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  • ItemOpen Access
    Land use influences on adjacent ecological systems: implications for conservation planning
    (Colorado State University. Libraries, 2009) Wade, Alisa Ann, author; Laituri, Melinda J., advisor; Theobald, David M., advisor; Hoeting, Jennifer A., advisor
    This research investigated the spatial relationships between land uses, primarily urbanization, and adjacent ecological systems. As anthropogenic stressors encroach on protected areas and aquatic systems, the ecological functioning of those systems is reduced, and this has implications for natural resource management and conservation. I conducted three separate studies to address different research questions relating to land use and land cover-ecological system linkages. I assessed the vulnerability of conservation lands throughout the U.S. to adjacent anthropogenic threats and identified protected lands that are likely threatened by human activities as well as unprotected lands that offer opportunities for future conservation action. I also quantified the amount of residential development encroachment surrounding protected lands in the U.S., and I quantified how encroachment has altered the landscape structure around conservation lands nationally from 1970 through 2000, and forecast changes for years 2000 through 2030. Results from these two studies showed that there are a number of protected areas that are vulnerable to neighboring threats and that development has both reduced the buffer surrounding and the connectedness between protected areas. However, results also suggested that there are a number of options for future conservation action, although continued urbanization will limit these options. These studies indicate that conservation planning must consider adjacent land uses. However, the final study presented in this dissertation illustrated that conservation scientists and land managers must recognize the limitations of their approach when modeling the relationships between ecological systems and adjacent land use. I used a conceptual model of how land cover at different upslope scales influences aquatic integrity to show how different modeling approaches can substantially alter resulting inference. Results suggest that a modeling approach that incorporates ecological knowledge may provide more relevant inference for management decisions. A finding applicable to all three studies is that a key conservation strategy will be to work cooperatively with adjacent land owners and mangers to successfully manage both protected areas and aquatic systems.
  • ItemOpen Access
    Predicting cumulative watershed effects in small forested watersheds
    (Colorado State University. Libraries, 2009) Litschert, Sandra E., author; MacDonald, Lee H., advisor
    Cumulative watershed effects (CWEs), such as the hydrologic and sedimentary changes due to management activities, are a critical concern for many land managers. The goal was to develop a suite of GIS-based models for assessing CWEs in forested watersheds. The models need to be easy-to-use, spatially and temporally explicit, and scientifically based. Delta-Q and FOREST (FORest Erosion Simulation Tools) are a series of empirical and conceptual models that calculate the annual changes in discharge (Delta-Q) and annual sediment production, delivery and yield (FOREST) from roads, fires, and forest management. Given the paucity of data on hillslope sediment delivery, a field study also was conducted to assess the connectivity of sediment pathways from timber harvest units. Delta-Q and FOREST were verified using data from the Eldorado National Forest in California. The models were evaluated using measured data from three experimental watersheds. The predicted and measured 50th percentile flows were closer than the 1st and 99th percentiles, as extreme flows are more sensitive to climatic fluctuations. While predicted bedload sediment yields usually fell within the range of measured values, suspended sediment yields were generally overpredicted. FOREST results were most sensitive to changes in mean annual precipitation and GIS data scale. FOREST generates GIS layers to show the hillslopes, roads, and stream reaches with the greatest risk for altering runoff and erosion, and inducing sedimentation. Both models can be used to compare management scenarios within a watershed or among watersheds. By design the models take a middle approach between commonly used, unvalidated empirical models, and complex physically based models that are rarely used by land managers. During the field study in the Sierra Nevada mountains of California, the downslope edges of nearly 200 timber harvest units were traversed. Only 19 rills or sediment plumes were found that originated from harvest units. Five of the six features that extended through the stream-side management zone to a stream channel were generated by runoff from skid trails. The results indicate that harvest units rarely deliver sediment to streams, but post-harvest skid trail treatments may be needed to reduce surface runoff and sediment delivery to streams.
  • ItemOpen Access
    Enhanced recovery from ancient carbonate ramps: lessons and analogs from Paleozoic successions and the Persian Gulf
    (Colorado State University. Libraries, 2009) Jaffri, Ali R., author; Egenhoff, Sven, advisor
    Satellite imagery of the Persian Gulf, fieldwork in Kuwait and Abu Dhabi, and data from published sources are integrated to develop a thorough understanding of large-scale stratal architecture in carbonate ramps. A section of this study deals with the identification of key-surfaces in homogeneous successions. An Ordovician carbonate ramp from Sweden is selected to illustrate the importance of trace fossils in identification of maximum regression surfaces. By comparing Ordovician trace fossils from Sweden with modern crab burrows in Kuwait, a sequence stratigraphic model that shows strata architecture is presented. Oolitic facies in ancient carbonate ramps in the Devonian-Mississippian Bakken Formation that have been previously ignored or considered subtidal sheet-like deposits have been reinterpreted as coastal embayment, eolian dunes on barrier islands, and tidal channel deposits. Geometric analyses of similar environments in the modern Persian Gulf reveal that none of the oolitic facies in the Bakken Formation would be conducive to a sheet-like morphology. This paper highlights the diversity in shapes and dimensions of modern oolitic tidal channels in the Persian Gulf. Tidal channels documented in satellite imagery are oriented parallel, perpendicular or oblique to the shoreline. Planforms are remarkably similar to terrestrial fluvial systems, and transitions between straight, meandering, anastomosing, and braided patterns occur. Wide, straight channels form where bank materials consist of non-cohesive oolitic-skeletal sands, whereas those with prolific cyanobacterial growth along banks are prone to sinuous channels. A section investigates the challenges that oil and gas companies face when attempting to strike a balance between appeasing authorities and exploiting hydrocarbons while maintaining sustainable development. It also recommends policies that include amendments regarding oil fields in Kurdish territory and healthy alternatives to Production Sharing Agreements which ensure the flow of oil from Iraq while maintaining sustainable development. These include exclusion of oil fields in the Kurdish territory, which constitute only 3 percent of Iraq's oil reserves, from article 5a of the Iraqi Oil and Gas Law. This study recommends the use of contracts, such as Technical Service Agreements, that satisfy both the foreign oil companies and the Iraqi populace.
  • ItemOpen Access
    The thermophysical and microstructural effects of an artificial ice layer in natural snow under kinetic growth metamorphism
    (Colorado State University. Libraries, 2007) Greene, Ethan M., author; Smith, Freeman, advisor; Elder, Kevin, advisor
    The macrostructure of a seasonal snow cover evolves with each new weather event. With wind and precipitation, layers of snow coat the old snow surface and the microstructure within these layers develops as a function of the environmental conditions. The thermal, mechanical and optical properties of snow are highly dependent on its microstructure. Many researchers have investigated metamorphism in homogenous snow, but little is known of snow metamorphism at the interface of two layers. In this study I observe the thermal and microstructural evolution of layered and non-layered samples of natural snow in kinetic growth metamorphism. The layered samples contain a 4 mm thick ice layer, which creates a large gradient in thermal conductivity and porosity. I collected samples of natural snow with a density range of 150-290 kg m-3 from the mountains of northern Colorado. In a cold laboratory, I subjected paired, treatment (layered) and control (non-layered), samples to a vertical temperature gradient of 60-110 K m-1 for a period of 5 days. During the experiment I measured the heat flux at the boundaries and the temperature profile within the sample. At the end of each experiment I cast the snow samples and performed serial sectioning and three-dimensional reconstruction of the snow microstructure. I also used the thermophysical data and microstructural data to simulate the evolution of the microstructure and the thermal state at the end of the experiment. The temperature profiles show snow in a steady-state thermal environment. There is no consistent signal from the ice layer in the temperature data. The microstructure within the snow samples undergoes a dramatic change during the experiments. In the control samples vertical chains of faceted and hollow particles develop and are responsible for transporting most of the thermal energy in the sample. Faceted structures grow off the bottom of the ice layer, while the upper surface erodes and becomes smooth and round. The presence of the ice layer affects thermal, mechanical and optical properties of the snow, these effects occur within several particles of the interface and would be difficult to detect with standard field techniques.
  • ItemOpen Access
    Substrate controlled interactions between hydraulics, sediment transport, and erosional forms in bedrock rivers
    (Colorado State University. Libraries, 2009) Goode, Jaime Ruth, author; Wohl, Ellen, advisor
    Bedrock rivers are important components of the landscape that are distinguished from alluvial rivers by high sediment transport capacity relative to supply, and a direct link between the underlying geology and forms and processes. This dissertation examines how independent substrate controls influence the interactions among bedrock channel morphology, hydraulics, sediment transport, and incision processes at inter-reach and intra-reach scales. The majority of this research was conducted on the Ocoee River, Tennessee, which flows through the Blue Ridge province of the southern Appalachians. Substrate differences correlate with variation in reach morphology (i.e., gradient, bedform orientation and amplitude), such that less erodible substrates are associated with steeper reach gradient and with transversely oriented ribs of greater amplitude. Increased hydraulic roughness in reaches having steeper bed slopes, greater rib amplitude, and less erodible substrate, points to the importance of positive and negative feedbacks in these systems: Greater substrate erosional resistance limits profile lowering, which likely creates steeper bed slopes and greater stream power, creating a self-enhancing feedback. This local increase in stream power is balanced by increased roughness resulting from the erosional processes that produce bedrock ribs, which represents a self-regulating feedback. The overall result reflects quantifiable adjustments between substrate resistance and hydraulic driving forces in bedrock channels. Transport distance for coarse sediment is not a significant function of grain size, as has been reported for alluvial channels. Instead, the highly complex bed topography in this system leads to widely varying coarse sediment transport dynamics. Reach-scale differences in channel morphology correlate with transport distance. Local topographic controls exert the strongest influence on coarse sediment transport dynamics. Complex interactions among gradient and bed roughness appear to govern reach-scale differences in the degree of alluvial cover. In reaches with more resistant rock and heterogeneous bed topography, pothole dimensions are larger and follow an aggregated spatial pattern. Intermediate bed elevations show the highest likelihood of pothole formation, suggesting that local hydraulics and tools versus cover relationships govern pothole formation and maintenance. At different spatial scales, substrate characteristics play a key role in controlling the forms and processes of the bedrock channels examined in this study.
  • ItemOpen Access
    Trends and processes of land cover change in the western High Plains ecoregion
    (Colorado State University. Libraries, 2007) Drummond, Mark A., author; Laituri, Melinda J., advisor
    The goal of this study was to better understand the agricultural land use processes and land cover changes affecting the semi-arid Western High Plains ecoregion in the United States Great Plains. Globally, the processes of agricultural expansion and loss have had widespread effects on land cover and ecosystems that are an ongoing concern of land change research. To improve the understanding of regional land change, three main topics were addressed: (1) the contemporary patterns and key processes of agricultural change in the conterminous United States; (2) the rates, causes, and processes of land cover change in the Western High Plains ecoregion between 1973 and 2000; and (3) the primary driving forces of contemporary land cover change in the Western High Plains, including the dynamics of water resource access. Land cover change estimates for the ecoregion were derived using a stratified random sample of 10 x 10 km blocks and remote sensing change detection. Land use was examined using the Census of Agriculture. Results of the study indicate that patterns of land change vary by region and time period depending on socioeconomic driving forces and environmental context. In the Western High Plains ecoregion, net grassland loss occurred from 1973 to 1986 as agricultural land expanded in response to market opportunities. Agricultural expansion affected 1.9% of the ecoregion. Processes of land change became substantially different after 1986. Between 1986 and 1992, grassland expanded and became the dominant land cover, driven in large part by the cropland retirement policies of the Conservation Reserve Program (CRP). Agricultural declines affected 7.3% of the ecoregion, primarily as cropland was converted to grassland cover. Between 1992 and 2000, net grassland expansion was less than 1%, although there was a high rate of gross change in the location of grassland and agriculture that had only a limited effect on net change. The primary driving forces of land cover change were enabled by water resource access, which had a substantial influence on grassland extent and pattern.
  • ItemOpen Access
    Physical modeling of jointed bedrock erosion by block quarrying
    (Colorado State University. Libraries, 2009) Dubinski, Ian Michael, author; Wohl, Ellen E., advisor
    The primary objective of this dissertation is to provide insight in erosional processes, types of channel geometry, and relative rates of incision and knickpoint retreat of channels formed on jointed, resistant rock in a controlled experimental flume setting. Jointed, resistant rock occurs primarily in crystalline lithologies such as granite, gneiss, quartzite, and basalt. These lithologies can be found in a wide range of climatic and tectonic settings. Channels in jointed bedrock may have distinctive erosional processes and geometry relative to channels formed in unjointed bedrock. Joints, fractures, and other discontinuities such as bedding planes in the bedrock are locally weakly resistant zones in contrast to the resistance of unjointed bedrock. These areas may be preferentially weathered to form weakly or completely detached blocks that may be mobilized by flows in the channel. Channels in jointed bedrock commonly have abrupt lateral or downstream discontinuities in bed elevation including steps and knickpoints. A physical model of jointed bedrock using concrete divided into discrete blocks was constructed in a flume and allowed to erode over time by primarily block quarrying. Experimental controls examined in the flume included discharge, channel width, and joint spacing. Observed changes in planform geometry were retreat of the downstream knickpoint with no development of anabranching channels. Erosion by block quarrying occurred with variation between runs of differing control variables. A force analysis of block quarrying combined with a statistical analysis of the erosion results in conjunction with the control variables, including joint spacing and stream power, provided insight into the process of block quarrying. Wider vertical joint spacing produced more easily eroded blocks than a narrower joint spacing with equal block height in each case when friction forces along the side of the blocks are considered. As blocks loosen over time, the side forces diminish. Without side forces resisting motion, blocks formed by the wider vertical joint spacing are less easily mobilized than the smaller blocks. The other important element in defining block erosion is the key block concept. Erosion of blocks occurred as either a few blocks at one time or a mass movement of blocks at roughly the same time. Mass movements sometimes occurred after removal of a few blocks. These movements of a few blocks were termed key block movements and formed a bimodal population in terms of event magnitude with the mass movements as the other sub-population. Comparison with joint spacing field data from observed anabranching, inner channel, and transitional reaches along the Orange River in South Africa generally concurred with the conclusions drawn from analysis of the model results. Block quarrying is controlled by the balance between block mobility and hydraulic conditions that change over time with periods of little block movement punctuated by mass movements.
  • ItemOpen Access
    Evaluating spatial and temporal controls on recharge fluxes in a stream-alluvial-bedrock aquifer system
    (Colorado State University. Libraries, 2023) Cognac, Kristen, author; Ronayne, Michael, advisor; Bailey, Ryan, committee member; Rathburn, Sara, committee member; Stright, Lisa, committee member
    The dynamics and timescales associated with natural and induced recharge to aquifers dictate whether and for how long groundwater resources are sustainable. This dissertation contains three studies which apply groundwater flow and geostatistical modeling to evaluate spatial and temporal controls on recharge fluxes in a stream-alluvial-bedrock system. Each study is based on a recharge mechanism that occurs within the Denver Basin aquifer system, a regionally significant water supply for which long-term pumping and active aquifer depletion call for improved characterizations of recharge. While recharge is the theme of this dissertation, I don't attempt to directly estimate recharge for the Denver Basin, but rather to investigate and expose dynamics of recharge that are essential for accurate conceptualizations and estimates of recharge. The first study investigates controls and timescales associated with streambed fluxes which are an important component of seepage recharge along mountain-front streams. Streambed fluxes are highly variable through time and space, having a range of implications for stream-aquifer processes. While spatial variations in streambed flux have been heavily characterized, temporal variability has been limited to short-term or low-frequency measurements. This study calculates high-frequency time series of Darcy-based streambed fluxes over a three-year period using water level and temperature inputs from shallow (<1.5m) nested streambed piezometers installed in two mountain-front streams in Colorado, USA. Results reveal important conclusions about controls and patterns of temporal variability. Three predominant temporal scales of variability, sub-daily (<1day), daily (>1d; <1y), and interannual (>1y), are quantified through statistical measures. Sub-daily variability was related to ET, temperature-induced changes in hydraulic conductivity, and variable stream stage while daily variability was highly seasonal and related to specific events on the channel (e.g., beaver dams). The magnitude of sub-daily variability was significant compared to daily variability (ratio 0.03 to 0.7). Annual median fluxes at each site varied across years, but typically remained consistent in order of magnitude and direction. A strong linear correlation characterizes the relationship between the daily variability and the median annual flux at individual sites, highlighting how sites with greater fluxes also exhibit greater temporal variability. The temporal flux variations documented in this study have important implications for calculations and interpretations of hyporheic exchange and groundwater recharge. Results provide a basis for quantifying temporal variations in streambed fluxes and highlight the extent to which fluxes vary over multiple timescales. Chapters 3 and 4 are organized to progress vertically downward within the system to investigate controls for inter-aquifer exchange between the alluvial and bedrock aquifer, an important component of recharge to the underlying bedrock aquifer system. In Chapter 3, the potential for and controls of hydraulic disconnection between the alluvial and bedrock aquifer are investigated. Hydraulic disconnection occurs when unsaturated conditions develop between a stream and water table causing seepage rates to stabilize with additional water table drawdown. In this study, I demonstrate that hydraulic disconnection can occur between an alluvial and bedrock aquifer when unsaturated conditions develop between the two water tables and inter-aquifer flow rates stabilize with subsequent drawdown. Variably saturated flow modeling is performed to simulate the effects of drawdown on alluvium to bedrock flow rates (A-B flow). Bedrock aquifer heterogeneity is represented through object-based geostatistical models that are conditioned to wellbore data from the Denver Basin aquifer system. The Monte Carlo framework includes 200 heterogeneity realizations across a range of sandstone fractions. Results document the formation of unsaturated regions beneath the alluvium in all models, particularly where sandstone channels underlie thinner low-permeability mudstones. Three-dimensional heterogeneity creates complex saturation patterns that result in localized flow paths, spatially varying disconnection, and a gradual transition to hydraulic disconnection as the regional water table is lowered. Successive changes in A-B flow decrease over the course of simulations by 80% to 99% and final rates approach stability as indicated by changes of <1% between successive stress periods. Of the 200 models, 190 reach full hydraulic disconnection and 10 conclude with a transitional flow regime. Dynamic connectivity metrics developed within the study strongly explain flow results. I also evaluate the aspects of heterogeneity that are most likely to produce disconnection, highlighting several factors that influence disconnection potential. Chapter 4 evaluates the potential for a beaver dam to drive flow across the alluvial-bedrock contact. Beavers construct dams which promote a range of surface and near-surface hydrologic processes, however, the potential for beavers to influence deeper aquifer dynamics is less often, if ever, considered. In this study I consider the potential for a beaver dam, specifically increased stream stage and width upstream of a dam, to drive deeper flow from an alluvial to bedrock aquifer. I utilize a numerical groundwater flow model to simulate the effects of the beaver dam on inter-aquifer exchange rates. The base case model is parameterized based on observations from a beaver dam constructed on Cherry Creek in 2020 and the stream-alluvial-bedrock aquifer sequence in the Denver Basin in previous chapters. I also test whether the influence of the beaver dam is sensitive to the alluvial-bedrock contact depth, beaver pond depth, and hydraulic properties by simulating flow across a range of sensitivity scenarios. Model results document an increase in alluvial to bedrock flow on the order of 0.5% to 4%, depending on the contact depth, beaver pond depth, and hydraulic properties. Changes in hydraulic head due to the dam propagate deep into the aquifer (>30m), highlighting the potential for deeper aquifer impacts. The effect of the beaver dam is greatest for shallow alluvial-bedrock contact depths, deeper pond depths, and lower hydraulic conductivity contrasts between the alluvial and bedrock aquifer. Overall, results document the potential for beavers to influence deeper aquifer fluxes where regional hydraulic gradients are downward, highlighting broader potential for beaver dams to enhance aquifer recharge in deeper aquifer settings.
  • ItemOpen Access
    Changes in water chemistry and fluvial geomorphology from arsenic contaminated floodplains of Whitewood Creek and Belle Fourche River, South Dakota
    (Colorado State University. Libraries, 2023) Marr, Alexander E., author; Sutton, Sally, advisor; Ridley, John, advisor; Ross, Matthew, committee member
    From 1877 to 1977 the Homestake Gold Mine in Lead, South Dakota released over 100 million megagrams (Mg) of arsenic rich mine waste into Whitewood Creek which joins the Belle Fourche River. The mine waste which contains arsenopyrite and other arsenic bearing minerals, is deposited along the floodplains of Whitewood Creek and the Belle Fourche River as overbank deposits and abandoned meander and channel fill. The introduction of mine tailings into these streams has impacted them chemically and geomorphologically for over 100 years. This study is a continuation of the work from Ji (2021) who focused on the long-term behavior of arsenic in the mine tailings. Her work involved sequential extractions of the tailings to determine the mineralogical setting of the arsenic and its rate of release. She also used statistical regression on historical data to estimate the physical and chemical removal of arsenic from Whitewood Creek's watershed. The focus of this study is to see how the tailings might have impacted the stream chemistry of Whitewood Creek and the Belle Fourche River by modeling mineral saturation indices of the stream and seep water through the geochemical modeling program, The Geochemist's Workbench. The Geochemist's Workbench was used to model the dissolution rate of arsenopyrite to calculate the rate of dissolved arsenic entering Whitewood Creek. Suspended arsenic entering Whitewood Creek was calculated using the dimensions of the creek bed, thickness of tailings, and density of arsenopyrite. In addition to chemistry, this study investigated the changes in the tailings and fluvial geomorphology of Whitewood Creek and the Belle Fourche River from 1948 to 2012. This was performed by using aerial photographs from 1971, which mapped locations of the tailings along the floodplains, and overlaying them with photographs from 1948, 1977, and 2012. Using GIS through ArcMap, the tailings and their portions that have been removed over time were digitized. Other fluvial parameters that have been determined and digitized are stream longitudinal profiles, sinuosity, contaminated floodplain width, channel migration, and total sediment deposition area. The mineral saturation indices of Whitewood Creek and the Belle Fourche River are similar to each other and differ at the most by around 2-3 orders of magnitude. The minerals that are supersaturated are mainly phyllosilicates (mostly clays), Fe, Cu and Al (hydr)oxides, and carbonates with minor sulfates and phosphates. Seep waters have lower mineral saturation indices, up to 10 orders of magnitude lower for Fe bearing minerals. The only arsenic bearing mineral that is calculated to be supersaturated is Ba3(AsO4)2; however, this mineral has not been observed in nature. Based on the range of possible arsenopyrite concentration in the contaminated sediment (15 to 0.11%), the calculation of dissolved arsenic being discharged out of Whitewood Creek ranges from 52 to 0.39 Mg per year. This range compares to Ji's (2021) daily dissolved arsenic rate range of 3.89-0.33 Mg/year. For a tailings width range of 0.6 to 3.5 m, the calculated rate of suspended arsenic being discharged ranges from 254 to 1.98 Mg per year. Although large, this range encompasses Ji's (2021) suspended arsenic transport rate range of 33 to 70 Mg per year. The overlap of values from Ji's (2021) statistical approach and this study's geochemical approach indicates that arsenopyrite may be to some degree significant in controlling As transportation in Whitewood Creek. Based on GIS results, the location and evolution of contaminated floodplains along Whitewood Creek and the Belle Fourche River are very complex. The streams are different from each other and behave as their own systems. In Whitewood Creek, locations with high tailings area and removal are controlled by a possible range of factors such as knickzone geomorphology, bedrock lithology, and changes in stream energy due to topography. In the Belle Fourche River, reaches with high tailings area and removal are found about 7 km from the Whitewood Creek confluence and a 30 km stretch where rapid floodplain reworking occurs due to neotectonics from Precambrian basement adjustments. Tailings removed area and contaminated floodplain width graphs show that the Belle Fourche River has larger storage for tailings and undergoes more floodplain reworking due to higher flood frequency and neotectonics. In contrast, Whitewood Creek has lower storage and erosion due to decreasing mine sediment load at least since 1948 and channel incision into shale bedrock in some reaches. While the reworking of tailings into the stream is lower in Whitewood Creek than the Belle Fourche River, the tailings will remain on the floodplains for many generations.
  • ItemOpen Access
    Neotectonic effects of glacial erosion and deglaciation on the Sangre de Cristo Mountains, southern Colorado
    (Colorado State University. Libraries, 2023) Hurtado, Cecilia, author; Gallen, Sean, advisor; Singleton, John, committee member; McGrath, Daniel, committee member; Denning, Scott, committee member
    Interrelations between climate and tectonics are important to the development of active mountain belts, but rarely are there natural examples that lend themselves to studying the effects of climate on tectonics. The Sangre de Cristo Mountains in southern Colorado provide an optimal natural laboratory to explore the effects of alpine valley glaciation on surface uplift of the footwall and on the active extensional normal fault system in the northern Rio Grande rift. This region has experienced changes in surface loads associated with long-term glacial erosion and sedimentation over the course of the Quaternary, as well as shorter-term deglaciation after the Last Glacial Maximum. These changing loads correspond with stress changes that affect the flexural isostatic response of the lithosphere, and further act as clamping or unclamping stresses on the Sangre de Cristo fault that bounds the western margin of the mountain range. This work quantifies the masses and spatial distributions of these various loads and models the associated flexural isostatic response to estimate potential uplift and subsidence patterns in the study area that could be attributed to climate-driven mechanisms. The glacially-scoured footwall material was estimated by using remnants of the fluvial reaches downstream of glaciated drainage basins, reconstructing the paleofluvial topography, and subtracting it from the modern topography. The quantification of the deposited sediment in the San Luis Basin was measured from an interpolated surface tethered by existing drill cores, geophysical data, and geologic maps. Lastly, the glacial extents and thicknesses were constructed using a simple numerical modeling tool, GlaRe, constrained by preserved depositional and erosional evidence of glaciers. Isostatic responses were calculated using a flexure model with two effective elastic thickness (Te) values, 2 km and 5 km, and stress changes on faults at depth were calculated using an analytical line load model. The results estimate ~29 m of footwall uplift and ~47 m of subsidence in the hanging wall for a realistic Te of 5 km, and footwall uplift of ~48 m and a hanging wall subsidence of ~80 m for an independently calibrated Te of 2 km. Importantly, while topographic reconstructions indicate an ~50 m reduction in mean footwall elevations, isostatic rebound pushes mountain peaks upward by tens of meters. Footwall uplift due to deglaciation has a response of 4 m and 6 m, for a Te of 5 km and a Te of 2 km, respectively. The Sangre de Cristo fault trace was mapped to quantify the offset of fault scarps on Quaternary alluvial fans to determine the spatial and temporal patterns of offset along-strike of the fault. Fault offset magnitudes correlate with glacial domains, and fault slip rates correlate with the post-glacial spatial pattern of isostatic uplift, indicating an unambiguous link between deglaciation and elevated fault activity. This work demonstrates that (1) differential glacial erosion reduces mean footwall elevations, but the associated isostatic response drives surface uplift of mountain peaks, and (2) seismicity along normal faults could be amplified by load changes associated with climate-driven mechanisms, which will become increasingly important as we continue in a period of anthropogenic warming and deglaciation.
  • ItemOpen Access
    Sequence stratigraphic framework for top seal development: examples from the Skull Creek and Graneros shales, Denver basin
    (Colorado State University. Libraries, 1999) Edwards, Kimberly, K., author; Sutton, Sally J., advisor; Ethridge, Frank G., advisor; Almon, William R., committee member
    In general, the distal open marine shelf setting, typified by the Graneros Shale produces a rock with a greater and more uniform seal capacity relative to the rocks of a proximal open marine shelf setting, such as those of the Skull Creek Shale. A distal setting, which usually corresponds to the time of maximum transgression, may produce better seals because there is less coarse clastic sediment input, which allows slow deposition of clays from suspension to be the dominant depositional process. In this study, the higher capacity seal rocks occur in the upper parts of the TST, either within the condensed section or below it. The Skull Creek locations show seal occurrence to be stratigraphically higher on depositional topographic highs, and lower in areas that were topographically low at the time of deposition. Top seal capacity was quantified with mercury injection capillary pressure (MICP) analysis. Other physical characteristics of these marine shales were studied but only porosity, permeability, total clay, and hydrogen index consistently demonstrated a significant correlation with seal capacity in both units. Shales that are well laminated with a high percentage of total clay and/or total organic carbon with a type I-II (marine) kerogen may or may not qualify as the best seal. Top seal capacity may be more a function of rock fabric rather than mineralogy. For example, two samples may have exactly the same amount of quartz, as shown by XRD analysis, but thin section examination reveals that the majority of quartz in one sample is present as grains and in the other sample as cement. Samples with cement usually provide a better seal because they decrease the pore throat diameter, thus increasing the amount of hydrocarbons that can be trapped. Seal quality in both the Skull Creek and Graneros Shales is quite variable throughout each of the facies within the TST deposits.
  • ItemOpen 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.
  • ItemOpen 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.
  • ItemOpen Access
    Geochemical modeling-based prediction of water-rock interaction during aquifer storage and recovery utilizing selected Colorado Front Range aquifers
    (Colorado State University. Libraries, 2023) Doherty, Amanda, author; Sutton, Sally, advisor; Sale, Thomas, committee member; Ronayne, Michael, committee member
    This study characterizes the Fountain Formation, Ingleside Formation, and sandstones of the Dakota Group and considers the potential of these three formations as hypothetical Aquifer Storage and Recovery (ASR) targets. Compositional data from surface rock samples, including major, minor and trace elements from bulk rock geochemical analysis and mineral identification from petrography are used to infer a generalized mineral suite to represent each of the formations of interest. Similarly, compositional analyses from domestic water well samples, including major anions and cations and selected metals, were used as generalized representations of native water from each formation of interest. Finally, compositional data from treated city water was obtained and used as a generalized representation of injection water. The generalized rock data along with the generalized native water data represent a hypothetical injection environment while the treated water composition represents a hypothetical injection water. All water and rock data were used to populate a Single Pass Mixing equilibria Model that simulated an ASR system using the USGS geochemical modeling computer program PHREEQC (PH REdox EQuilibrium). Model results include mixed solution compositions, mineral saturation indices and estimates of mineral mass precipitation during simulated injection. Results of modeling suggest there is limited geochemical water-rock interaction during ASR in the hypothetical environment in this study. Model results indicate that the mixed solution composition is controlled more by the injected solution than by reactions occurring between the injection fluid and aquifer host material. Specifically, as greater volumes of hypothetical injection water are introduced with each model step, the compositions of the resulting mixed solutions increasingly resemble those of the injected water. The model predicted the precipitation of hematite, kaolinite and quartz during injection of the hypothetical injection water. Because aluminum was below detection in the water analyses and an arbitrary value less than the detection limit was used in the model, the prediction of kaolinite precipitation is not meaningful. Further, the model was constrained to not permit mineral dissolution, limiting the applicability of the model only to the consideration of mineral precipitation. In addition, benchtop leaching experiments were performed on rock samples to provide additional information about potential water-rock interaction. Benchtop experiment results are presented, but the focus of the study is primarily on geochemical modeling results. Water analysis results presented here suggest that the formations of interest currently contain good quality water. Modeling results suggest that injection of treated water would likely not lead to volumetrically important precipitation of minerals in the formations.
  • ItemOpen Access
    Modeling of channel stacking patterns controlled by near wellbore modeling
    (Colorado State University. Libraries, 2023) Escobar Arenas, Luis Carlos, author; Stright, Lisa, advisor; Ronayne, Michael, committee member; Barnes, Elizabeth, committee member
    Reservoir models of deep-water channels rely upon low-resolution but spatially extensive seismic data, high vertical resolution but spatially sparse well log data and geomodeling methods. The results cannot predict architecture below seismic resolution or between well logs. Usually, the data and interpretations that provide constraints for modeling workflows do not capture sub-seismic scale architecture. Therefore, standard modeling methods do not generate models that include details that can impact hydrocarbon flow and recovery. Constraining models to well and seismic data is problematic. Employing measured sections in the Tres Pasos Fm. (Magallanes Basin, Chile) is feasible to predict deep-water channel architecture, specifically channel stacking patterns with 1D information analogous to well data. This research performed near-wellbore modeling to generate multiple scenarios of channel stacking patterns constrained by machine learning-derived probabilities using (i) conditional Monte Carlo simulation with soft probabilities per channel element within the measured section choosing the highest probabilities for each element (ii) conditional Monte Carlo simulation of channel stacking, (iii) template-based modeling, (iv) forward modeling with Markov transition probabilities without matching to thickness and (v) conditional Monte Carlo simulation constrained to measured section thickness. Machine learning workflows generate channel position probabilities (i.e., axis, off-axis, margin) within a measured section given the interpreted top/bases of channel elements. These probabilities constitute the input for Monte Carlo simulations capturing channel element stacking patterns at the measured section locations. The most likely 2D channel stacking pattern scenarios defined channel centerline points, and volumes of the individual channel elements can be generated connecting them. Surface-based modeling offers a way to depict reservoirs of hydrocarbons, water or low-enthalpy geothermal systems in which small-scale heterogeneity needs to be captured explicitly by bounding surfaces because it impacts fluid flow, improving our forecasts of resource exploitation. Furthermore, predicting heterogeneity controlled by depositional architecture is critical for transport and storage capacity in CO2 reservoirs. The dataset provided and the advent of these flexible and accurate methods to depict the subsurface offer the opportunity to overcome the historical limitations of grid-based models and allow us to assess multi-scale architecture that controls fluid flow. This research aims to show the results of modeling deep-water channels, including a 1D identification of architectural positions and a 2D arrangement of channel stacking patterns.
  • ItemOpen Access
    Quantifying aspect-dependent snowpack response to high-elevation wildfire in the southern Rocky Mountains
    (Colorado State University. Libraries, 2023) Reis, Wyatt, author; McGrath, Daniel, advisor; Kampf, Stephanie, committee member; Ronayne, Michael, committee member
    Seasonal snow is a critically important water resource for the western U.S., providing water for human consumption, hydropower, agricultural uses, and sustaining ecological biodiversity. However, due to a changing climate, seasonal snowpacks have declined by ~20% in the last century and the timing of annual runoff is occurring 1–3 weeks earlier than the historical normal. Wildfires are an additional disturbance that are impacting high-elevation seasonal snowpacks at significantly greater rates since 2000. The impacts of increased wildfire altered area introduces considerable water resources challenges due to the ways wildfire directly changes the mass and energy balances of seasonal snowpacks for years to decades following the disturbance. While the impacts of wildfire on seasonal snowpack are increasingly well documented, there is a lack of understanding in how impacts might vary across the complex terrain that characterizes these mountainous environments. Utilizing burn-condition paired automated weather stations, regularly repeated burn-condition and aspect paired snow pits and snow depth transects, and snow depth measurements from time-lapse cameras within the 2020 Cameron Peak burn area during the second winter post-wildfire, I found no significant difference in peak snow water equivalent (SWE) between burned and unburned areas on both north and south aspects. Peak SWE was comparably greater (~100%) on north aspects in both burned and unburned areas. On burned south aspects, peak SWE occurred 22 days prior to burned north and all unburned areas. During the spring melt, snow melted 147% faster on burned south aspects compared to unburned south aspects, while on burned north aspects, melt rates increased by ~60% relative to unburned slopes. The increase in melt rates on burned slopes was the result of energy balance differences, with the median daily net shortwave radiation increasing by 170%, while median daily longwave radiation fell by ~205%. However, the net energy evolved over the winter, with the sign of the daily net energy flipping in late March for both burned and unburned areas. In both instances, the magnitude of the net energy was greater in the burned area throughout the observed period. From late march through snow disappearance at the burned site, the net energy was ~60% greater at the burned site than the unburned weather station. My research provides a more nuanced understanding of wildfire impacts on seasonal snowpacks compared to previous work, as this work identified clear aspect-dependent differences in the response. These findings can be incorporated into physical models so water managers can better predict the timing and quantity of melt from these critical water resources in fire-impacted regions.
  • ItemOpen 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.
  • ItemOpen Access
    Quaternary alluvial lineaments in the Atacama Desert, northern Chile: morphology, relationships to bedrock structures, and link to the seismic cycle of the Andean subduction margin
    (Colorado State University. Libraries, 2023) Perman, Emily A., author; Singleton, John, advisor; Gallen, Sean, advisor; Bhaskar, Aditi, committee member
    Located in the upper plate of the modern Nazca-South American subduction zone, the hyperarid Atacama Desert is an ideal place to study forearc deformation through surface geomorphology. We studied neotectonic lineaments in alluvium between ~25.5° and 26° S in the Coastal Cordillera with the goal of understanding modern forearc strain. Visible in satellite imagery, these lineaments are defined by linear to curvilinear structures consisting of subparallel ridges that range from tens of meters to kilometers in length. Field observations and 10 cm-resolution digital elevation models (DEMs) derived from drone imagery record a consistent, asymmetrical "ridge-trough-ridge" morphology that commonly traces into ~1–2 m-wide bedrock fissures containing gypsum and calcite. Most lineaments in this region trend ~N-S to NW-SE, parallel to the dominant bedrock structural grain and the Cretaceous Atacama and Taltal fault systems. Small-scale faults found in the lineament ridges have cm- to mm-scale apparent normal-sense displacement and consistently dip moderately to steeply (50–70°) towards the lineament troughs, defining a graben-like structure. In outcrop, these normal fault zones are enhanced by differential erosion, and in thin section faulted material is distinguishable by increased cementation within fractures penetrating grain boundaries. A tuff deposit within an alluvial fan containing several lineaments yields a zircon U-Pb age of 2.2 ± 0.1 Ma, indicating that lineaments in this fan are Quaternary in age and likely related to upper plate strain due to modern subduction along the Nazca-South American plate boundary. Older alluvial surfaces tend to have lineaments with broader and taller ridges than those formed on relatively younger alluvial surfaces, indicating that these structures formed progressively through time. In addition, profile data gathered from DEMs show a weak linear correlation between lineament trough width and ridge height, meaning that wider lineaments tend to have taller ridges. Along the flanks of two trenched ridges, we observed shallowly-dipping planes that resemble thrust faults, suggesting the ridges may have formed in response to contractional deformation. We propose the lineaments record alternating forearc shortening and extension related to interseismic and coseismic phases of the earthquake cycle, with the development of ridges and thrust faults recording interseismic shortening and normal faults and fissures which form the central troughs recording coseismic extension.
  • ItemOpen Access
    Geologic mapping and kinematic analysis of the Independence Mine shear zone in the Sangre de Cristo Range, southern Colorado: extensional reactivation of a Laramide reverse fault
    (Colorado State University. Libraries, 2023) Sitar, Michael C., author; Singleton, John, advisor; Ridley, John, committee member; Leisz, Stephen, committee member; Caine, Jonathan Saul, committee member
    The Sangre de Cristo Range in southern Colorado records some of the deepest Cenozoic structural levels in the Rocky Mountain region. Exposures of Laramide-age contractional mylonites show evidence for brittle-plastic extensional overprinting associated with the Rio Grande rift. This study examines the relation between Laramide contraction and Rio Grande rift extension by detailed geologic mapping and kinematic, geochronological, and geochemical analyses in a 50 km2 area centered on the Independence Mine shear zone (IMSZ), formerly called the Independence Mine thrust. The IMSZ is a 15- to 100-meter-thick, shallow-to-moderately (25°–62°), WSW-dipping brittle-plastic shear zone near the topographic base of the western flank of the range. It displays microstructural evidence for initiation as a top-NE contractional mylonite zone consistent with Laramide kinematics but is pervasively overprinted by deformation fabrics indicating top-SW extensional reactivation. Top-SW microstructures are characterized by phyllosilicate-lined C- and C'-shear bands and mixed brittle-plastic deformation of quartz. Mapping shows that the IMSZ is the thickest member of a system of mylonitic shear zones that dip shallowly to moderately (25°–67°) to the WSW and are hosted primarily within Proterozoic gneiss. Shear zones in amphibole-rich gneiss are commonly dominated by chlorite whereas those in quartzo-feldspathic gneiss have abundant white mica. Many of the thinner shear zones also record top-SW overprinting of top-NE fabrics. Though both top-NE and top-SW shear fabrics involve cataclasis and quartz dislocation creep, extensional overprinting appears to be mostly restricted to mylonites where secondary phyllosilicates form an interconnected weak phase. These relations are interpreted as fluid-mediated, reaction-weakening gradients where lithologically controlled rheological contrasts were variably sensitive to extensional reactivation. One top-SW shear zone adjacent to the IMSZ cuts a gabbro stock that was dated at 25.7 ± 0.7 Ma using LA-ICP-MS zircon U-Pb geochronometry. Synkinematic monazite grains in two samples of the IMSZ yield LA-ICP-MS U-Pb and U-Th-Pb ages of 24.9 ± 3.0 Ma and 22.2 ± 0.7Ma, respectively. These data are consistent with extensional reactivation occurring during Late Oligocene to Early Miocene time. The IMSZ and associated reactivated shear zones may represent mid-crustal extension that was widespread in the earliest stages of Rio Grande rifting before extension shifted to high-angle brittle-regime normal faults along the range front.
  • ItemOpen Access
    Exploring new approaches to understanding channel width and erosion rates in bedrock rivers, Puerto Rico, USA
    (Colorado State University. Libraries, 2022) Eidmann, Johanna Sophie, author; Gallen, Sean, advisor; Rathburn, Sara, committee member; Hughes, Kenneth Stephen, committee member; Ham, Jay, committee member
    Earth system dynamics produce constant adjustments to sea level, tectonics, and climate. Bedrock rivers communicate these changes throughout mountains by driving landscape and erosional responses that facilitate topographic change. It follows that an improved understanding of bedrock rivers can help us better model and reconstruct the interplay of changes to base level, uplift, and climate from landscapes. Although bedrock channel width plays a first-order role in river stream power and stream power-based landscape evolution models, because of the physical challenges associated with acquiring these data, channel width is often estimated and introduces uncertainty. In addition, the lack of bedrock channel width data has limited our understanding of what factors control channel width. In this dissertation (Chapter 2), I leverage high-resolution topographic data, Mean Annual Precipitation information, and use the HEC-RAS river modeling software to remotely derive bedrock channel width at desired flow scenarios. The accuracy of modeling results is verified for rivers in Puerto Rico using USGS gauging station field measurements, as well as my own channel width field measurements associated with 1-year recurrence interval discharges. As a next step, (Chapter 3) I implement the bedrock width modeling method derived in Chapter 2 to obtain >4,000 channel width measurements from reaches across Puerto Rico. I then compare these bedrock river width values to various factors (e.g. rock type and rock strength, drainage area, Ecozone, and grain size) that have been identified in the literature to scale with or influence channel width. My analyses indicate that, in Puerto Rico, rock type is a dominant control of bedrock channel width in small (≤6-10 km2) drainage areas. Contrary to patterns of rock strength and bedrock width documented in the literature (e.g. Montgomery and Gran, 2001), I find that width doesn't appear to correlate with proxies for bedrock channel strength. Strong granodiorites have the widest low-order channels and the strong volcaniclastics and weak serpentinites have comparably narrow low-order channels. Analysis of limited grain size measurements shows a discernable difference in the coarse grain size distribution between the three rock types, with the volcaniclastic and serpentinite draining rivers having coarser sediment than granodiorite draining streams. These findings suggest that bedrock channel width may be influenced by unmeasured lithological parameters that impact the size of grains delivered to river channels from adjacent hillslopes (i.e. rock fracture density and spacing, as well as weathering). Lastly, (Chapter 4) I spatially analyze in-situ cosmogenic nuclide (10Be in quartz and 36Cl in magnetite) concentrations and find that bedrock erosion rates are higher in the central part of Puerto Rico than toward the east. Analysis of erosion rates compared to other parameters reveals that channel steepness, rather than precipitation or rock type, is positively associated with erosion rates. I further apply these erosion rate data to test the accuracy of four incision models of varying complexity. Model comparisons reveal that drainage area is a better predictor of incision rates in Puerto Rico than a precipitation-weighted drainage area parameter. In addition, whereas an increase in model complexity slightly improves model performance, the model only explains ~35% of the variability in erosion rates. It follows that current incision models are still missing many controlling factors of river incision rates in Puerto Rico.