Mountain Scholar :: Browsing by Author "Morrison, Ryan, committee member"

Browsing by Author "Morrison, Ryan, committee member"

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Open Access
A catchment is more than the sum of its reaches: post-fire resilience at multiple spatial scales
(Colorado State University. Libraries, 2024) Triantafillou, Shayla P., author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Morrison, Ryan, committee member
As wildfires are projected to increase in frequency and severity, there is a growing interest in understanding river resilience to the wildfire disturbance cascade. Numerous 3rd-order mountain catchments within the Cache la Poudre (Poudre) River basin in the Colorado Front Range, USA burned severely and extensively during the 2020 Cameron Peak fire. Many of these catchments experienced debris flows and flash floods triggered by convective storms after the fire. The downstream effects of the debris flow sediment varied along a continuum from attenuated and largely contained within the catchment, through contributing to a pre-existing debris fan at the catchment outlet, to releasing substantial volumes of water and sediment to the Poudre River. I conceptualize these catchments as exhibiting decreasing resilience to post-fire disturbance along the continuum described above based on the geomorphic evidence of relative sediment export. The characteristics affecting resilience and magnitude of response to disturbance span multiple spatial scales from the catchment to stream corridor reaches hundreds of meters in length. I conceptualize characteristics on different spatial scales as driving or resisting response to disturbance and therefore impacting the resilience outcome of the catchment. As the magnitude of resisting characteristics increases at the catchment, inter- and intra- reach scales, I hypothesize that a catchment will be more resilient to the wildfire disturbance cascade. At the catchment scale I consider geomorphic, burn, vegetation, and precipitation characteristics. I conducted longitudinally continuous surveys to measure reach-scale characteristics within each study catchment. I focus on the reach-scale geomorphic, vegetation, and burn characteristics, with a particular focus on elements that introduce inter- and intra-reach spatial heterogeneity including channel planform, beaver-modified topography, the distribution of channel and floodplain logjam distribution density, and the floodplain width/channel width ratio for the population of reaches within each catchment. The floods observed at the study catchments illustrate fire lifting the elevation above which rainfall-induced flooding occurs due to the efficient conveyance of water from hillslopes to channels after wildfire. Results suggest that inter- and intra-reach spatial heterogeneity are better descriptors of resilience than catchment-scale characteristics: resilience is associated with greater longitudinal variations in floodplain/channel width and more reaches with wide floodplains, low channel gradients, beaver-modified topography, and multi-stem deciduous vegetation.
Open Access
Assessing and managing urban riverscapes: integrating physical processes and social-ecological values
(Colorado State University. Libraries, 2022) Murphy, Brian Michael, author; Nelson, Peter, advisor; Wohl, Ellen, committee member; Grigg, Neil, committee member; Morrison, Ryan, committee member
In the age of the Anthropocene, human influence has spread far and wide across our planet affecting the physical, chemical, and biological condition of the rivers, streams, and floodplains in the urban environment, our "urban riverscapes." The human connection to urban riverscapes includes both the built environment created and accessed by people and the intangible community values that humans place upon flowing water. The value of these benefits encourages stewardship of our waterways by integrating experiential, aesthetic, and cultural attributes that foster appreciation for streams as natural systems in the built environment. However, when poorly managed, human activities adversely impact our natural ecosystems, resulting in less resilient stream systems, poor aesthetics, and unsafe conditions. The research presented in this dissertation asks the following overarching research question: How can managers and practitioners apply multi-scale social-ecological, hydrologic, geomorphologic, and riparian ecological remote sensing and field data to advance urban riverscape management? Four chapters follow from this hypothesis: urban riverscape problems lie on a spectrum of complexity where solutions are often conceivable but difficult to implement. Integrating diverse perspectives and knowledge extends the scope of stakeholder perspectives so that social-ecological context is considered alongside the physical processes that typically characterize riverscapes. This approach entails leveraging existing and new methods to create frameworks that integrate the multi-scale assessment of physical conditions and social-ecological qualities underlying applied riverscape management. I explore the integration of diverse knowledge to enhance management outcomes through the concept of "wicked problems." I analyze the connections between diverse types of knowledge and practices through numerous case studies. My analysis shows how systematically characterizing project attributes, such as the prominence of local government and technical knowledge or the weakness of academia and indigenous knowledge, requires an approach that builds capacity and collaboration within transdisciplinary stakeholder groups. I find that the importance of integrating communities, including under-represented knowledge bases, into urban riverscape management can generate equitable and incremental solutions. To evaluate connections between social values, ecological conditions, and hydrogeomorphic processes, I outline a framework for urban riverscape assessment that advances the practice of managing urban riverscapes facing complex problems. The framework is based upon evaluation across four foundational categories, or facets, critical to the management of urban riverscapes: (1) human connections and values, (2) hydrologic processes and hydraulic characteristics, (3) geomorphic forms and processes, and (4) ecological structure and processes. I structure the framework around three tiers of actionable steps, which tackle the questions: Why are we assessing this riverscape (Tier 1)? What do we need to understand in and along this riverscape (Tier 2)? How will we assess the riverscape to develop that understanding (Tier 3)? I find that the answer to the first question is context-based and dependent upon integrating diverse types of knowledge, while the response to the second question involves examining the functions and values of urban riverscapes through the lens of the four facets and their inter-related processes. Answering the third question requires developing and testing a novel assessment method – the "Urban Riverscape conditions-Based Assessment for management Needs" (URBAN). I base URBAN on riverscape context and on integrating the assessment of facets at multiple scales. I apply the method to a test data set of publicly available and site-specific data across a study area in the Denver metropolitan region to illustrate its overall performance, including its ability to evaluate specific riverscape physical conditions and social-ecological qualities. I find reach typologies combined with urban riverscape characteristics provide tangible management strategies that managers can use to inform planning and decision making. The overarching conclusion of this dissertation is that managing urban riverscapes requires assessment methods that consider scale (spatial, temporal, and topical) and context (both physical and social characteristics), and the use of indicators and metrics that directly support decision-making among interdisciplinary stakeholders. It is possible to move toward this vision by using remote-sensed and field data that provides both social and physical information, to assess the relationship between physical condition and social-ecological values, and to use that information to determine where and how to prioritize management strategies for urban riverscapes.
Open Access
Beyond the case study: characterizing natural floodplain heterogeneity in the United States
(Colorado State University. Libraries, 2023) Iskin, Emily Paige, author; Wohl, Ellen, advisor; Morrison, Ryan, committee member; McGrath, Daniel, committee member; Ronayne, Michael, committee member
With human degradation of natural river corridors, the number of natural, functional floodplains is rapidly decreasing due to dams, diversions, artificial levees, draining, development, agriculture, and invasive species. At the same time, small- to large-scale interest in and implementation of river restoration is expanding, with floodplain restoration soon to take a starring role. To properly manage and restore processes to floodplains, we first need a broad understanding of what they look like and why. A key component of natural river-floodplain systems is heterogeneity, defined as the spatial variation of geomorphic and vegetation classes and patches across a floodplain. Heterogeneity of floodplains both reflects and influences the fluvial processes acting on floodplains and can help shape our understanding of the form and function of floodplains. To begin characterizing floodplain spatial heterogeneity, I present in this dissertation: 1) the development of a method to combine field measurements and remote sensing data products to calculate integrative landscape-scale metrics of floodplain spatial heterogeneity, and the demonstration of which metrics from landscape ecology are likely to be useful for identifying qualities of natural floodplains at four case study sites; 2) a sensitivity analysis to determine whether and how the values of the heterogeneity metrics change when spatial and spectral resolution of the input data are increased, and the extraction of underlying data from the classification results to determine whether using higher resolution data allows identification of the resulting unsupervised classes in relation to field and remote data at four case study sites; and 3) quantification of floodplain spatial heterogeneity, evaluation of whether statistically significant patterns are present, and interpretation of the statistical analyses with respect to the influence of channel lateral mobility and valley-floor space available using a complete dataset of 15 sites representing diverse floodplains across the continental United States. I found that "stacking" Sentinel-2A multispectral satellite imagery and digital elevation model topographic data allows for unsupervised classification of floodplains, and that metrics from landscape ecology can differentiate between different floodplain types. I also found via a sensitivity analysis that increasing the spatial resolution of the topographic data to finer than 10 m and including band ratios related to vegetation improves the classification results. Comparison of the field classes with the remote sensing classes allows for general interpretation of the results, but it is the heterogeneity within the broad classes that I expect is most important to these ecosystems. Lastly, through classification of 15 diverse river corridors across the United States, calculation of five heterogeneity metrics, and completion of a comparative analysis, I found that these natural floodplains have moderate aggregation of classes (median aggregation index = 58.8%), high evenness (median Shannon's evenness index = 0.934) and intermixing of classes (median interspersion and juxtaposition index = 74.9%), and a wide range of patch densities (range of patch density = 491–1866 patches/100 ha). I also found that the river corridor characteristics of drainage area, floodplain width ratio (space available), and elevation, precipitation, total sinuosity, large wood volume, planform, and flow regime (channel mobility) emerge as important variables to understanding floodplain heterogeneity.
Open Access
Characteristics of current and future flood-producing storms in the continental United States
(Colorado State University. Libraries, 2020) Dougherty, Erin M., author; Rasmussen, Kristen, advisor; Schumacher, Russ, committee member; Maloney, Eric, committee member; Morrison, Ryan, committee member
Understanding the changes to extremes in the hydrologic cycle in a future, warmer climate is important for better managing water resources and preventing detrimental impacts to society. The goal of this dissertation is to contribute to this understanding by examining the precipitation characteristics of flood-producing storms in the current climate over the continental United States (CONUS) and how these will change in a future, warmer climate. Numerous storm types are responsible for floods over the CONUS, so quantifying how their characteristics will change among a large number of flood-producing storms in the future provides a spectrum of possible changes and impacts to flood-prone regions across the country. To understand flood-producing storms in the current climate over the CONUS, a climatology of these storms from 2002–2013 is created by merging storm reports, streamflow-indicated floods, and Stage-IV precipitation data (Chapter 2). From this climatology, it is observed that flash flood-producing storms preferentially occur in the warm-season in the Mississippi River Basin, with intense rain rates and short durations. Slow-rise floods occur mostly during the cool-season, concentrated in the Ohio River Valley and Pacific Northwest, and are long-duration, low-intensity rainfall events. Hybrid floods, having characteristics of both flash and slow-rise flood-producing storms, tend to occur in the spring and summer notably in the central CONUS and Northeast, with moderate durations and rain rates. Examining these floods on a sub-basin scale in the Wabash and Willamette basins, precipitation and instantaneous streamflow correlations are spatially variable, with strong positive correlations in areas of complex terrain and urbanization (Chapter 3). These studies show that in the current climate, flood-producing storm precipitation characteristics and their hydrologic response is nuanced, which is critical to document in order to understand their behavior in a future climate. A subset of nearly 600 flash flood-producing storms from the Chapter 2 climatology are examined using high-resolution convection-permitting simulations over the CONUS to understand how these historical storms might change in a future, warmer climate (Chapter 4). Both precipitation and runoff show widespread increases in the future over the CONUS, increasing by 21% and 50%, respectively, with maximum hourly rain rates becoming more intense by 7.5% K−1. In California, 45 flood-producing storms associated with atmospheric rivers also display a future increase (decrease) in precipitation (snow water equivalent) leading to increased runoff, particularly over the Sierra Nevada Mountains, implying a shift in future water resources in California (Chapter 5). In the Mississippi River Basin–a flash flood hotspot in the CONUS––nearly 500 flash flood-producing storms exhibit a 17% average increase in precipitation and 32% average increase in runoff primarily associated with warm-season convection, and to a lesser extent, tropical cyclones (Chapter 6). When stratified by vertical velocity, the storms with the strongest vertical velocity in the current climate exhibit the greatest (least) increase (decrease) in future rainfall (vertical velocity), suggesting a potential role of storm dynamics in modulating future rainfall changes.
Open Access
Comparing precipitation estimates, model forecasts, and random forest based predictions for excessive rainfall
(Colorado State University. Libraries, 2023) James, Eric, author; Schumacher, Russ, advisor; Bell, Michael, committee member; Van Leeuwen, Peter Jan, committee member; Morrison, Ryan, committee member
Flash flooding is an important societal challenge, and improved tools are needed for both real-time analysis and short-range forecasts. We present an evaluation of threshold exceedances of quantitative precipitation estimate (QPE) and forecast (QPF) datasets in terms of their degree of correspondence with observed flash flood events over a seven-year period. We find that major uncertainties persist in QPE for heavy rainfall. In general, comparison with flash flood guidance (FFG) thresholds provides the best correspondence, but fixed thresholds and average recurrence interval thresholds provide the best correspondence in certain regions of the contiguous US (CONUS). QPF threshold exceedances from the High-Resolution Rapid Refresh (HRRR) generally do not correspond as well as QPE exceedances with observed flash floods, except for the 1-h duration in the southwestern CONUS; this suggests that high-resolution model QPF may be a better indicator of flash flooding than QPE in some poorly observed regions. Subsequently, we describe a new random forest (RF) based excessive rainfall forecast system using predictor information from the 3-km operational HRRR. Experiments exploring the use of spatial predictor information reveal the importance of averaging HRRR predictor fields across a spatial radius rather than using only information from sparse input grid points for regimes with small-scale excessive rain events. Tree interpreter results indicate that the forecast benefits of spatial aggregation stem from greater contributions provided by storm attribute predictors. Forecasts are slightly degraded when there is a mismatch between the trained RF model and the daily HRRR forecasts to which the model is applied, both in terms of initialization time and HRRR model version. Use of FFG as an additional predictor leads to forecast improvements, highlighting the potential of hydrologic information to contribute to forecast skill. In addition, averaging predictor information across several HRRR initializations leads to a statistically significant improvement in forecasts relative to using predictor fields from a single HRRR initialization. The HRRR-based RF has been evaluated at the annual Flash Flood and Intense Rainfall Experiment (FFaIR) over the past three years, with year-over-year improvements stemming from the results of sensitivity experiments. The HRRR-based RF represents an important baseline for future machine learning based excessive rainfall forecasts based on convection-allowing models.
Open Access
Dammed ponds! A study of post-fire sediment and carbon dynamics in beaver ponds and their contributions to watershed resilience
(Colorado State University. Libraries, 2023) Dunn, Sarah B., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Morrison, Ryan, committee member
Excess sediment generated by wildfires threatens stream water quality, riparian habitat, and infrastructure. Beavers construct dams that pool water and capture sediment. Beaver ponds may bolster watershed resilience by providing sediment and carbon storage following wildfire. I tested the hypotheses that (1) burned ponds store greater relative volumes of sediment compared to unburned ponds, (2) post-fire sedimentation rates exceed pre-fire and unburned rates, and (3) post-fire sediment stored in beaver ponds is coarser and has a higher abundance of organic carbon relative to pre-fire sediment. I surveyed 48 beaver ponds in the Colorado Rocky Mountains. Approximately half of the ponds are in areas that burned in 2020 wildfires, whereas the other half remain unburned. Sites also spanned a range of geomorphic, vegetation, and individual pond characteristics. I conducted sediment probe surveys and collected sediment cores to quantify pond sediment storage and characterize sediment composition. Stratigraphic units present in sediment cores were analyzed for grain size and total organic carbon (TOC). Results indicate that beaver ponds in the Rocky Mountains store high volumes of sediment (mean = 796 m3). Burned ponds contain statistically significantly more relative sediment storage and have higher sedimentation rates than unburned ponds. Beaver ponds recorded high post-fire sedimentation rates (median = 19.8 cm/yr). Moreover, post-fire sedimentation rates are an order of magnitude higher than pre-fire rates in ponds with both pre- and post-fire sediments. Total sediment volume, sedimentation rates, grain size, and TOC content did not vary significantly between burned and unburned ponds. Geomorphology, vegetation, and pond characteristics exert additional influences on pond sediment dynamics. Pond characteristics determine the sediment trapping efficiency of ponds. Larger ponds store greater volumes of sediment, as do off-channel and older ponds. Ponds abandoned by beaver store greater volumes of sediment than actively maintained or human- constructed dams. Beaver activity and dam maintenance is critical for maintaining storage availability in ponds. Additionally, sedimentation rates are higher in ponds that are on-channel and recently constructed compared to off-channel and older ponds. These findings indicate that beaver-based restoration can be implemented prior to fire to provide critical post-fire sediment storage, thus enhancing watershed resilience and recovery.
Open Access
Evaluating post-fire geomorphic change on paired mulched and unmulched catchments using repeat drone surveys
(Colorado State University. Libraries, 2023) Hayter, Lindsey, author; Nelson, Peter, advisor; Kampf, Stephanie, committee member; Morrison, Ryan, committee member
Sediment redistribution after wildfire can dramatically alter a catchment and pose risks to local infrastructure and water quality. Mulch application is increasingly being used to mitigate post-fire hillslope runoff and erosion, although relatively little is known about its effects at the catchment scale. In this study we used repeat drone surveys to measure erosion and deposition across 6 small (0.5-1.5 km2) catchments, 3 mulched and 3 unmulched, in the 2020 Colorado Cameron Peak Fire burn scar. The objectives were to (1) quantify sediment volumes and spatial patterns of erosion and deposition on a catchment and channel scale, (2) compare geomorphic change to mulch coverage, vegetation cover, precipitation intensity, burn severity, and morphologic metrics, and (3) identify conditions in which mulch may be most appropriate based on findings. Initial drone surveys were gathered in the spring of 2022 shortly after mulching and were differenced to surveys collected in fall of 2022, capturing the erosional effects of a Colorado monsoon season within a 6.4 cm horizontal resolution DEM of Difference (DoD). Structure from motion (SfM) errors were thresholded out of the DoD to yield maximum and mean levels of detection at 14 cm and 5 cm respectively. Vegetation was filtered from the DoD by supervised classification of vegetation in the drone imagery. We found hillslope erosion dominated the sediment budget, with the mulched catchments eroding 141% more per area than the unmulched. A regression model suggested erosion to be most influenced by vegetation, hillslope length, and maximum 60-minute rainfall intensity. Channels were overall net depositional, and patterns of erosion and deposition in channels were controlled by changes in slope and stream power as well as local morphologic metrics. Our analysis does not find a significant impact of mulch at the catchment scale especially when coverage is low (~22%) and highlights the importance of understanding catchment attributes and processes when making post-fire treatment decisions.
Open Access
Flume study of mechanisms responsible for particle sorting in gravel-bed meandering channels
(Colorado State University. Libraries, 2019) White, Daniel, author; Nelson, Peter, advisor; Morrison, Ryan, committee member; Wohl, Ellen, committee member
Meandering gravel-bed rivers tend to exhibit bed surface sorting patterns with coarse particles located in pools and fine particles on bar tops. The mechanism by which these patterns emerge has been explored in sand-bed reaches; however, for gravel-bed meandering channels it remains poorly understood. Here we present results from a flume experiment in which bed morphology, velocity, sediment sorting patterns, and bed load transport were intensively documented in a single-bend meandering channel. The experimental channel is 1.35 m wide, 15.2 m long, and its centerline follows a sine-generated curve with a crossing angle of 20 degrees. Water and sediment input were held constant throughout the experiment at 104.8 L/s and 230 kg/h, respectively, and measurements were collected under quasi-equilibrium conditions once the sediment input and output were approximately equal and the bed was essentially unchanging. Measurements of the three-dimensional velocity field indicate the development of a helical flow where near-bed velocity is directed toward the inner bank and flow at the surface is directed toward the outer bend. Calculated cross-stream bed load transport rates show that the trajectories of fine and coarse particles cross downstream of the bend apex, with fine sediment directed inward toward the point bar and coarse sediment directed toward the outer pool. Boundary shear stress, calculated from near-bed velocity measurements, indicates that in a channel with mild sinuosity, deposition of fine particles on bars is a result of divergent shear stress at the inside bend of the channel just downstream of the apex. The strong inward secondary currents that developed near the outside bend of the channel have little impact on the fine sediment deposition occurring on the bar under the conditions of this study. Boundary shear stress at equilibrium in the upstream half of the pool was below the critical value for coarse particles (>8 mm), which were only found in the pool. Selective transport toward the sloped region connecting the pool and bar top was responsible for winnowing of fine particles in the pool. Similarly, boundary shear stress near the bar front at equilibrium was below the critical value for particles near the D₅₀ of the bulk sediment feed (≤4 mm). Here, only fine particles were mobilized and transported downstream to the bar top. Fine and coarse sediment followed essentially identical trajectories through the meander bend, which contrasts earlier studies of sand-bedded meanders where fine and coarse particles cross paths. This suggests a different sorting mechanism for gravel bends. This experiment shows that a complex interaction of quasi-equilibrium bed topography, selective sediment transport, and currents that develop as a result of curved channel geometry are responsible for the sorting patterns seen in gravel bed, meandering channels.
Open Access
Form and function: quantifying geomorphic heterogeneity and drivers in dryland non-perennial river corridors
(Colorado State University. Libraries, 2023) Scamardo, Julianne E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
Non-perennial rivers, including intermittent rivers and ephemeral streams, comprise the majority of drainage networks globally. However, ephemeral streams remain understudied compared to perennial counterparts, and the majority of extant studies focus on in-channel dynamics. Floodplains along perennial streams are known to host a high density of ecosystem functions, including the attenuation of downstream fluxes and provision of habitat to diverse flora and fauna. These functions are thought to be correlated to geomorphic heterogeneity, and studies of floodplain heterogeneity are emerging on perennial rivers. Here, I extend the conceptualization of floodplain function and heterogeneity commonly focused in perennial watersheds to dryland, ephemeral streams. Based on a synthesis of current literature identifying ephemeral stream floodplain characteristics in drylands, a set of floodplain styles emerge dependent on confinement and the presence of channelized flow. Functions related to attenuation and storage are typically concentrated in unconfined and channeled floodplains. The temporary storage of sediment and sub-surface water in ephemeral stream floodplains make them hotspots for biogeochemical cycling and hosts to richer, denser, and more diverse vegetation communities compared to surrounding uplands. Many functions of ephemeral stream floodplains are also found in perennial counterparts, but flashy flow regimes and high sediment loads in ephemeral streams can potentially impact rates and magnitudes of comparable processes and functions. Similar to perennial rivers, the diverse physical and ecological functions in ephemeral stream floodplains are thought to be related to spatial geomorphic heterogeneity. Although studies on the characteristics and drivers of geomorphic heterogeneity exist for perennial streams, similar studies in ephemeral streams are lacking. Geomorphic heterogeneity was therefore quantified along with potential drivers – including metrics related to geomorphic context and proxies for flood disturbance – to understand underlying processes in ephemeral river corridors. Geomorphic units were mapped in 30 unconfined river corridors within six non-perennial watersheds in Utah and Arizona, U.S. Landscape heterogeneity metrics – Shannon's Diversity Index, Shannon's Evenness Index, and patch density – were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, channel change and sediment storage times. Although heterogeneity positively correlated with metrics for morphology and disturbance (i.e., channel change and storage), statistical models suggest that morphologic context, particularly floodplain width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that non-perennial rivers with greater geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long-term temporal heterogeneity are needed. The lack of direct flux observations could also be restricting insight into how floods interact with large wood and vegetation, which are known to have complex relationships with geomorphic heterogeneity in perennial rivers. In the absence of flood observations, a hydro-morphodynamic model was developed to investigate changes to channel and floodplain morphology due to wood and vegetation in an ephemeral river corridor in southeastern Arizona, U.S. Three scenarios were modeled: the actual configuration of the river corridor; an experiment in which jams were removed; and an experiment in which vegetation was removed. Both large wood and vegetation effectively confined flow to the main, unvegetated channel, which became wider and deeper over the course of a single moderate flood. When isolating the impact of large wood, model results show that wood increases the magnitude of channel change created by vegetation, resulting in ±0.1 to 0.3 m of additional scour or aggradation. The simulated removal of vegetation resulted in more channel change than the removal of wood alone, partially because vegetation occupies a much greater area within the stream corridor than large wood. I propose a conceptual framework in which large wood could mediate sedimentation as well as the recruitment and growth of vegetation in ephemeral streams, contributing to the evolution of ephemeral stream morphology over time. Due to the ubiquity of dryland ephemeral streams, results of this research have the potential to influence watershed management globally. Wide, unconfined ephemeral stream floodplains and riparian forests could be targets for protection and restoration similar to current efforts in perennial rivers. Particularly in the context of future climate and land use changes, understanding the natural character, function, and heterogeneity of ephemeral stream floodplains highlights their physical and ecological importance in dryland landscapes.
Open Access
From rain gauges to retweets: using diverse datasets to explore overlapping hazards and human experiences in landfalling tropical cyclones
(Colorado State University. Libraries, 2021) Mazurek, Alexandra C., author; Schumacher, Russ, advisor; Henderson, Jen, committee member; Morrison, Ryan, committee member; Rasmussen, Kristen, committee member
Landfalling tropical cyclones (LTCs) are responsible for numerous hazards, including damaging winds, storm surge, inland flooding, and tornadoes. Furthermore, multiple hazards may threaten an area at the same time, which raises challenges from a prediction, warning operations, and human impacts standpoint. Previous research has approached overlapping tornado and flash flood events—which exemplify these challenges because the recommended protective actions can be in conflict—in continental systems from multidisciplinary perspectives, but less work has been done to explore these phenomena in LTC environments. Because LTCs also introduce other hazards, additional complexities may exacerbate already challenging circumstances. This work integrates meteorological and social sciences to broadly advance the understanding and implications of simultaneous flash flood and tornado events in LTCs. Part I of this thesis investigates the relationship between two predecessors to tornadoes and flash floods—meso- to storm-scale rotation and heavy rainfall rates, respectively—using observations. Motivated by previous work that has drawn linkages between these two processes in continental convective storms, this connection is explored in Tropical Storm Imelda, a system that was among the wettest LTCs on record to impact the contiguous United States (CONUS), producing rainfall accumulations in excess of 1000 mm when it made landfall on the western Gulf Coast in September 2019. First, a synoptic and mesoscale overview of the tropical cyclone (TC) is presented as motivation for its utility in examining overlapping embedded rotation and extreme rainfall rates. Then, rain gauges from a high-density observing network in southeast Texas are analyzed alongside polarimetric radar data to compare rainfall rates that occur in the presence of embedded rotation to those that occur when no rotation is evident on radar. According to these results, 5-minute rainfall rates that followed subjectively-identified meso- to storm-scale rotation on radar tended to be statistically significantly greater, and when accumulated over time, more than twice as much rainfall was recorded at gauge sites when rotation was present near the gauge compared to when there was no rotation located nearby. To further quantify the spatial and temporal relationships of embedded rotation and heavy rainfall rates, quantitative precipitation estimates (QPE) and rotation tracks from the Multi-Radar Multi-Sensor system are compared in time and space. A positive correlation was found to exist between the hourly-accumulated 0-2 km rotation tracks and hourly local gauge bias-corrected QPE, suggesting that more rain tends to fall in the presence of low-level rotation. In Part II of this thesis, social science methods are used to investigate another LTC: Hurricane Harvey (2017)—an unprecedented event that became the wettest LTC on record to impact CONUS and spawned over 50 tornadoes when it affected the western Gulf Coast. This work aims to explore the notion of experience as it evolves on Twitter in real-time during Harvey among a group of users who were located in areas that were impacted by the LTC and its overlapping hazards. Though a significant amount of research has investigated experience through surveying and interview techniques after LTCs occur, much less work has been done to study experience as it is shared live during an event or through the lens of social media. Using this motivation and drawing on the overarching theme of concurrent hazards, this research begins with a database of tweets composed during the period surrounding Hurricane Harvey that reference tornadoes and flash flooding. The sample is refined through a multi-step querying process, ultimately resulting in a group of 39 users who shared 158 tweets about "past events"—that is, events related to LTCs and/or the hazards that are associated with them. These tweets are thematically analyzed by individual users, by individual past events, and over time. The results of these analyses show that Twitter users referenced past events during Harvey for two main reasons: first, because the user has a personal connection to the event and second, because the past event is helping them to make sense of various aspects of the situation that is unfolding around them. Understanding what roles past events may play in a real-time crisis is useful to leaders and decision-makers, such as meteorologists, local politicians, and emergency managers, as it provides insight on the evolving needs and concerns of the public that they serve as they change and are modulated by various events that unfold throughout the overarching crisis.
Open Access
Genetic analysis reveals bidirectional fish movement across the Continental Divide via an interbasin water transfer
(Colorado State University. Libraries, 2021) Harris, Audrey Catherine, author; Kanno, Yoichiro, advisor; Winkelman, Dana, advisor; Oyler-McCance, Sara, committee member; Morrison, Ryan, committee member
Interbasin water transfers are becoming an increasingly common tool to satisfy municipal and agricultural water demand, but their impacts on the movement and gene flow of aquatic organisms are poorly understood. The Grand Ditch is an interbasin water transfer that diverts water from tributaries of the upper Colorado River on the west side of the Continental Divide to the upper Cache la Poudre River on the east side of the Continental Divide. I used single nucleotide polymorphisms to characterize population genetic structure in cutthroat trout (Oncorhynchus clarkii) and determine if fish utilize the Grand Ditch as a movement corridor. Samples were collected from two sites on the west side and three sites on the east side of the Continental Divide. I identified two genetic clusters, but they did not align with the west and east sides of the Continental Divide. Spatial distributions of admixed individuals indicated that the Grand Ditch facilitated bidirectional fish movement across the Continental Divide, a major biogeographic barrier. Many others have demonstrated the ecological impacts of interbasin water transfers, but this study is one of the first to utilize genetics to understand how interbasin water transfers affect connectivity between previously isolated watersheds. I also discuss implications on native trout management and the need for balancing water demand and biodiversity conservation.
Open Access
Geomorphic effects of increased wood loading on hyporheic exchange flow
(Colorado State University. Libraries, 2019) Ader, Ethan, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Morrison, Ryan, committee member
Much of the recent scientific literature in the field of fluvial geomorphology has documented the benefits of the presence of large wood in rivers. One of these benefits is enhanced hyporheic exchange flow (HEF). Enhanced HEF has numerous benefits and therefore plays an important role in stream health. While the science of hyporheic exchange has progressed over the past few decades, studies thus far have focused on single pieces of wood or single jams. There have not yet been studies that examine whether multiple consecutive jams have an additive or nonlinear effect on HEF. This study focuses on the impacts of increased wood loading on geomorphic complexity and HEF. We examined relations among wood load, geomorphic complexity, and HEF by studying four different reaches along Little Beaver Creek, a 3rd order tributary to the Cache la Poudre River in the Colorado Front Range within the Arapaho and Roosevelt National Forest: 1) a single channel with no logjams, 2) a single channel with limited logjams, 3) an anabranching channel with limited logjams, and 4) an anabranching channel with abundant logjams. Pearson correlations were used to analyze the relationship between HEF, wood loading, and geomorphic complexity. We found that increased wood loading increases the volume of both pools and accumulated fine sediment at the reach level. Additionally, HEF positively correlates with geomorphic complexity and wood loading. The metrics that most strongly correlated with enhanced HEF all represent factors expected to increase connectivity from the channel to the hyporheic zone. These preliminary results suggest that it is through this mechanism of increasing hyporheic zone connectivity that HEF is enhanced.
Open Access
Geospatial analysis of specific degradation in South Korea
(Colorado State University. Libraries, 2019) Kang, Woochul, author; Julien, Pierre Y., advisor; Grigg, Neil S., committee member; Morrison, Ryan, committee member; Kampf, Stephanie, committee member
South Korea experienced many local and concentrated sediment problems such as landslides, upland erosion, rills and valleys, aggradation/degradation, and flood plain sediment deposition. These problems vary in space and time, therefore a reliable and consistent approach to model sediment processes is desirable. In contrast to sediment yield at the basin scale, Specific Degradation (SD) is defined as the ratio of the sediment yield divided by the watershed area. Field measurements of discharge and sediment concentration are analyzed at 70 stations in South Korea. Half of the sampled river basins (35 stations) represent streams in mountain regions and the other half represent rivers. The Modified Einstein Procedure (MEP) was used to determine the total sediment load at all stations. The Flow Duration – Sediment Rating Curve (FD-SRC) method was used to determine the sediment yield and specific degradation for all gauging stations. The annual sediment yield of 70 rivers and streams in South Korea ranged from 10 to 1,000 tons/km2▪yr. The application of three existing models from the literature showed Root Mean Square Errors (RMSE) in excess of 1,400 tons/km2▪yr and gave negative values of the Nash-Sutcliffe Efficiency coefficient (NSE) for existing models, which indicates that the observed mean is a better predictor than the model. The main characteristics of each watershed were analyzed using GIS tools such as ArcGIS version 10.3.1. The data used for the analysis included: (1) daily precipitation data at 60 stations from the Korea Meteorological Administration (KMA); (2) a detailed soil map from the National Institute of Agriculture Sciences; (3) a 5m by 5m resolution Digital Elevation Model (DEM); and (4) land cover raster data at a 10 m resolution from the Ministry of Environment (ME). Seven regression models based on these watershed characteristics are proposed to estimate the mean annual sediment yield and specific degradation. In decreasing order of importance, the meaningful parameters are: (1) drainage area; (2) mean annual precipitation; (3) percentage of urbanized area; (4) percentage of sand of the surface soil (upper 50cm); (5) percentage of wetland and water; and (6) morphometric parameters such as watershed average slope and two parameters of the hypsometric curve. The RMSE for the newly developed models decreased to 90 tons/km2▪yr and the NSE increased from -50 to 0.5, which shows good agreement between the model and the measured sediment yield on these watersheds. The calculated specific degradation and mean annual soil loss of mountain streams were larger than alluvial rivers. Erosion loss mapping at 5m, 30m and 90m was also developed from the Revised Universal Soil Loss Equation (RUSLE). Satellite images and aerial photos were used to better represent geospatial features affecting erosion and sedimentation. Long-term reservoir sedimentation measurements were available to determine the Sediment Delivery Ratio (SDR). An important finding from this analysis is that the percentage of the area covered with wetland and water is well-correlated with the estimated sediment delivery ratios. It suggests that the transfer of sediment to the rivers is affected by wetlands located near alluvial rivers. The erosion maps at 5m resolution could clearly show unique erosion features (i.e. hill slopes, croplands, and construction sites) and locate areas for sediment deposition (i.e. wetlands and agricultural reservoirs). In comparison, the gross erosion rates at 90 m resolution were highly distorted and could not delineate the areas with high upland erosion rates. Sustainable sediment management with these methodologies could be helpful to solve various erosion and sedimentation problems.
Open Access
Islands in the stream: spatial and temporal patterns of logjam-induced river corridor dynamics
(Colorado State University. Libraries, 2024) Marshall, Anna E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
Spatial and temporal variations in water and sediment fluxes moving within the river corridor drive changes in the three-dimensional geometry of channels and floodplains. In forested river corridors, pieces of large wood (> 10 cm diameter and 1 m length) and logjams (≥ 3 pieces of large wood) become an integral part of the interactions among water, sediment, and the resulting river corridor form and function. The net effect of logjams stored at least temporarily in the river corridor is to increase spatial heterogeneity, or patchiness, via processes such as channel avulsion and formation/abandonment of secondary channels, increased channel-floodplain connectivity, and greater instream aggradation. The importance of spatial heterogeneity, logjams, and secondary channels/islands to river corridor function has been well documented, but a lack of existing quantitative underpinning creates knowledge gaps in the processes driving island formation and persistence, the role of wood in facilitating these processes, and the complex interactions between flow, sediment, and wood in dynamic river corridors. This dissertation addresses some of the existing knowledge gaps around how logjams interact in a river corridor to create heterogeneity at different spatial and temporal scales by characterizing the patterns, processes, and interactions occurring in a naturally dynamic system. The topics explored here focus on research primarily conducted along the Swan River in the Northern Rocky Mountains of Montana with mention of sites in the Southern Rocky Mountains of Colorado. These locations represent some of the few remaining river corridors in the contiguous U.S. with natural flow, sediment, and wood regimes, but represent former widespread conditions. In the work that follows, Chapter 2 explores the processes driving spatial patterns in bifurcations induced by logjams. I find that logjam-induced bifurcations exist as a continuum of different patterns and the position of a river segment along this continuum correlates with the ratio of erosive force to erosional resistance. Chapter 3 builds on this by investigating how accretionary and avulsive processes shape bifurcations over time, emphasizing a temporal progression of logjam-induced features using 14C and tree ring data. I find that islands tend to grow through upstream migration – the presence of buried logs with contemporary trees growing on them indicates this process – and lateral accretion. Chapter 4 dives deeper into the interactions between process and form, demonstrating the relationship between channel dynamism, logjam presence, and spatial heterogeneity at larger temporal and spatial scales. I find that logjams and channel movement through time interact in a cascade of processes and feedbacks that foster increased spatial heterogeneity. Wood preferentially accumulates in more geomorphically heterogeneous portions of the river corridor that provide sites capable of trapping and retaining wood. Logjams can then drive greater total sinuosity and the formation of secondary channels that result in further wood trapping, greater heterogeneity of floodplain vegetation, and ideal habitat for beaver that further modify river corridor heterogeneity. I also find that bifurcations and spatial heterogeneity persist even after logjam is no longer present. These results have implications for river management. If sections of the river corridor with more logjams and more beaver meadows display higher spatial heterogeneity, creating and protecting wood-rich heterogeneous retention zones within a river corridor is an important component to emphasize for river resilience. If physical effects persist even after a logjam is no longer present, than wood reintroduced to the river corridor as individual pieces or engineered logjams does not have to be anchored in place to facilitate formation of geomorphic heterogeneity within the river corridor. By dissecting the complexities of processes governing naturally dynamic river corridors, this work adds quantitative insight to the diverse functionality of heterogenous river systems in forested or historically forested regions and provides a launching point for future river management aimed at fostering river corridor function and resilience.
Open Access
Large-scale remote sensing of geomorphic change in mulched and unmulched watersheds burned in the 2020 East Troublesome Fire, Colorado
(Colorado State University. Libraries, 2023) Murray, John Thomas, author; Nelson, Peter, advisor; Kampf, Stephanie, committee member; Morrison, Ryan, committee member
Elevated levels of sediment transport in post-wildfire landscapes can degrade the hydrologic and geomorphic processes of a river system, damage aquatic habitat, and pose a threat to downstream infrastructure. Hillslope mulching applications have proven to be effective at mitigating runoff and erosion at plot and hillslope scales but the impacts of mulching at the watershed scale remain generally unknown. We conducted repeat aerial surveys of one unmulched and five partially mulched watershed outlets (0.61-1.44 km2) to quantify erosion and deposition in the East Troublesome Fire burn scar. The objectives of the study were (1) to quantify volumes of erosion and deposition for hillslopes and channels for a variety of sites at a range of elevations (2) to identify and quantify the drivers of erosion and deposition and their relative contributions within and across watersheds (3) to determine the impact of slope, width, and vegetation cover on sediment storage and transport within watersheds; and (4) to assess the impacts of a large-scale aerial mulching operation at scales from hillslopes to watersheds. Multiple drone flights were conducted for each study site between July and October 2022. The earliest and latest surveys were differenced to produce DEM of Difference (DoD), with spatial resolutions ranging from 3.8 to 4.4 cm. Vertical uncertainties calculated from measurement uncertainty and Structure from Motion (SfM) errors were filtered out of the DoD at a 95% confidence interval (CI), resulting in maximum and mean detection thresholds of 11 and 4 cm, respectively. A supervised classification algorithm was used to filter out changes due to vegetation growth and decay, which varied in effectiveness across the six study sites. Hillslope erosion and deposition volumes were at least three times higher than near-channel volumes, with most sites being an order of magnitude higher. However, near-channel erosion and deposition magnitudes normalized by area were higher than normalized hillslope magnitudes at all sites. A bootstrap forest regression model was used to determine relationships between various site-specific parameters and erosion and deposition for each watershed individually, and for all six sites combined. The model indicated mean slope, absence of vegetation, mean differenced normalized burn ration (dNBR), and hillslope length to be strong drivers of erosion and deposition for the individual models. Total precipitation accumulation and maximum 60-minute rainfall intensity were stronger contributors in the combined models. Near-channel storage and transport was influenced by local relationships between width, stream power, and absence of vegetation. Mulch coverage area was found to be weakly correlated with erosion and deposition at the watershed scale, with contributions possibly being dependent on coverage rate. These findings emphasize the importance of applying mulch in areas where it is both necessary and can have a measurable impact on reducing erosion rates.
Open Access
Linking morphodynamic processes and Silvery Minnow habitat conditions in the Middle Rio Grande- Isleta Reach, New Mexico
(Colorado State University. Libraries, 2020) Fogarty, Caitlin, author; Julien, Pierre, advisor; Morrison, Ryan, committee member; Wohl, Ellen, committee member
The Middle Rio Grande, located in central New Mexico, is home to the Rio Grande Silvery Minnow (RGSM), an endangered species of fish. Much of the RGSM's historical range has been lost due to natural and human-caused alterations to the river. For this study, the availability of RGSM habitat is analyzed in the Isleta reach, a segment of the Middle Rio Grande extending approximately 42 miles from Isleta Diversion dam to the confluence of Rio Puerco. To better understand spatial and temporal trends in morphology and channel geometry, the Isleta reach is delineated into six subreaches (I1, I2, I3, I4, I5, and I6). The purpose of this study is to identify connections between hydraulics, geomorphology, and biology to better explain the changing biological conditions in the river. To assess changes in geomorphology along the Isleta reach, the geomorphic conceptual model developed by Massong et al. (2010) was applied to representative cross-sections in each subreach. The model proposes two pathways that changes in the Middle Rio Grande can follow: aggrading (A) or migrating (M). Through inspection of aerial imagery and cross-sectional geometry data, it appears that the Isleta reach is in stage 3 and migrating stages, M4-M8, indicating high sediment transport capacity. River form was further classified using Cluer and Thorne's (2013) stream evolution model. In 2012, all subreaches were in stage 3 (i.e. degradation) of the model. One-dimensional modeling techniques were used to assess habitat availability for the RGSM from 1962 to 2012. Using the Hydrologic Engineering Center's River Analysis System (HEC-RAS), flow distribution slices were used to compute velocity and depth along a cross-section. Hydraulically suitable RGSM habitat for larvae, juvenile, and adult stages is determined using velocity and depth criteria for the fish proposed by Mortensen et al. (2019). The results suggest that habitat availability follows three typical patterns. Earlier years (1962 and 1972) showed "rounded" habitat curves, while later years (1992, 2002, and 2012) showed "step" and "hook" habitat curves. Detailed maps were produced in ArcMap that aid in the visualization of where RGSM habitat is located within the Isleta reach. These maps suggest that subreaches I1 to I3 contain the most habitat for all life stages. However, much of the habitat is disconnected and far away from the main channel, making it inaccessible to the fish. Through an analysis of restoration potential, it was determined that subreaches I2 to I4 may be areas of focus for river management to increase RGSM habitat. Time-integrated habitat metrics, originally developed by Doidge et al. (2020), is a method of interpolating cumulative RGSM habitat for each year between 1992 and 2019. This method requires input of annual habitat curves and daily discharge data. These inputs are used in a summation of simple linear equations that results in habitat metrics for each of the RGSM's life stages. The results show that larval and juvenile habitat metrics are more sensitive to changes in daily discharge than adult habitat metrics. Ecological relationships were inferred based on plots created by Mortensen et al. (2020) that relate habitat metrics, discharge, occurrence probability and lognormal density. Overall, larvae proved to be strong predictors of population dynamics.
Open Access
Logjam attenuation of annual sediment waves in eolian-fluvial environments, North Park, Colorado
(Colorado State University. Libraries, 2020) Grabowski, Julia, author; Wohl, Ellen, advisor; McGrath, Daniel, committee member; Morrison, Ryan, committee member
Sediment waves, a term that describes the fluvial transport of a discrete sediment influx, have long been studied in regard to channel response to infrequent, catastrophic events, such as mass movements or dam removal. However, few researchers have studied (1) the potential presence of sediment waves of annual or sub-annual scale in mixed eolian-fluvial geomorphic environments or (2) the role of large wood in sediment wave dispersal. This study addresses both topics through observations of North Sand Creek and East Sand Creek, which flow alongside the active sand dunes of North Sand Hills and East Sand Hills, respectively, in North Park, Colorado. The creeks experience similar seasonal, asynchronous cycles of eolian influx and fluvial transport, although North Sand Creek likely receives a greater volume of eolian sand due to intensive Off-Highway Vehicle (OHV) recreation on the North Sand Hills dunefield. Linear spectral unmixing of Landsat imagery from 1984-2019 is used to determine whether OHV recreation has resulted in vegetation loss, typically associated with elevated eolian flux, on North Sand Hills. Repeat photography and repeat measurement of terrace-like structures are used to determine whether each creek experiences a sediment wave, and repeat measurement of logjam sand wedge volume is used to examine changes in sand storage associated with logjams over time. Results indicate that North Sand Hills hast lost vegetative cover in areas not fenced-off to OHV users at a rate of ~800 m2/year, and that North Sand Creek experiences a highly translative sediment wave that is attenuated by logjams. East Sand Hills, on the other hand, has gained vegetative cover throughout the dunefield, and East Sand Creek does not experience a sediment wave. The sediment wave at North Sand Creek translates rapidly through the area of channel outside of the logjam backwater and translates more slowly through logjam backwater areas—principally through reduction in the length of logjam sand wedges, rather than reduction in depth.
Embargo
Mixed populations flood frequency analysis in the mid-Atlantic region of the United States
(Colorado State University. Libraries, 2023) Breverman, Avital, author; Arabi, Mazdak, advisor; Morrison, Ryan, committee member; Fassnacht, Steven, committee member
In many parts of the United States, floods at a single site are caused by multiple mechanisms. Flood mechanisms can broadly be classified as meteorologic, land surface processes, and disturbances. These non-homogeneous flood series are typically referred to as mixed populations. While the two latest revisions of federal flood frequency guidelines, published in 1982 and 2019, identified the treatment of mixed populations as an area of future research, no quantitative guidance exists on the classification of flood events or the incorporation of flood types into frequency analyses. Without quantitative guidance on the treatment of mixed populations in flood frequency analyses, there is the potential for considerable variability in frequency-based flood estimates. The treatment of a flood series resulting from a mixed population violates the assumption that floods at a site are independent and identically distributed. To avoid this issue, separate statistical models should be fit to floods arising from different mechanisms and the resulting curves should be combined to produce flood quantiles. Mixed population and flood typing literature has focused primarily on the western United States. In comparison, the Mid-Atlantic region of the United States is characterized by complex meteorology and numerous flood causal mechanisms but has been studied less frequently in mixed population literature. In addition to the absence of guidance of mixed populations, flood frequency approaches in the United States use the annual maximum series, or the maximum flow each year. The peaks-over-threshold, or partial duration series, approach to selecting flood events has the potential to increase the information content in a flood series which is important when subdividing flood events by causal mechanism. The objectives of the study are to examine: (1) flood typing based on gridded meteorologic products, (2) the advantages of using partial duration series over annual maximum series when performing a flood frequency analysis with consideration of flood type, and (3) the difference in flood quantiles from the proposed combined population methodology compared to those from the current mixed flood frequency analysis. An automated flood classification procedure was developed using gridded meteorologic products. The automated classification procedure was validated manually using historic storm publications. Flood frequency analyses were performed using both partial duration and annual maximum flood series. The method is applied within the Lehigh River watershed in eastern Pennsylvania. While the flood frequency analysis results varied across the watershed, separation of flood series by causal mechanism generally resulted in higher flood quantiles than those obtained from mixed flood series. Design floods based on the treatment of flood series as homogeneous are likely underpredicting event magnitudes. In addition, quantitative guidance on separation of flood events by causal mechanism and treatment of flood type subsets within frequency analyses is needed to produce more reliable flood estimates.
Open Access
Monitoring heterogeneity and carbon sequestration of restored river-wetland corridors
(Colorado State University. Libraries, 2022) Hinshaw, Sarah Kathleen, author; Wohl, Ellen, advisor; Rathburn, Sarah, committee member; Morrison, Ryan, committee member; McGrath, Dan, committee member
Innovation of new stream restoration strategies over the past three decades has added much-needed geomorphic complexity and ecological consideration to the practice of stream restoration. With the modernization of stream restoration to include biologically driven feedbacks, methods in monitoring must be simultaneously created to match the current progress. In addition, modern stream restoration practices offer significant opportunity to store carbon in restored river-wetland corridors by increasing carbon sequestration potential of the affected landscapes via rewetting valley bottoms and enhancing fluvial deposition. To address the need for monitoring techniques that capture complex river corridor restoration and carbon sequestration, I present in this dissertation: 1) the development of a geomorphic monitoring strategy that is applied to a valley-scale floodplain enhancement project that involved regrading of the valley bottom and abundant large wood placement, 2) a conceptual framework and protocol for estimating carbon sequestration potential in restored river-wetland corridors, and 3) the application of the latter protocol across multiple restoration projects in the western USA. With the monitoring protocol, which is based on plots rather than river cross-sections, I found finer substrate grain sizes, reduced canopy cover, spatial patterns in particulate organic matter, and the initial signatures of expected changes in heterogeneity at the valley-scale floodplain enhancement project over two years after project implementation. In the context of carbon sequestration among eight sites combined, I found the majority of pre-restoration or degraded condition sites to have significantly fewer carbon stocks than restoration projects or reference conditions, and the highest carbon stocks in reference condition sites. The chapters of this dissertation are not only intended to provide context and methods to measure stream restoration projects, but also to examine the state and trends of stream restoration in general and to contribute to the understanding of rivers in the global carbon cycle.
Open Access
Multi-scalar response of an experimental fixed-wall meandering channel to a sediment supply increase
(Colorado State University. Libraries, 2020) Cortese, David, author; Nelson, Peter, advisor; Morrison, Ryan, committee member; Wohl, Ellen, committee member
Meandering river planforms are prevalent and well-studied features in the natural landscape. These rivers commonly exhibit a characteristic morphology of fine-grained point bars along the inner banks of meander bends with coarser pools along the outer banks. If subjected to a change in sediment supply, these rivers are likely to respond at various spatial and temporal scales through adjustments to sorting patterns, cross-sectional shape, and reach-scale morphology. In this study, a flume experiment was conducted to document the temporal progression of responses across scales of a fixed-wall meandering channel to a sediment supply increase. The 0.344 m wide experimental channel consisted of four meander bends following a sine-generated trace with a 20-degree crossing angle, meander wavelength of 2.75 m, and a unimodal sediment mixture with median grain size of 0.62 mm. The channel was provided constant flow and sediment supply until an initial equilibrium was established, after which the sediment supply was doubled until a new equilibrium state was reached. The experimental channel developed characteristic bar-pool morphologies and sorting patterns with superimposed, mobile, scaled gravel-dune bed forms during both phases of the experiment. After the sediment supply increase, dynamic adjustments occurring from smaller to larger scales took place. Initially, the dunes essentially disappeared, after which the relief of the bars decreased. Both of these sub-reach-scale responses were temporary, however, and ultimately the dunes and bar-pool morphology returned to their conditions at the beginning of the sediment supply increase. The long-term and largest-scale response to the supply increase was a 44% increase in bed slope. To explain these observations, we propose a conceptual model wherein the channel undergoes a temporal progression of responses from smaller to larger spatial scales, with the total response potential at each scale related to the conditions and constraints at that scale. This conceptual understanding allows us to reconcile seemingly divergent outcomes from previous research on how meandering rivers adjust to sediment supply changes.