
As one of the three major Asian marginal seas in the western Pacific, the SCS occupies less than 1% total ocean area while accommodating 15% atoll (25434.6 km2) in the globe (GSA, 2009), which mainly distribute in the Xisha, Zhongsha and Nansha Islands. Atolls in the SCS are generally ellipse-shaped with a longer axis extending in the NE-SW direction and a wider southwest reef platform compared to the northeast. One possible explanation ascribed such features to the monsoon circulation (northeast and southwest monsoons blow alternatively in winter and summer) over the SCS (Zeng, 1984). Waves and currents influence the atoll development by (1) sediment suspension and transportation that can influence the transparency of the water, thus the symbiotic algae and the coral growth, (2) supply of dissolved oxygen and nutrient and (3) removal of metabolic wastes under normal weathers, while storm waves can cause large-scaled breakage, transportation and reconfiguration of reefs (e.g. Chappell, 1980; Storlazzi et al., 2005). Yet, little data was available regarding the hydrodynamic conditions of the forereef of the SCS atolls. Here, we conducted in situ tripod mooring observations (ADCP, ADV & CTD) for at least one tide cycle in 15-18 m water depth at the southeast forereef of three typical atolls – Xiaonanxun (NX), Anda (AD) and Kugui (KG) Reef – in the SCS, respectively, and collected coral sediment samples at different zonation of atolls in September 2017. During the observation periods, tide elevations varied by ca.1 m in all the three sites, with the highest 1.16 m in AD and lowest 0.96 m in KG. Mean flow velocity turns out to be as weak as about 0.1 m/s, with the weakest ~0.05 m/s in KG. Wave influence appears to be strongest in NX, with the significant wave height of ~1 m, in contrast to the 0.6 m and 0.4 m in AD and KG, respectively. The hydrodynamic observations under normal weathers should be able to transport the fine reef debris alone, with limited sediment transport rates of 0.61, 0.01 and 0.64 m3/m per tidal period in the observations in NX, AD and KG, respectively. Coarse coral rubbles and gravels might be only transported during extreme weathers. More observations and modeling work are needed, e.g. simulations of waves’ influence on atoll sedimentary systems’ development with XBeach.
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As part of the Mediterranean Landscape Dynamics (MedLand) project to create a modeling laboratory for human-landscape interaction, we have developed a suite of landscape evolution tools in the GRASS GIS environment. The core of this tool set is a Python script to estimate sediment transport for hillslopes, gullies/rills, and small channels, and simulate resulting terrain change for high-resolution 3D digital landscapes. Because it takes advantage of raster-optimized routines in GRASS, it is very fast on normal desktop systems, making it ideal for simulating long-term landscape change resulting from human activity, climate change, or other drivers. We provide examples of how this landscape evolution model is being used in the MedLand project. +
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Assessing the tsunami hazard in regions with infrequent or no instrumental or historical records of tsunamis is a challenge for emergency managers. In the absence of these records, coastal geologists rely on evidence of past tsunami inundation from buried sedimentary deposits to identify the presence of a tsunami hazard and to determine the recurrence of past events. One persistent challenge in assessing tsunami hazard from sandy coastal deposits is inferring the relative magnitude of past tsunamis from characteristics of the deposits. Recent reanalysis of field data from the 2011 Tohoku-oki earthquake and tsunami show that the volume of onshore sandy tsunami deposits is highly correlated with offshore tsunami magnitude, seafloor deformation, and fault slip. To further explore these relationships, we employ a Delft3D-FLOW hydrodynamic and sediment transport model to simulate onshore tsunami deposit volume from offshore slip of the 2011 Tohoku-oki earthquake and tsunami. We use the Satake et al. (2013) tsunami source model to derive the hydrodynamic boundary conditions for the sediment transport simulations. The Delft3D-FLOW model uses van Rijn (2007) sediment transport formulations and coefficients and a two-dimensional, vertically layered grid to model sediment transport with the effect of suspended-sediment induced density stratification on the vertical turbulent mixing. We model how variation in offshore slip affects tsunami deposit volume for a wide range of sediment sources, offshore and onshore slopes, and boundary roughness conditions. Model results show a strong correlation between onshore tsunami deposit volume and adjacent offshore co-seismic slip if ample sediment is available in the model to be eroded and transported. These results are consistent with data from the 2011 Tohoku tsunami at sites with sufficiently wide beaches and without shoreline armoring. We continue to test the model to evaluate sensitivity to parameters that may not be well known for paleo-tsunamis such as width of fault rupture, paleo-topography, and changes in sea level. Ultimately, this approach may be able to reconstruct past tsunami magnitudes and improve assessment of tsunami hazard.
* Satake, K., Fujii, Y., Harada, T., & Namegaya, Y. (2013). Time and space distribution of coseismic slip of the 2011 Tohoku earthquake as inferred from tsunami waveform data. Bulletin of the seismological society of America, 103(2B), 1473-1492.
At a global scale, deltas significantly concentrate people by providing diverse ecosystem services and benefits for their populations. At the same time, deltas are also recognized as one of the most vulnerable coastal environments, due to a range of adverse drivers operating at multiple scales. These include global climate change and sea-level rise, catchment changes, deltaic-scale subsidence and land cover changes, such as rice to aquaculture. These drivers threaten deltas and their ecosystem services, which often provide livelihoods for the poorest communities in these regions. Responding to these issues presents a development challenge: how to develop deltaic areas in ways that are sustainable, and benefit all residents? In response to this broad question we have developed an integrated framework to analyze ecosystem services in deltas and their linkages to human well-being. The main study area is part of the world’s most populated delta, the Ganges-Brahmaputra-Meghna Delta within Bangladesh. The framework adopts a systemic perspective to represent the principal biophysical and socio-ecological components and their interaction. A range of methods are integrated within a quantitative framework, including biophysical and socio-economic modelling, as well as analysis of governance through scenario development. The approach is iterative, with learning both within the project team and with national policy-making stakeholders. The analysis allows the exploration of biophysical and social outcomes for the delta under different scenarios and policy choices. Some example results will be presented as well as some thoughts on the next steps. +
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At the Visual World Investigation Lab of the Nature Research Center, we are developing a module where museum visitors investigate geomorphic and land-use scenarios through a landscape evolution model. Visitors use touchscreen computers to select simplified inputs for the CHILD model. Model visualizations will be produced for each trial in which they run the scenario. For example, visitors can explore the impact of the percentage of impervious surfaces in a section of urbanized Raleigh that will be adjusted by scaling infiltration parameters, and how the headwaters of the Little Tennessee River would differ if the southern Appalachians were still undergoing tectonic uplift. These scenarios provide relatable experiences to visitors, an opportunity to educate them upon the science behind the scenarios, and the purpose and limitations of models. We will first develop the framework of the module to be able to accept scenarios and its inputs, including digital elevation models, such that others can contribute scenarios. This module is early in its conception, thus we will present our initial framework with the intent to elicit feedback from the community. +
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At the catchment scale, alluvial rivers co-adjust their planform, cross-sectional, and longitudinal geometries in response to changing water and sediment inputs, base level and the transport of this sediment through the fluvial system. In this study, we derive a simple, physics-based model to understand and predict sand-bed river long-profile form and evolution. This model links sediment transport and river morphodynamics, following an analogous approach to that taken by Wickert and Schildgen (2019) for gravel-bed rivers. It allows for planform (width) adjustments as a function of excess shear stress by following Parker (1978); this linearizes the sediment-transport response to changing river discharge, and ultimately suggests a diffusive form for sand-bed river long-profile evolution. Here, we also present model results of gravel- and sand-bed river long profiles under a variety of water- and sediment-supply and base-level conditions to discuss how these may help us to better interpret the geological and geomorphological context of alluvial rivers, and better predict their changes over time. This expression for the long-profile evolution of transport-limited sand-bed rivers provides forward momentum to merge theory and models for gravel-bed and sand-bed river systems, to look at the alluvial river system response as a whole (from bedrock-alluvial transition to the point at which backwater effects become significant) over both human and geological time scales, and to decipher the long-term rate and magnitude of this response to facilitate a better understanding of the evolution of fluvial landscapes. +
At the margins of many glaciers, we observe visually-striking layers of concentrated sediment incorporated into ice near the base of the glacier. Despite the prevalence of these ice-sediment facies, sediment transported in basal ice is rarely quantified in the overall sediment transport budget for glacial systems. Previous facies descriptions have been linked to formation mechanisms that depend on specific configurations of the topography or hydrology beneath a glacier, which remains inconsistent with observations of similar facies across disparate regions, climate zones, and geologic settings. Here, we use detailed descriptions of ice-sediment facies from Mendenhall glacier, Alaska, to inform a numerical model of sediment entrainment in basal ice. We find that the overall volume of entrained sediment is strongly related to the glacier’s thermal regime near the ice-sediment interface. Further, we present a likely mechanism for the formation of dispersed ice facies that explains the natural variability in sediment characteristics observed at Mendenhall glacier and other alpine systems. These results show that ice-sediment facies are a plausible archive for understanding the subglacial environment, even in the absence of additional constraints on temperature or hydrologic connectivity at the bed. +
Barrier island response to sea level rise depends on their ability to transgress and move sediment to the back barrier, either through flood-tidal delta deposition, or via storm overwash. Our understanding of these processes over decadal to centennial time scales, however, is limited and poorly constrained.
We have developed a new barrier inlet environment (BRIE) model to better understand the interplay between tidal dynamics, overwash fluxes, and sea-level rise on barrier evolution. The BRIE model combines existing overwash and shoreface formulations with alongshore sediment transport, inlet stability, inlet migration and flood-tidal delta deposition. Within BRIE, inlets can open, close, migrate, merge with other inlets, and build flood-tidal delta deposits. The model accounts for feedbacks between overwash and inlets through their mutual dependence on barrier geometry.<br><br>Model results suggest that when flood-tidal delta deposition is sufficiently large, barriers require less storm overwash to transgress and aggrade during sea level rise. In particular in micro-tidal environments with asymmetric wave climates and high alongshore sediment transport, tidal inlets are effective in depositing flood-tidal deltas and constitute the majority of the transgressive sediment flux. Additionally, we show that artificial inlet stabilization (via jetty construction or maintenance dredging) can make barrier islands more vulnerable to sea level rise. +
Barrier islands and other coastal landforms are highly dynamic systems, changing in response a spectrum of disturbances from multi-decadal ‘press’ disturbances like sea-level rise (SLR) to often more intense episodic perturbations like storms. As a result, multiple stable ecomorphological states exist on barrier islands. In this study, we use a probabilistic Bayesian network approach to investigate the likelihood of shifts among alternative equilibrium states on Fire Island, New York under three scenarios of shoreline change driven by sea-level rise (SLR). Specifically, we highlight areas that are most likely (i) to become inundated, (ii) to shift from one non-inundated state (or landcover type) to another (e.g., a forest becomes beach), or (iii) to remain in the current landcover state. We explore the effects of these changes on the availability of coastal ecosystem types, piping plover habitat, and anthropogenic development. +
Barrier islands, which comprise ~10% of shorelines worldwide, are ecologically and economically important coastal systems. They also provide numerous ecosystem goods and services, acting as critical buffers that protect the mainland from storms, erosion, and other natural hazards. However, the dynamic nature of barrier island geomorphology and the processes that sustain them create complex coastal management challenges, particularly in response to more intense and frequent storms and rising sea levels. These challenges contribute to infrastructure vulnerability, habitat loss, and increasing maintenance costs for management actions like beach nourishment, negatively impacting coastal communities. Thus, understanding the interplay between natural processes and management decisions is essential for predicting the future of developed coastlines. Here, we apply the CoAStal Community-lAnDscape Evolution (CASCADE) model, a coupled landscape and human dynamics modeling framework, tailoring it to simulate geomorphic change on Hatteras Island, North Carolina — a barrier island in the Outer Banks experiencing severe erosion that threatens both properties and transportation routes along the NC-12 highway. Following a hindcast calibration and test, we assess the likely range of future island behavior under a range of different climate and management scenarios. Our approach integrates geomorphic and human decision-making processes and incorporates diverse datasets, such as LiDAR-derived elevations, historic shoreline change rates, storm records, sea-level rise projections, and management scenarios currently under consideration. This study demonstrates the utility of CASCADE as a tool for understanding coupled human-natural systems and provides a framework for assessing long-term coastal resilience and adaptation strategies under changing environmental conditions in other similar settings. +
Beach ridges are common landforms found along coasts undergoing isostatic rebound or other forms of relative sea-level fall. The development of individual ridges has been attributed to storms, tidal cycles, and even the change in the rate of relative sea-level. However, few studies have investigated the role of autogenic processes in the development of individual ridges. In this study, we modify the existing code for modeling beach/foredune-ridge and swale morphology to examine the development of beach ridges during conditions of falling relative sea-level and constant sediment supply. We show that individual beach ridges can form in the absence of changes in the rate of sea-level change, tidal cycle, sediment supply, and storms. New beach ridges form as the shoreline moves seaward due to relative sea level fall, removing older beach ridges from their source of sediment, thus nucleating new beach ridges. Furthermore, we find that beach ridges grow higher and more frequent with increased rates of sediment supply. This study highlights the importance autogenic processes play in beach ridge development and has significant implications for the ability to decipher between environmental signals using beach ridges as historical archives. +
Bedload flux is notoriously challenging to measure and model with its dynamics, therefore, remains largely unknown in most fluvial systems worldwide. We present a global scale bedload flux model as part of the WBMsed modeling framework. The results show that the model can very well predict the distribution of water discharge and suspended sediment and well predict bedload. Bedload predictions’ sensitivity to river slope, particle size, discharge, river width, and suspended sediment were analyzed, showing that the model is most responsive to spatial dynamics in river discharge and slope. The relationship between bedload and total sediment flux is analyzed globally and in representative longitudinal river profiles (Amazon, Mississippi, and Lena Rivers). The results show that while, as expected, the proportion of bedload is decreasing from headwater to the coasts, there is considerable variability between basins and along river corridors. The topographic and hydrological longitudinal profiles of rivers are shown to be the key driver of bedload longitudinal trends with fluctuations in slope controlling its more local dynamics. Differences in bedload dynamics between major river basins are attributed to the level of anthropogenic modifications, flow regimes, and topographic characteristics. +
Bedrock lithology has been shown to strongly influence how rivers and landscapes respond to tectonic perturbations, yet the specific variables and mechanisms that set how lithology controls river erosion are poorly understood. Recent field and modeling work suggests that one important lithologic control on channel response may be the delivery of large, generally immobile boulders from hillslopes to channels. This raises the possibility that differences in boulder delivery rates between lithologies may cause substantial differences in how landscapes respond to tectonics. An intriguing recent study suggested that in the Mendocino Triple Junction (MTJ) region of northern California, bedrock lithology might control the frequency and size of boulders delivered to channels, and therefore govern channel steepness and river evolution (Bennett et al., 2016). We further test this hypothesis here.
The Central Belt of the Franciscan Complex, a mix of sheared graywacke and mudstone, contains large blocks of more resistant serpentinite, greenstone, and amphibolite that are delivered to channels by earthflows. The adjacent Coastal Belt generally lacks such boulders, and sediment delivery to channels is dominated by shallow landsliding. This geologic setting provides a unique opportunity to test whether boulder abundance exerts a first-order control on landscape form. We use a landscape-scale analysis of channel steepness and active width indices, local topographic relief, lithology, and mapped boulder occurrence to understand the differences between the catchments eroding the Central Belt and those eroding the Coastal Belt. We find that channels are steeper in the Central Belt than in the Coastal Belt, both across the whole MTJ region and when averaged over 10-50 km2 subcatchments. Channels are also generally narrower in the Central Belt. This result could reflect lithologic controls or spatial heterogeneity in erosion rates. To control for the latter, we construct clusters of neighboring subcatchments that are free of knickpoints to explore possible controls of lithologic makeup (percent of a subcatchment underlain by Central Belt rocks) on channel steepness independent of erosion rate variations. We find inconsistent relationships between lithologic makeup and channel steepness within a given cluster of catchments with similar baselevel history. Finally, we compared channel segments adjacent to hillslope failures with segments far from failures. Central Belt channels show greater absolute increases in steepness adjacent to hillslope failures, but relative increases in steepness are consistent between the Central Belt and Coastal Belt.
Our preliminary results suggest that Central Belt channels are steeper and narrower than Coastal Belt channels, but that the lithological influence on steepness is difficult to disentangle from the effects of spatially variable erosion rates. We are continuing to map in-channel boulder size distributions to assess the relative importance of intra- vs. inter-lithologic variability in setting boulder concentrations and landscape form.
