Browsing by Subject "Geomorphology"

This dissertation investigates the limnology of the southwestern Peruvian Amazon centered on the Madre de Dios department by examining first the geomorphology and then the ecology and biogeochemistry of the region's fluvial systems.

Madre de Dios, Peru is world renowned for its prolific biodiversity and its location within the Andes biodiversity hotspot. It is also a site of study regarding the development of the Fitzcarrald Arch and that feature's geomorphological importance as the drainage center for the headwaters of the Madeira River - the Amazon's largest tributary and as well as its role as a physical divider of genetic evolution in the Amazon. Though each of these has been studied by a variety of prominent researchers, the ability to investigate all the aspects of this unique region is hampered by the lack of a regional geomorphological map. This study aims to fill that gap by using remote sensing techniques on digital elevation models, satellite imagery and soil, geology and geoecological maps already in publication to create a geomorphological map. The resulting map contains ten distinct landform types that exemplify the dominance of fluvial processes in shaping this landscape. The river terraces of the Madre de Dios River are delineated in their entirety as well as the various dissected relief units and previously undefined units. The demarcation of the boundaries of these geomorphic units will provide invaluable assistance to the selection of field sites by future researchers as well as insights into the origin of the high biodiversity indices of this region and aid in planning for biodiversity conservation.

Secondly this study examines 25 tropical floodplain lakes along 300 km of the Manu River within the Manu National Park in the Madre de Dios department. Alternative stable state and regime shifts in shallow lakes typically have been examined in lakes in temperate and boreal regions and within anthropogenically disturbed basins but have rarely been studied in tropical or in undisturbed regions. In contrast this study focuses on a tropical region of virtually no human disturbance and evaluates the effects of hydrological variability on ecosystem structure and dynamics. Using satellite imagery a 23 yr timeline of ecological regime shifts in Amazon oxbow lakes or "cochas" is reconstructed. The study shows that almost 25% of the river's floodplain lakes experience periodic abrupt vegetative changes with an average 3.4% existing in an alternative stable state in any given year. State changes typically occur from a stable phytoplankton-dominated state to a short lived, <3 yr, floating macrophytic state and often occur independent of regional flooding. We theorize that multiple dynamics, both internal and external, drive vegetative regime shifts in the Manu but insufficient data yet exists in this remote region to identify the key processes.

To complete the investigation of tropical limnology the third study compares and contrasts the nutrient-productivity ration of floodplain and non-floodplain lakes globally and regionally. For over 70 years a strong positive relationship between sestonic chlorophyll-a (Chl-a) and total phosphorus (TP) has been established with phosphorus generally viewed as the most limiting factor to productivity. Most of these studies, however, have focused on northern, temperate regions where the lakes are typically postglacial, isolated and fed by small streams. Relatively little work has been done on floodplain lakes which are semi or permanently connected to the river. This study examines the relationship between nutrients and productivity in floodplain lakes globally through an extensive literature synthesis. Values for total phosphorus, total nitrogen and chlorophyll-a were collected for 523 floodplain lakes, represented by 288 data points while 551 data points were collected for 5444 non-floodplain lakes. Analysis revealed that globally, floodplain lakes do not show any significant difference in the total phosphorus/chlorophyll-a relationship from that found in non-floodplain lakes but significant differences are seen between tropical and temperate lakes. We propose that the term `floodplain' lake should serve as purely a geographical descriptor and that it is lacking as an ecological indicator. Instead factors such as precipitation seasonality, hydrological connectivity and regional flooding regimes are better indicators of high or low productivity in floodplain lakes.

The topographical properties of a landscape and their time evolution are key features of the Earth's surface, regulating ecosystem functioning in terms of soil properties as well as water and energy budgets, and creating visually diverse and striking patterns across various spatial scales. Furthermore, the natural evolution of a topography under the influence of geologic erosion can be greatly altered by anthropogenic disturbances (e.g., through agriculture, mining, deforestation), with the potential of accelerating soil erosion, causing land degradation and soil fertility losses. Hence, understanding the geomorphological processes driving the evolution of landscapes under natural and disturbed conditions is key not only to define the main factors and feedbacks shaping the Earth's topography, but also to foresee the consequences of intensive land use and implement optimal strategies of land management and recovery.

This dissertation addresses some key aspects of landscape evolution and stability, with a focus on the statistical description and modeling of hillslope morphologies under natural and disturbed conditions, the theoretical definition of drainage area at regular and non-regular points of the watershed, and the formation of spatially organized ridge and valley patterns.

We start from the analysis of topographic slopes under natural and accelerated soil erosion. Using large topographic datasets from mountain ranges worldwide, we show that the approximate age of a landscape is fingerprinted in the tails of its slope distributions. We then explore the role of the different processes driving this dynamic smoothing over geologic time scales by means of numerical experiments, showing that the relaxation process is mainly dominated by diffusion. The effect of agricultural-driven soil erosion on hillslope morphology is then investigated, highlighting how the natural aging process can be altered by intensive land use which, at smaller scales, produces key differences in the slope distribution tails. Furthermore, theoretical solutions are derived for the hillslope profile and the associated soil creep and runoff erosion fluxes, and used to link the observed differences in the morphological features of disturbed and undisturbed areas to a disruption of the natural balance between soil creep and runoff erosion mechanisms.

We then move the analysis to the drainage area, an important nonlocal morphometric variable used in a large number of geomorphological and ecohydrological applications. A nonlinear differential equation whose validity is limited to regular points of the watershed is obtained from a continuity equation, and the theory is then extended to critical and singular points by means of both Gauss' theorem and dynamical systems concepts. Such a link between the drainage area and a continuity equation sets the basis for the subsequent analysis of organized ridge and valley patterns and channel forming instability. The formation of ridge/valley patterns is analyzed by means of numerical experiments in detachment limited conditions, with the identification of various regimes as a function of diffusive soil creep, runoff erosion, and tectonic uplift as well as the specific geomorphic transport law assumed. Lastly, a linear stability analysis of the coupled water and landscape evolution dynamics is outlined to investigate the critical conditions triggering channel formation and the emergence of characteristic valley spacings in relation to the main geomorphological processes involved.

Coherent, large-scale shapes and patterns are evident in many landscapes, and evolve according to climate and hydrological forces. For large-scale, sandy coastlines, these shapes depend on wave climate forcing. The wave climate is influenced by storm patterns, which are expected to change with the warming climate, and the associated changes in coastline shape are likely to increase rates of shoreline change in many places. Humans have historically responded to coastline change by manipulating various coastal processes, consequently affecting long-term, large-scale coastline shape change. Especially in the context of changing climate forcing and increasing human presence on the coast, the interaction of the human and climate-driven components of large-scale coastline evolution are becoming increasingly intertwined.

This dissertation explores how climate shapes coastlines, and how the effects of humans altering the landscape interact with the effects of a changing climate. Because the coastline is a spatially extended, nonlinear system, I use a simple numerical modeling approach to gain a basic theoretical understanding of its dynamics, incorporating simplified representations of the human components of coastline change in a previously developed model for the physical system.

Chapter 1 addresses how local shoreline stabilization affects the large scale morphology of a cuspate-cape type of coastline, and associated large-scale patterns of shoreline change, in the context of changing wave climate, comparing two fundamentally different approaches to shoreline stabilization: beach nourishment (in which sediment is added to a coastline at a long-term rate that counteracts the background erosion), and hard structures (including seawalls and groynes). The results show that although both approaches have surprisingly long-range effects with spatially heterogeneous distributions, the pattern of shoreline changes attributable to a single local stabilization effort contrast greatly, with nourishment producing less erosion when the stabilization-related shoreline change is summed alongshore.

Chapter 2 presents new basic understanding of the dynamics that produce a contrasting coastline type: convex headland-spit systems. Results show that the coastline shapes and spatially-uniform erosion rates emerge from two way influences between the headland and spit components, and how these interactions are mediated by wave climate, and the alongshore scale of the system. Chapter 2 also shows that one type of wave-climate change (altering the proportion of `high-angle' waves) leads to changes in coastline shape, while another type (altering wave-climate asymmetry) tends to reorient a coastline while preserving its shape.

Chapter 3 builds on chapter 2, by adding the effects of human shoreline stabilization along such a convex coastline. Results show that in the context of increasing costs for stabilization, abandonment of shoreline stabilization at one location triggers a cascade of abandonments and associated coastline-shape changes, and that both the qualitative spatial patterns and alongshore speed of the propagating cascades depends on the relationship between patterns of economic heterogeneity and the asymmetry of the wave-climate change--although alterations to the proportion of high-angle waves in the climate only affects the time scales for coupled morphologic/economic cascades.

Coastal ecosystems provide a wide range of services including protecting the mainland from the destructive effects of storms, nutrient cycling, water filtration, nurseries for fish and crustaceans, and carbon sequestration. These zones are threatened by human impacts and climate change through more frequent intense storms and sea level rise with a projected increase of up to 16 mm/yr for the last two decades of the 21st century. However, it is not fully understood what mechanisms control the formation and degradation of these landforms, and determine their resilience to environmental change. In this work, I highlight the role of various physical characteristics and environmental parameters that contribute to the formation and stability of coastal environments.

First, I develop and use remote sensing analyses to quantitatively characterize coastal dune eco-topographic patterns by simultaneously identifying the spatial distribution of topographic elevation and vegetation biomass in order to understand the coupled dynamics of vegetation and coastal dunes. LiDAR-derived leaf area index and hyperspectral-derived normalized difference vegetation index patterns yield vegetation distributions at the whole-system scale which are in agreement with each other and with field observations. LiDAR-derived concurrent quantifications of biomass and topography show that plants more favorably develop on the landward side of the foredune crest and that the foredune crestline marks the position of an ecotone, which is interpreted as the result of a sheltering effect sharply changing local environmental conditions. The findings reveal that the position of the foredune crestline is a chief ecomorphodynamic feature resulting from the two-way interaction between vegetation and topography.

Next, to shed light on the vertical depositional dynamics of salt marshes in response to sea level rise, I investigate the hypothesis that competing effects between biomass production and aeration/decomposition determine an approximately spatially constant contribution of soil organic matter (SOM) to total accretion. I use concurrent observations of SOM and decomposition rates from marshes in North Carolina. The results are coherent with the notion that SOM does not significantly vary in space and suggest that this may be the result of an at least partial compensation of opposing trends in biomass productivity and decomposed organic matter. The analyses show that deeper soil layers are characterized by lower decomposition rates and higher stabilization factors than shallower layers, likely because of differences in inundation duration. However, overall, decomposition processes are sufficiently rapid that the labile material in the fresh biomass is completely decomposed before it can be buried and stabilized. The findings point to the importance of the fraction of initially refractory material and of the stabilization processes in determining the final distribution of SOM within the soil column.

Finally, I develop a process-based model to evaluate the relative role of watershed, estuarine, and oceanic controls on salt marsh depositional/erosional dynamics and define how these factors interact to determine salt marsh resilience to environmental change at the estuary scale. The results show that under some circumstances, vertical depositional dynamics can lead to transitions between salt marsh and tidal flat equilibrium states that occur much more rapidly than marsh/tidal flat boundary erosion or accretion could. Additionally, the analyses reveal that river inputs affect the existence and extent of marsh/tidal flat equilibria by both modulating exchanges with the ocean (by partially “filling” the basin) and by providing suspended sediment.

The Andes Mountains provide a unique setting to study the interplay between climate and geomorphology. The mechanism proposed to describe the evolution of Andean topography is a feedback loop where precipitation erodes the surface, causing the earth's crust to thin and, through buoyancy, uplift the surface. The uplifted surface acts as a barrier which in turn increases precipitation and reinforces the feedback. Demonstrating this feedback is difficult due to the long temporal scales involved. To overcome this challenge, we consider current topographic constraints and climate regimes as a means to evaluate geomorphologic behavior. Initial data analysis leads to the identification of qualitative similarities in the distributions of outlets and precipitation events by elevation, which suggest a link between climatic and fluvial erosion and a strong interaction between orography and precipitation. To explore impacts of this link on regional geomorphology, we estimate uplift rates under a Bayesian hierarchical modeling framework based on the stream power erosion law (SPEL). We specify model parameters using slope and area data generated from a high-resolution, digital elevation map and mean annual precipitation (MAP) derived from 14 years of TRMM 3B42 v.7 precipitation rainfall rates, supplemented with rain gauge data from the Kospinata network in Peru. A key component of the analysis is the development of a natural spatial scale which captures the qualitative similarities observed in the region and provides a means to compare estimated uplift rates to the geomorphologic behavior of each basin. The estimated uplift values recovered from the analysis range from 0.81 to 11.59 mm/yr and thus fall within a physically-reasonable range for the central Andes region. These estimates also are in strong agreement with basin hypsometry. The analysis further reveals a pattern of spatially dependent uplift, which is consistent with the differential tectonic forcing imposed on the basins by the subducting Nazca plate. The adaptation of the physical-statistical model represents a novel method for quantifying the relationship between climate and orography and estimating key parameters of SPEL.

Complex feedbacks dominate landscape dynamics over large spatial scales (10s – 100s km) and over the long-term (10s – 100s yrs). These interactions and feedbacks are particularly strong at land-water boundaries, such as coastlines, marshes, and rivers. Water, although necessary for life and agriculture, threatens humans and infrastructure during natural disasters (e.g., floods, hurricanes) and through sea-level rise. The goal of this dissertation is to better understand landscape morphodynamics in these settings, and in some cases, to investigate how humans have influenced these landscapes (e.g., through climate or land-use change). In this work, I use innovative numerical models to study the larger-scale emergent interactions and most critical variables of these systems, allowing me to clarify the most important feedbacks and explore large space and time scales.

Chapter 1 focuses on understanding the shoreline dynamics of pocket (embayed) beaches, which are positioned between rocky headlands and adorn about half the world’s coastlines. Previous work suggested that seasonality or oscillations in climate indices control erosion and accretion along these shorelines; however, using the Coastline Evolution Model (CEM), I find that patterns of shoreline change can be found without systematic shifts in wave forcings. Using Principal Component Analysis (PCA), I identify two main modes of sediment transport dynamics: a shoreline rotation mode, which had been previously studied, and a shoreline “breathing” mode, which is newly discovered. Using wavelet analysis of the PCA mode time series, I find characteristic time scales of these modes, which emerge from internal system dynamics (rather than changes in the wave forcing; e.g., seasonality). To confirm the breathing mode’s existence, I retroactively identified this mode in observations of pocket beach shoreline change from different parts of the world. Characterization of these modes, as well as their timescales, better informs risk assessment and coastal management decisions along thinning shorelines, especially as climate change affects storminess and wave energy variations across the world.

Chapter 2 moves slightly inland to examine how coastal marshes, which provide numerous ecosystem services and are an important carbon sink, respond to climate change and anthropogenic influences. Specifically, I focus on how increasing concentrations of atmospheric CO2 affect marsh resilience to increased rates of sea-level rise relative to inorganic sediment availability and elevated nitrogen levels. Using a meta-analysis of the available literature for marsh plant biomass response to elevated levels of CO2 and nitrogen, I incorporated these effects into a coupled model of marsh vegetation and morphodynamics. Although nitrogen’s effect on biomass and marsh accretion rates is less clear, elevated CO2 causes a fertilization effect, increasing plant biomass, which enhances marsh accretion rates (through increased rates of both in- organic and organic sedimentation). Findings from the model experiments suggest that the CO2 fertilization effect significantly increases marsh resilience to sea-level rise; however, reduced inorganic sediment supply (e.g., through land-use change or damming) still remains a serious threat to marsh survival).

Almost half a billion people live on or near river deltas, which are flat, fertile landscapes that have long been ideal for human settlement, but are increasingly vulnerable to flooding. These landscapes are formed by the repeated stacking of sedimentary lobes, the location and size of which are formed by river channel avulsions, which occur when the river changes course relatively rapidly. Despite the importance of avulsions to delta morphodynamics, we do not fully understand their dynamics(specifically, avulsion location and timing). In order to investigate the relative influence of rivers and waves on delta morphology and avulsion processes, I develop the River Avulsion and Floodplain Evolution Model (RAFEM) and couple it to CEM to create a new morphodynamic river delta model.

In Chapter 3, I use the new coupled fluvial-coastal model to examine the upstream location of avulsions over a range of sea-level rise rates and wave energies. In model experiments, the longitudinal river profile adjusts as the river progrades, causing a preferential avulsion location where the river aggradation relative to the floodplain topography is most rapid. This avulsion length scale is a function of the amount of in-channel sedimentation required to trigger an avulsion, where a larger amount of aggradation required necessitates a greater amount of pre-avulsion progradation. If an avulsion is triggered once aggradation reaches half bankfull channel depth, the preferential length scale is around a backwater length, which scales well with laboratory and field observations.

In Chapter 4, I explore how a wide range sea-level rise rates and wave climates affect both delta morphology and avulsion dynamics with the coupled model. Surprisingly, I find that increasing sea-level rise rates do not always accelerate avulsions. In river-dominated deltas, avulsion time scales tend not to decrease, as upslope river mouth transgression counteracts base-level driven aggradation. I also find that both the sign and magnitude of the wave climate diffusivity affects both avulsion dynamics and large-scale delta morphology. My findings highlight not only important differences between river and wave-dominated deltas, but also prototypical deltas and those created in the lab. Because the wave climate, sea-level rise rate, and amount of in-channel aggradation required to trigger an avulsion all affect rates of autogenic variability operating within the delta, each of these forcings has important implication for avulsion dynamics and stratigraphic interpretation of paleo-deltaic deposits.

The main conclusion of this dissertation is that global H2 production within young ocean crust (<10 Mya) is higher than currently recognized, in part because current estimates of H2 production accompanying the serpentinization of peridotite may be too low (Chapter 2) and in part because a number of abiogenic H2-producing processes have heretofore gone unquantified (Chapter 3). The importance of free H2 to a range of geochemical processes makes the quantitative understanding of H2 production advanced in this dissertation pertinent to an array of open research questions across the geosciences (e.g. the origin and evolution of life and the oxidation of the Earth’s atmosphere and oceans).

The first component of this dissertation (Chapter 2) examines H2 produced within young ocean crust [e.g. near the mid-ocean ridge (MOR)] by serpentinization. In the presence of water, olivine-rich rocks (peridotites) undergo serpentinization (hydration) at temperatures of up to ~500°C but only produce H2 at temperatures up to ~350°C. A simple analytical model is presented that mechanistically ties the process to seafloor spreading and explicitly accounts for the importance of temperature in H2 formation. The model suggests that H2 production increases with the rate of seafloor spreading and the net thickness of serpentinized peridotite (S-P) in a column of lithosphere. The model is applied globally to the MOR using conservative estimates for the net thickness of lithospheric S-P, our least certain model input. Despite the large uncertainties surrounding the amount of serpentinized peridotite within oceanic crust, conservative model parameters suggest a magnitude of H2 production (~1012 moles H2/y) that is larger than the most widely cited previous estimates (~1011 although previous estimates range from 1010-1012 moles H2/y). Certain model relationships are also consistent with what has been established through field studies, for example that the highest H2 fluxes (moles H2/km2 seafloor) are produced near slower-spreading ridges (<20 mm/y). Other modeled relationships are new and represent testable predictions. Principal among these is that about half of the H2 produced globally is produced off-axis beneath faster-spreading seafloor (>20 mm/y), a region where only one measurement of H2 has been made thus far and is ripe for future investigation.

In the second part of this dissertation (Chapter 3), I construct the first budget for free H2 in young ocean crust that quantifies and compares all currently recognized H2 sources and H2 sinks. First global estimates of budget components are proposed in instances where previous estimate(s) could not be located provided that the literature on that specific budget component was not too sparse to do so. Results suggest that the nine known H2 sources, listed in order of quantitative importance, are: Crystallization (6x1012 moles H2/y or 61% of total H2 production), serpentinization (2x1012 moles H2/y or 21%), magmatic degassing (7x1011 moles H2/y or 7%), lava-seawater interaction (5x1011 moles H2/y or 5%), low-temperature alteration of basalt (5x1011 moles H2/y or 5%), high-temperature alteration of basalt (3x1010 moles H2/y or <1%), catalysis (3x108 moles H2/y or <<1%), radiolysis (2x108 moles H2/y or <<1%), and pyrite formation (3x106 moles H2/y or <<1%). Next we consider two well-known H2 sinks, H2 lost to the ocean and H2 occluded within rock minerals, and our analysis suggests that both are of similar size (both are 6x1011 moles H2/y). Budgeting results suggest a large difference between H2 sources (total production = 1x1013 moles H2/y) and H2 sinks (total losses = 1x1011 moles H2/y). Assuming this large difference represents H2 consumed by microbes (total consumption = 9x1011 moles H2/y), we explore rates of primary production by the chemosynthetic, sub-seafloor biosphere. Although the numbers presented require further examination and future modifications, the analysis suggests that the sub-seafloor H2 budget is similar to the sub-seafloor CH4 budget in the sense that globally significant quantities of both of these reduced gases are produced beneath the seafloor but never escape the seafloor due to microbial consumption.

The third and final component of this dissertation (Chapter 4) explores the self-organization of barchan sand dune fields. In nature, barchan dunes typically exist as members of larger dune fields that display striking, enigmatic structures that cannot be readily explained by examining the dynamics at the scale of single dunes, or by appealing to patterns in external forcing. To explore the possibility that observed structures emerge spontaneously as a collective result of many dunes interacting with each other, we built a numerical model that treats barchans as discrete entities that interact with one another according to simplified rules derived from theoretical and numerical work, and from field observations: Dunes exchange sand through the fluxes that leak from the downwind side of each dune and are captured on their upstream sides; when dunes become sufficiently large, small dunes are born on their downwind sides (“calving”); and when dunes collide directly enough, they merge. Results show that these relatively simple interactions provide potential explanations for a range of field-scale phenomena including isolated patches of dunes and heterogeneous arrangements of similarly sized dunes in denser fields. The results also suggest that (1) dune field characteristics depend on the sand flux fed into the upwind boundary, although (2) moving downwind, the system approaches a common attracting state in which the memory of the upwind conditions vanishes. This work supports the hypothesis that calving exerts a first order control on field-scale phenomena; it prevents individual dunes from growing without bound, as single-dune analyses suggest, and allows the formation of roughly realistic, persistent dune field patterns.

Watersheds are complex, three dimensional structures that partition water between the components of the water balance and multiple storage pools within the watershed. This central function, however, remains poorly understood in a broadly transferable way despite decades of research. Perhaps one reason for this is the disciplinary bias towards studying pristine, mountainous watersheds with steep terrain and shallow soil. Although the relative simplicity of such systems has made them ideal hydrologic laboratories, understanding how watersheds function globally will require the incorporation of new types of landscapes into the studies of hillslope and watershed hydrology.

The Southern Piedmont region of the United States is situated between the Appalachian mountains and Atlantic coastal plains and stretches from Alabama to Maryland. It’s generally rolling terrain if underlain by deeply weathered and highly stratified soil characterized by relatively shallow argillic Bt horizons while weathered saprolite can extend tens of meters deep. Although it is a low-relief landscape, headwaters are often highly dissected with steep narrow valleys containing temporary streams surrounded by diverse topography. The region represents an ideal opportunity to incorporate more diverse landscapes into our studies of watershed hydrology.

As part of the NSF funded Calhoun Critical Zone Observatory, we intensively instrumented a 6.9 ha headwater (watershed 4, WS4), along with other targeted sensor locations including discharge in the 322 ha watershed that contains it (Holcombe’s Branch, HLCM), a nearby meteorological station, a deep groundwater well on a relatively flat interfluve, and a small network of wells in the buried floodplain. Sensors were continuously monitored for over 3 years while logging at 5 minute intervals. This sensor network allowed us to quantify the timing and magnitude of runoff, precipitation, deep and shallow groundwater levels distributed across a watershed, and soil moisture at multiple depths and hillslope positions. By doing so we were able to 1) describe the interactions between water balance components in WS4, 2) compare these watershed-scale measurements to internal hydrologic dynamics to determine what parts of the watershed are responsible for distinct watershed functions, and 3) explore how headwaters connect to higher order streams.

Using the monthly water balance in WS4, we calculated changes in integrated watershed storage and then derived a cumulative monthly storage time series from its running integral. We found that storage changes within the year by hundreds of millimeters (~25% of annual precipitation) in conjunction with seasonal peaks in evapotranspiration. Additionally, of all the potential variables that correlated to runoff magnitudes at the watershed scale, we found storage to be the best, particularly above a threshold value which remained remarkably consistent across all three years even with substantial differences in precipitation.

However, despite the storage threshold dependence of runoff, when we calculated daily storage we found that while runoff increased primarily in response to major precipitation events and then decreased again shortly thereafter, storage primarily wet up once from its low point at the end of the growing season and then drained starting at the growing season and continuing through the summer. Similarly, individual measurements of internal watershed hydrology like soil moisture or water table level displayed either seasonal or event-scale changes. We determined that measurements taken at watershed positions with more convergent hillslopes, or farther from the watershed divide, or installed deeper in the soil are more likely to display seasonal changes, and vice versa for event-scale changes. These three gradients are essentially proxies for vertical, lateral, and longitudinal distances, and so it appeared that the underlying gradient being measured was actually contributing volume. We determined the functions of different landscape components based on this analysis, and came to understand that storage-linked sites wet up first and then stay consistently so, making conditions for runoff. Subsequently, when runoff-linked sites wet-up, they mobilize significant runoff fluxes either by hydraulic displacement, or interflow, or a transmissivity feedback, or likely some combination of them all. During these times a substantial portion of the watershed is connected before drying down again with the exception of more storage-linked locations.

The result of this threshold setting followed by large runoff events is extremely flashy outputs from WS4. In contrast, we found HLCM to be far less flashy and relatively less sensitive to year to year fluctuations in precipitation. Further, we observed that except in the most extreme storms, surface flow from WS4 across the former floodplain in between it and HLCM always fully infiltrates into the sandy, legacy sediments deposited along the entire former floodplain. These sediments are the legacy of centuries of intensive and poorly managed agriculture across the Southern Piedmont. Wells in these sediments revealed a highly dynamic water table that was very responsive to outflow from WS4. A simple geometric simplification of the shape of these sediments and an estimate of their porosity revealed that these sediments had ~900 m3 of available storage space, space that was constantly filling and draining. Interestingly, that available storage volume level was sufficient to absorb discharge from WS4 on 97% of the days we measured. Through most of WS4 flow states, this storage served to buffer HLCM from flashier runoff coming from WS4, and then subsequently releasing it much more slowly and drawn out as shallow subsurfaceflow. However, when it reached volumes within 15% of maximum, usually in conjunction with large fluxes coming from WS4, runoff in HLCM reacted closely with WS4. So the storage volume in legacy sediments serves as an effective buffer from flashy upstream hydrology, but when the reach or approach saturation they become effective at transmitting surface flow, likely via saturation excess. Although we observed this phenomenon in only one alluvial fan, we have reason to think that such features are quite common locally and regionally, and represent a heretofore underappreciated legacy of historic agriculture.

Taken together, these findings describe a hydrologic system that is much more dynamic than its abundant rainfall and surface water resources would suggest. Further, they indicate that even a century or more after agricultural land abandonment and forest regrowth, legacies of the 18th and 19th century remain in the landforms and soils of the region. We feel that these findings are strong support for continued and expanded hydrologic study at the CCZO and in the Southern Piedmont in general.

Sorted bedforms are spatial extensive (100 m-km) features present on many inner continental shelves with subtle bathymetric relief (cm-m) and localized, abrupt variations in grain size (fine sand to coarse sand/gravel). Sorted bedforms provide nursery habitat for fish, are a control on benthic biodiversity, function as sediment reservoirs, and influence nearshore waves and currents. Research suggests these bedforms are a consequence of a sediment sorting feedback as opposed to the more common flow-bathymetry interaction. This dissertation addresses three topics related to sorted bedforms: 1) Modeling the long-term evolution of bedform patterns, 2) Refinement of morphological and sediment transport relations used in the sorted bedform model with `machine learning'; 3) Development of a new sorted bedform model using these new `data-driven' components.

Chapter 1 focuses on modeling the long term evolution of sorted bedforms. A range of sorted bedform model behaviors is possible in the long term, from pattern persistence to spatial-temporal intermittency. Vertical sorting (a result of pattern maturation processes) causes the burial of coarse material until a critical state of seabed coarseness is reached. This critical state causes a local cessation of the sorting feedback, leading to a self-organized spatially intermittent pattern, a hallmark of observed sorted bedforms. Various patterns emerge when numerical experiments include erosion, deposition, and storm events.

Modeling of sorted bedforms relies on the parameterization of processes that lack deterministic descriptions. When large datasets exist, machine learning (optimization tools from computer science) can be used to develop parameterizations directly from data. Using genetic programming (a machine learning technique) and large multisetting datasets I develop smooth, physically meaningful predictors for ripple morphology (wavelength, height, and steepness; Chapter 2) and near bed suspended sediment reference concentration under unbroken waves (Chapter 3). The new predictors perform better than existing empirical formulations.

In Chapter 3, the new components derived from machine learning are integrated into the sorted bedform model to create a `hybrid' model: a novel way to incorporate observational data into a numerical model. Results suggest that the new hybrid model is able to capture dynamics absent from previous models, specifically, the two observed end-member pattern modes of sorted bedforms (i.e., coarse material on updrift bedform flanks or coarse material in bedform troughs). However, caveats exist when data driven components do not have parity with traditional theoretical components of morphodynamic models, and I address the challenges of integrating these disparate pieces and the future of this type of `hybrid' modeling.

This dissertation asks how anthropogenic disturbances are subsequently modified by pedogenic processes over century and decadal-time scales in two soil systems that have, at best, a modicum of previous study. The first system is a bottomland floodplain with legacy sediment in a low-gradient Piedmont watershed that has experienced a century of pedogenesis following accelerated sedimentation. The second is the urban soil of a mid-sized city in the Piedmont, where the elapse of half a century has transformed soil lead concentrations from gasoline and lead-based paint. This dissertation broadens the focus of land-use legacies in the Southern Piedmont and its results bear upon decisions in floodplain restoration and soil Pb exposure.

The dynamics of bottomland floodplains laden with legacy sediments are interrogated over two chapters. Chapter 2 constrains the timing of legacy sediment deposition with radioactive isotopes and demonstrates that legacy sediment soils limit accumulation of mineral-associated soil carbon, suggesting changes to the floodplain regime since sediment deposition. Chapter 3 asks whether legacy sediments drive loss of floodplain function by resolving belowground dynamics of soil moisture and redox regimes over 18 months of field-level measurements. These findings shed light on the mechanisms by which widespread legacy sediment deposition has transformed Piedmont floodplains to be drier and more aerobic. Urban soil Pb legacies are examined in Chapter 3, which conducts the first city-wide sampling of soil Pb in North Carolina. This work reconciles the immobility of soil Pb, reflected in resolvable patterns still present today, with the mobility and decreasing concentrations of soil Pb enabled by human activities and soil redistribution processes. Overall, these findings show how soil processes transform environmental legacies from human-driven disturbances over time and contribute to pedology’s pursuit of centuries- and decades-scale soil change.

In this dissertation, human-critical zone (CZ) dynamics are explored at the Calhoun Critical Zone Observatory (CCZO). The 190km2 CCZO is part of a broader landscape in the southeastern US which was subjected to extensive agricultural degradation for approximately two hundred years before cultivation was abandoned sixty or more years ago. The physical and functional dynamics of land abandonment were explored at three spatial scales herein: 1) landscape geomorphology and spatial patterns of CZ processes, 2) microtopographic roughness of hillslopes being diagnostic of landuse history and, 3) plot-based soil investigations of three chronosequence landuse histories approximating the temporal successional progression from pre-disturbance forested landcover into deforested agricultural management and finally into secondary old-field mixed pine forests which typify the post-agricultural landscape of the Southeastern US.

In the first chapter it was observed that most of the landscape area consists of hillslopes and that quantifying the spatial patterns and connections of hillslopes is important for understanding landscape function and evolution. Landscape network structure was described and quantified with demonstration of how it drives the landscape processes of soil erosion, bedrock weathering, and landcover. In this work, interfluve (hilltop) networks were ordered according to Hortonian methodology at the CCZO in South Carolina with corresponding “hillshed” areas delineated and bounded at the base by valleys and streams. At the CCZO, low-order interfluves are abundant, small, and steep with low elevation, while high-order interfluves are fewer, broad, and relatively flat with high elevation. We further estimated that geologic erosion rates of 1st order interfluves were two orders of magnitude higher than on those of 4th or 5th orders, bedrock weathering is modeled to be deepest and most spatially variable on 1st order interfluves with depth and depth-variability decreasing as interfluve-order increases. It is further shown that agriculture and private land ownership has concentrated and persisted to the greatest extent on high-order interfluves where erosion has been far less serious than on low-order interfluves. We conclude with an assertion that land-management and many fields of environmental research can benefit from ordered-interfluve networks and corresponding hillsheds much as they have benefited from the concepts of stream-orders and watersheds.

The Calhoun Critical Zone Observatory in the Piedmont region of South Carolina is an ancient, highly weathered landscape which was transformed by historic agricultural erosion. Following the conversion of hardwood forests to cultivated fields and pastures for ~200 years, excess runoff from fields led to extreme sheet, rill, and gully erosion across the landscape. Roads, terraces, and a variety of other human disturbances also increased the landscape’s surface roughness. By the 1950s, cultivation-based agriculture had been abandoned in most of the Southern Piedmont due to soil erosion, declining agricultural productivity, and shifting agricultural markets. Forests dominated by loblolly and shortleaf pine species, have since reforested much of the landscape. There are, however, isolated hardwood forest stands and even entire small watersheds dotting the landscape which are believed to have never been clear-cut or plowed. These rare forests are expected to have special aesthetic and scientific research value as a Piedmont pre-disturbance reference condition. Hardwood reference forests may be of interest to hydrologists, environmental historians, biogeochemists, geomorphologists, geologists, pedologists, and others interested in understanding the legacy of landuse history in this severely altered environment. In this work we demonstrated how Light Detection And Ranging (LiDAR) digital elevation model (DEM) data and microtopographic terrain roughness analysis (MTRA) of fine scale variation in terrain slope can be used in concert with historic aerial photography, contemporary remote sensing data, and field ecological interpretation to identify low human-impact, minimally eroded, reference hardwood stands, hillslopes, and even small watersheds for study and conservation.

Following the identification and selection of reference hardwood forests as a landuse history treatment for intensive study, novel method development was needed in order to accurately and effectively study the belowground dynamics of these forests and other landuse histories of interest. One of the analytical tools necessary for this was the development of effective and reliable Field-Portable Gas Analysis tools (FPGA). FPGAs have been developed at the Calhoun Critical Zone observatory for the measurement and monitoring of deep soil O2 and CO2 under different landuse history treatments. The tools and methodology developed and presented here are extremely cost-effective, are physically very light, compact, and robust for field deployment and reliable soil gas monitoring. The FPGA platform integrates off-the-shelf components including Vaisala™ non-dispersive infrared (NDIR) CO2 probes and electro-chemical Apogee™ O2 meters for flow-through gas analyses of soil gas using a Cole-Parmer™ vacuum-pressure pump. More than 1600 soil gas measurements have been made using these devices over more than 2 years of observations. Measurement accuracy of the FPGA appears very consistent compared with conventional bench-top gas chromatography and time series representations of paired CO2 and O2 measurement under hardwood forests at the CCZO indicate the ability to observe and track seasonal and climatic patterns and events with this technology. Further, the ability to analyze the apparent respiratory quotient, the ratio of CO2 production divided by O2 depletion relative to the aboveground atmosphere, indicates a high degree of sophisticated analyses are made possible with the FPGA platform. The high accuracy and reliability of the FPGA platform for soil gas monitoring allows for temporally extensive and spatially expansive studies of soil respiration.

With the development of the FPGA and other empirical analyses, plot-based field investigations were undertaken to determine how different critical zone measurements are able to quantify the vertical propagation of forest regeneration downward into soil profiles. The below-ground effects of 60-80 years of old-field forest succession in the CCZO were explored via chronosequence landuse history investigation. Chronosequence plots consist of reference hardwood forests, plowed agricultural fields, and old-field secondary pine forests. In this framework, reference hardwood soil profiles are minimally degraded in terms of erosion, soil structure, and soil biogeochemistry while currently cultivated agricultural plots are maximally impacted. Old-field secondary pine forests are considered to be intermediate and partially regenerated in terms of soil structure and function as reforestation has occurred and proceeded for decades, though the degree of regeneration was uncertain. Each landuse history comparison plot has been studied and soil structural regeneration of bulk density, macropores >0.075mm in diameter via X-ray Computed Tomography, and soil aggregate stability. Deeper monitoring of soil CO2 and O2 using FPGAs characterized belowground forest functioning down to 5m soil depth.

Hardwood forest soils have been observed to have higher CO2 concentrations and lower O¬2 below 2m soil depth, past the soil B-horizon, than either agricultural plots or old-field pine forests. Results indicate that while there has been a high degree of soil regeneration above the B-horizon in old-field secondary forest soils in terms of rooting, respiration dynamics, and soil structure, deep CZ processes below 2m remain significantly altered under old-field pine forests. This suggests that there may be a lag in below-ground regeneration relative to surficial soil regeneration. This appears to be due the hindering of root and macropore regeneration by a thick, low permeability, B-horizon. Abiotic CZ processes like storms which affected all treatments were also investigated as rapid declines in CO2 concentrations were observed deep in soil profiles during periods of intense precipitation. This is presumably due to CO2 dissolution and export into groundwater and indicates great potential to advance the fundamental understanding of the linkages between upland management and landcover, aerobic respiration, and deep critical zone processes like mineral weathering and the export of terrestrially-derived CO2 to streams.

This aim of this dissertation is to investigate "pedogenesis" (soil formation and change over multi-millennial timescales with minimal human impact) and "anthropedogenesis" (centurial and decadal soil formation and change through the Holocene with increased human influence) in the highly weathered, upland soils of the Southern Piedmont physiographic region in the southeastern United States. I start by combining an analysis of the cosmogenic nuclide meteoric beryllium-10 (10Be) with a mass balance analysis of pedogenic 9Be loss to estimate how long the Southern Piedmont Ultisol have been residing at Earth's surface. This coupled analysis indicates that pedogenesis has been operating in these highly weathered Ultisols for much, if not all, of the Quaternary; considerably longer than previously thought. Next, I utilize traditional soil analyses alongside iron stable isotope measurements to investigate how one century of reforestation after agricultural land abandonment impacts the coupled carbon -iron cycle in these ancient subsoils. This project suggests that widespread patterns of anthropogenic land use change in the Southern Piedmont have caused significant subsoil changes that impact carbon storage and the distribution of iron deep below ground. Finally, I analyze over 50 years or repeated soil and forest ecosystem observations from the Calhoun Experimental Forest to investigate the relationship between soil macronutrient contents aboveground forest ecosystem development in the region. These long term observations suggest that decadal patterns of secondary forest growth and decline fundamentally alters the role that soils plays in individual ecosystem nutrient cycles and that the potential for ecosystem nutrient loss is highly nutrient dependent, despite well-established ecological theory.

This dissertation focuses on the riverine water column and the lentic (i.e. lake like) nature of rivers in the context of predominant themes in river science: spatial heterogeneity and scale. River science has developed many concepts to describe and understand the hydrologic, geomorphic, and ecological structure and function of rivers. While these core concepts largely grapple with spatial heterogeneity and scale, they have generally not conceptualized the water column as unit of study nor have they integrated lakes and rivers as one hydrologic system. Understanding the spatial heterogeneity and scaling patterns within the water column itself and how lakes fit into river networks will advance our understanding of geomorphic and ecological processes of entire networks.

The study approach includes field campaigns using in-situ sensors, analysis of large data sets, and conceptual modeling. Chapter 2 develops an analytical model implemented with empirical data to find the location along a river where there is more sediment surface area in the water column than the benthic zone. Chapter 3 integrates flowpath and fixed-site measurement approaches to characterize the spatial and temporal scales of variability in water column light regimes. Chapter 4 analyzes large datasets to understand the scaling patterns of lake abundance, lake size, and lake spacing with river size across the conterminous US.

Conclusions from this research have theoretical and practical implications. First, rivers larger than ~5th order had more sediment surface area in the water column than the benthic zone. This suggests material processing may occur largely within the water column in large rivers. Studying large rivers may therefore require different conceptual and methodological approaches, and it may be inappropriate to scale up measurements from small streams. Second, large rivers had an expanding and contracting photic volume over multiple temporal and spatial scales. Photo-reactive processes in the water column are therefore limited by the size of these light and dark zones and turbulent fluctuations along flowpaths through the river. Third, river networks are, in-fact, river-lake networks that have characteristic scaling patterns that describe lake abundance, size, and spacing. This suggests the default conceptual model of river networks should be river-lake networks.

Rivers transform more than half of the organic inputs they receive from terrestrial systems through metabolic processes. In addition to providing the energetic base to sustain stream food webs, these transformations are linked to multiple elemental cycles and result in the release of greenhouse gasses to the atmosphere, oxygen depletion, and the mobilization of trace metals. Despite this central importance we lack a sufficient understanding of what drives processes within these ecosystems to make broad scale predictions about metabolic rates and biogeochemical cycles. This dissertation addresses how local controls interact with hydrologic and geomorphic constraints to determine the nature and magnitude of stream carbon and energy cycling. Through field observations and ecosystem process models, this study characterizes the spatial and seasonal biogeochemical dynamics of a single study system, New Hope Creek, a low gradient 3rd order river in the North Carolina Piedmont. I present a mechanistic explanation of stream ecosystem functioning from three different perspectives and use the insights gained to confront existing conceptual models and methods in river science. In chapter two, I examined the frequency and dynamics of river hypoxia, or depleted dissolved oxygen, in the North Carolina Piedmont by synthesizing state monitoring records since the 1960s and by collecting high resolution measurements of oxygen along a 20 km section of New Hope Creek. In chapter three, I monitored dissolved oxygen concentrations and modeled the metabolic regimes across six river segments over the course of three years. I compared the rates and seasonality of annual metabolism across space and time and examined the impact of climatic, hydrologic, and geomorphic drivers. Using the scales of this variation for context, I compared the metabolic regime in New Hope Creek today to a historical study of annual metabolism collected fifty years ago in the same site. In chapter four, I measured dissolved greenhouse gas concentrations from the autumn to the following spring at the six study locations from chapter three. I calculated the rate of exchange of CO2, CH4, N2O and O2 with the atmosphere, estimated the fraction of this flux that was attributable to instream metabolic processes, and determined the best predictors for each concentration and flux rate. The results from this research have implications for both conceptual models and methodological approaches. First, I found that hypoxia is widespread throughout New Hope Creek. It can arise through oxygen supply limitation due to seasonal low flows and warm water, even in the absence of high organic matter and nutrient loading. Hypoxia is most frequent at night and in pools, and is systematically underrepresented by state-wide monitoring records which rely on point samples and avoid both night and pools. Second, warmer water temperatures shift stream ecosystems toward more net heterotrophy. The effects of rising temperatures on stream ecosystem respiration are contingent upon organic matter inputs and storage, each of which are strongly constrained by flow regime and local geomorphology. Hydrologic settings in which storms are more frequent or more severe will drive high variability in the transport and fate of organic matter processing in streams. Third, the geomorphology and hydrology of New Hope Creek create conditions under which stream metabolic cycling is the dominant control on greenhouse gas concentrations and flux rates. These findings challenge preconceived notions about how rivers work and encourage us to reconsider conceptual models. Rivers are lotic ecosystems, which are defined by their advective flow, with the implication that they are well aerated and rarely hypoxic. Methods in stream ecology are not suited to study the conditions that arise during lentic, or non-flowing, time periods and as a result, the geomorphologies that create these conditions are avoided and understudied. However, these results suggest that they are control points of biogeochemical activity and that they may be more susceptible to change in response to anthropogenic drivers. Collectively, this study calls into question the binary distinction between lotic and lentic ecosystem dynamics with which we tend to categorize and study freshwater ecosystems. To understand the full range of variability of inland waters we must broaden our conceptual frameworks and adapt our methods of study to encompass ecosystems that span this divide.

Coastal wetlands are complex biogeomorphic systems that provide important ecosystem services, but our current understanding of salt marsh evolution and persistence is based on models and empirical studies of limited spatial and temporal extent. In this dissertation, I ask: How and why do coastal wetlands form and persist? Using a geospatial framework of publicly available datasets, I analyzed drivers of wetland extent along the Atlantic and Gulf coasts of the United States. Results establish that distinct modes of wetland extent (fringing and basin full wetland extent) occur at spatial scales (approximately 10^0 to 10^2 km^2) predicted by theoretical models of local feedbacks among fetch, wind erosion, and marsh building. Marsh distributions reflect interactions between these local biogeomorphic feedbacks and macroscale drivers that set boundary conditions, including estuarine-scale morphology that governs wave energy, and riverine influence affecting sediment availability and transport. These relationships varied among regions by a regionally characteristic set of factors: estuary shape complexity, depth, estuary area and relative dominance of riverine to estuary volume. Using new and existing sediment cores from tidal marshes along the Atlantic and Gulf coasts, I analyzed the timing and spatial variability of wetland formation. Although most cores formed after the stabilization of sea level subsequent to the last ice age (approximately 4000 to 6000 ybp), overwash events, connection to major riverine systems, riverine morphology and timing of peak agriculture post European settlement all created spatial and temporal variability in the age of marshes. Historic sea level rise studies dominated the literature found for this study, pointing to the need for targeted investigations of drivers of tidal marsh formation. By reimagining tidal marshes in a macroscale framework, I investigated both the spatial and temporal drivers of land-water linkages and coastal wetland formation and persistence, elucidating ultimate drivers and future impacts on coastal wetlands from environmental pressures.

Low-lying coastal landforms such as barrier islands and river deltas are attractive sites for human habitation and infrastructure. They are also highly vulnerable to both climate change impacts such as rising sea levels or increases in storm intensity and anthropogenic impacts such as changes in sediment supply. In this dissertation I aim to improve understanding of some of the primary drivers of the evolution of low-lying coastal landforms over varying space (1-100s km) and time (decadal to millennial) scales. I focus in Chapter 2 on the influence of shoreline curvature and resulting gradients in alongshore sediment transport on shoreline change; in Chapter 3 on the influence of wave-edge erosion on back-barrier marsh resilience; and in Chapters 4 and 5 on the cohesive effects of vegetation on river deltas.

Sandy coastlines, often associated with low-lying barrier islands that are highly vulnerable to sea level rise and storms, can experience high rates of shoreline change. However, they also attract human habitation, recreation, and infrastructure. Previous research to understand and quantify contributions to shoreline erosion has considered factors such as grain size, underlying geology, regional geography, sea level rise, and anthropogenic modifications. Shoreline curvature is often not considered in such analyses, but subtle shoreline curvature (and associated alongshore variation in relative offshore wave angles) can result in gradients in net alongshore transport which can cause significant erosion or accretion. In Chapter 2, we conducted a spatially extensive analysis of the correlation between shoreline curvature and shoreline change rates for the sandy shorelines of the US East and Gulf coasts. For wave-dominated, sandy coasts where nourishment and shoreline stabilization do not dominate the shoreline change signal, we find a significant negative correlation between shoreline curvature and shoreline change rates over decadal to centurial and 1-5 km temporal and spatial scales. This indicates that some of the coastal erosion observed in these areas can be explained by the smoothing of subtle shoreline curvature by gradients in alongshore transport. In other settings, this signal can be obscured by tidal, anthropogenic, or geologic processes which also influence shoreline erosion. While limited in practical application to long, sandy shorelines with limited human stabilization, these results have widespread implications for the inclusion of shoreline curvature as an important variable in modelling and risk assessment of long-term coastal erosion on sandy, wave-dominated coastlines.

The marshes and bays in the back-barrier environment between barrier islands and the mainland can also experience wave-driven erosion, and their dynamics are coupled to those of barrier islands. Previous results show that overwash provides an important sediment source to back-barrier marshes, sustaining a narrow marsh state under conditions in which marsh drowning would otherwise occur. In Chapter 3, I expand the coupled barrier island-marsh evolution model GEOMBEST+ to explore the effects of wind waves on back-barrier marshes. I find that the addition of marsh-edge erosion leads to wider, more resilient marshes and that horizontal erosion of the marsh edge is a more efficient sediment source than vertical erosion of the marsh surface as it drowns. Where marshes and bays are vertically keeping up with sea level, and the net rate of sediment imported to (or exported from) the basin is known, the rate of marsh-edge erosion or progradation can be predicted knowing only the present basin geometry, sea-level rise rate, and the net rate of sediment input (without considering the erosion or progradation mechanisms). If the rate of sediment input/export is known, this relationship applies whether sediment exchange with the open ocean is negligible (as in basins dominated by riverine sediment input), or is significant (including the loss of sediment remobilized by waves in the bay). Analysis of these results reveals that geometry and stratigraphy can exert a first order control on back-barrier marsh evolution and on the marsh-barrier island system as a whole, and provides new insights into the resilience of back-barrier marshes and on the interconnectedness of the barrier-marsh system.

Coastal wetlands such as marshes are also an important component of river deltas. Like barrier islands, these low-lying landscapes are both attractive to human settlement (providing fertile farmland, fisheries, hydrocarbon reserves, and many other services) and prone to hazards such as flooding and land loss. Delta evolution is governed by complex interactions between coastal, marine, and fluvial processes, many of which are still not well understood. In Chapters 4 and 5, I use the delta-building model DeltaRCM to explore the effects of vegetation, specifically its ability to introduce cohesion, on delta morphology and the dynamics of delta distributary networks. The use of this rule-based model allows me to simplify vegetation dynamics and effects in order to enhance the clarity of potential insights into which processes or interactions may be most important in the context of vegetation as a cohesive agent.

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. In Chapter 4, I use DeltaRCM to directly explore two cohesive effects of vegetation: decreasing lateral transport and increasing flow resistance. I find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g. more rugose shorelines, deeper, narrower, less mobile channels), but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. My results suggest that sediment composition is a first-order control on delta morphology but vegetation has a stronger influence on channel mobility timescales. To fully understand the cohesive influences acting on a delta, the influence of vegetation should be considered in addition to fine sediment.

In Chapter 5, I explore the cohesive effects of vegetation on delta evolution under different environmental conditions. The dynamics and evolution of deltas and their channel networks are controlled by interactions between a number of factors, including water and sediment discharge, cohesion from fine sediment and vegetation, and sea level rise rates. Vegetation’s influence on the delta is likely to be significantly impacted by other environmental factors. For example, increasing sea level or sediment discharge increases aggradation rates on the delta, and may result in sediment transport processes such as deposition and erosion, both of which can kill vegetation, happening more rapidly than vegetation growth. I conduct two sets of experiments; in the first, I explore the interactions between vegetation and sea level rise rate, and in the second, between vegetation and rate of sediment and water discharge. As expected, I find that sea level rise decreases vegetation’s ability to stabilize channels but that vegetation can still exert a strong influence on the delta at low rates of sea level rise. This limit appears to be higher for channel dynamics than delta morphology, supporting the findings of Chapter 4. In addition, I propose two new insights into delta evolution under different discharge conditions with and without vegetation. First, without vegetation, I observe a shift in avulsion dynamics with increasing water discharge: from a few active channels supplemented by overbank flow and undergoing episodic avulsion (with low discharge) to many active channels experiencing frequent local and partial avulsions (with high discharge). Second, with vegetation, increased sediment discharge and associated aggradation results in more frequent switching of the dominant channels, but also prevents vegetation from establishing in non-dominant channels, resulting in more frequent channel reoccupation and therefore in channel network planform stability. These insights have important implications for understanding the distribution of water, sediment, and nutrients on deltas in the face of future changes in climate, human modifications of fluxes of sediment and water to the coast, and especially for restored or engineered deltas with controlled water or sediment discharges.

Modeling studies predict that droughts and hotter water and air temperatures caused by climate warming will reduce the efficiency (η) of thermoelectric plants by 0.12-0.45% for each 1°C of warming. In Chapter 2, we evaluate these predictions using historical performance data for 39 open- and closed-loop, coal and natural gas plants from across the U.S., which operated under daily and seasonal temperature fluctuations multiples greater than future average warming projections. Seven to fourteen years of hourly water (Tw), dry-bulb air (Ta) and wet-bulb air (Twb) temperature recordings collected near each plant are regressed against efficiency to attain estimates of ∆η per 1°C increase. We find reductions in η with increased Tw (for open-loop plants) up to an order of magnitude less than previous estimates. We also find that changes in η associated with changes in Ta (open-loop plants) or Twb (closed-loop plants) are not only smaller than previous estimates but also variable, i.e. η rises with Ta or Twb for some plants and falls for others. Our findings suggest that thermoelectric plants, particularly closed-loop plants, should be more resilient to climate warming than previously expected. Moreover, our results raise questions regarding the relative impacts of climate change-induced drops in water availability versus increases in ambient temperatures on the ability of thermoelectric power plants to generate power.

In Chapter 3, we explore and compare the effects of decreased water availability and increased water temperature on once-through power plants, which are expected to suffer more of the impacts of climate change than recirculating plants. Currently, little is known about which of the constraints, water temperature or availability, has a greater impact on power generation, and how these impacts and trends may vary with plant age, nameplate capacity, fuel type, generator technology, and location. We apply seven years of historical data from 20 once-through coal and natural gas plants into a thermoelectric power generation model to simulate how changes in various external parameters (water temperature, temperature regulations, and water availability) can affect the usable capacity of these plants. We find that depending on the plant, streamflow can contribute to 0-35% of the capacity reduction, while temperature can contribute 0-17% and regulations 48-100%. We also observe that power plants located on smaller water bodies (i.e., <3000 m^3/s in this study) are more likely to be severely impacted in future climate extreme events than plants located in other areas, regardless of power plant technology.

The fourth and final chapter of this dissertation diverges from the previous chapters and examines the processes that influence the evolution of fluvio-deltaic systems at passive continental margins. Depositional and erosional patterns that were previously believed to be entirely produced by externally-derived (allogenic) processes are now being recognized as patterns that can develop from autogenic interactions (e.g., channel avulsion and delta lobe switching). In this work, we are interested in understanding how traditional, allogenically-based interpretations of these systems change when we incorporate the impacts of autogenic processes. We introduce a novel first-order numerical basin-filling model to address this question. This model differs from previous work in that external inputs (i.e., subsidence rate, base level change, sediment supply) as well as streamwise and cross-stream transport coefficients can be adjusted to simulate basins that are dominated by allogenic processes (i.e., subsidence, eustasy, and sediment supply), laterally-moving autogenic processes, or a combination of both. Because of this, the model can be used to track how autogenic and allogenic processes interact to impact the evolution of fluvio-deltaic systems as more and more autogenic forcing is introduced. Our ability to identify, separate, and understand the geomorphic and stratigraphic signals of internally-derived processes from those of external controls is critical for better understanding shelf development.