Theses and Dissertations

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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
A channel stability assessment and logistic regression model for a reach of muddy creek below Wolford Mountain Reservoir, in north-central Colorado
(Colorado State University. Libraries, 2014) Williams, Cory A., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Bledsoe, Brian, committee member
Water resource managers face increasing pressure to meet community water needs while responsibly managing resource infrastructure and preserving aquatic and riparian ecosystems. Management of many western rivers involves multiple uses for multiple stakeholders, especially rivers downstream from dams impounding water-supply reservoirs. These resource management issues arise in a reach of Muddy Creek near Kremmling, Colorado, which serves multiple functions including: (1) wetland mitigation, (2) private, recreational fishing, (3) and agriculture. Alteration of streamflow from reservoir operations and loss of upstream sediment supply, in conjunction with legacy management effects, have resulted in channel instability and increased streambank erosion in Muddy Creek below Wolford Mountain Reservoir. A typical response to channel instability on managed rivers includes installation of erosion-control structures. However, installation of these structures to protect property and infrastructure is expensive and can have unintended consequences at adjacent locations, highlighting the need for resource managers to better understand the underlying geomorphic processes controlling channel adjustment within the reach. To address these issues, field reconnaissance and channel surveying completed during base-flow conditions were used to (1) determine the dominant erosive and resistive processes within the reach that contribute to channel stability and response, and (2) assess the validity of using logistic regression techniques as an analytical framework and to estimate the probability or risk of localized streambank erosion. These findings can be used in conjunction with local management objectives to evaluate or gage acceptable risk to current infrastructure and to target and prioritize where monitoring or remediation should be conducted. Understanding the geomorphic processes and reach characteristics driving streambank erosion can be used to guide management and operational decisions within the reach to minimize impacts. A map of probability of erosion, for each streambank, is presented which shows risk (as a probability of streambank erosion, ranging from < 3 to 80 percent) based on significant explanatory variables from the logistic regression model. The study found that stream-induced scour and undercutting have differing effects within the reach due to changes in the erosive power of the stream and relative difference in streambank and riparian characteristics. Areas most susceptible to streambank erosion occurred in wider cross sections where fluvial energy was oriented into the streambanks, not necessarily in areas with the greatest fluvial energy and potential erosive power (i.e., areas with the steepest bed slope). This suggests additional, localized conditions within the reach need to be considered. Differences in streambank and riparian characteristics were shown to have varying levels of resistance to streambank erosion within the study reach. Larger streambank heights increased the probability of streambank erosion when these streambanks were not supported by bedrock outcrops of Pierre Shale or alluvial fans and talus slopes. Erosion-control structures decreased the probability of streambank erosion where structures retained original positions relative to flow. Where changes to flow orientation occurred, the probability of streambank erosion around these structures increased substantially. Riparian vegetation type also influenced streambank erosion as well as channel top-width. Streambanks covered in willows were found to decrease the risk of streambank erosion, whereas areas dominated by grasses increased streambank erosion potential as well as increased channel top-widths relative to areas dominated by willows. Additional effects from reach-scale characteristics were evaluated. Areas of greater sinuosity and wider valley widths show increased probability of streambank erosion, as well as in areas located downstream of an irrigation diversion structure. This may be due to a combination of effects from further confinement of the reach as valley widths decrease, increased streamflow from tributaries, and/or the occurrence of increased seepage along areas near an irrigation ditch. Streambanks with observed saturated soils were also found to have between 20-44 percent increased odds of streambank erosion. A linkage between proximity of streambank seeps and unlined irrigation ditches and irrigation turn-outs was highly significant, with potential effects extending to distances up to 250 m from the irrigation water sources.
Open Access
A characterization of Colorado Front Range and Denver basin aquifer system water stable isotope signatures
(Colorado State University. Libraries, 2024) Ulate, Isabella, author; Rugenstein, Jeremy K. C., advisor; Ronayne, Michael, committee member; Ross, Matthew, committee member
The Denver Basin Aquifer System (DBAS) is an important groundwater resource for Front Range communities and is currently experiencing increasing demand as populations grow and surface water supplies remain limited. It is necessary to better constrain aquifer recharge mechanisms to enable sustainable use of this resource. In other sedimentary basin aquifer systems, mountain front recharge has been shown to be a significant contributor to local basin groundwater recharge. In the DBAS, inputs from the mountain block are poorly understood, and previous numerical models have treated large segments of the mountain-front boundary as impermeable. However, there exist potential connections between the mountain block and the DBAS, either by direct contact of permeable units, which would facilitate underflow recharge into the basin, or by surface water infiltration to the aquifer units where they outcrop near the mountain front. To observe spatial and temporal relationships between mountain block water and DBAS water, we use water stable isotopes and characterize the δ2H and δ18O of monthly precipitation, seasonal surface waters, and groundwaters in and around the Front Range and Denver Basin. The goal of this study is to determine if differences in the isotopic composition of waters across the Front Range permit the use of δ18O and δ2H as tracers of water flow between Front Range streams and groundwater and the DBAS. We analyzed the unique signature of mountain-block water to compare with DBAS water stable isotope data collected from Castle Rock Water municipal wells. Stable isotope ratios varied spatially and temporally, with the greatest temporal variance observed in precipitation. Streams showed great spatial variance, and less significant seasonal variance between the three seasonal sampling events conducted. Groundwaters showed very little temporal variance but had great spatial variance both between the aquifer units of the DBAS and between different locations within the mountain block crystalline aquifer. The lowest δ2H and δ18O ratios were measured in winter precipitation, winter streams, and groundwater samples collected from the high-elevation Front Range. Samples of DBAS groundwaters with the lowest δ2H and δ18O ratios indicate potential hydrogeologic connection to the mountain block. Interpreted mixing lines on a d-excess versus δ18O plot support the potential DBAS-mountain block connection. The deepest aquifer units of the DBAS (Arapahoe and Laramie-Fox Hills) show the least relationship with meteoric or surface waters on both a δ2H and δ18O plot and the d-excess versus δ18O plot and have higher δ18O values than would be predicted based on their previously measured recharge ages and paleoclimate data from the region. Characterizing the spatial and temporal variations in water stable isotope signatures of the Front Range and DBAS region enhances understanding of the region's hydrology and hydrogeology. Additionally, these results help to better inform models of aquifer recharge and promote sustainable use of the DBAS resource.
Open Access
A debris flow chronology and analysis of controls on debris flow occurrence in the Upper Colorado River valley, Rocky Mountain National Park, CO
(Colorado State University. Libraries, 2012) Grimsley, Kyle J., author; Rathburn, Sara, advisor; Wohl, Ellen, advisor; Bledsoe, Brian, committee member
The role of debris flows along the Upper Colorado River was recently highlighted when the Grand Ditch, a 19th-century water-conveyance ditch, overtopped from snowmelt in 2003 and triggered a large debris flow along Lulu Creek, a tributary of the Colorado. Historical aerial photographs indicate that at least two other debris flows have been triggered from the Grand Ditch over the last century. This study examines the natural regime of debris flows in the Colorado River headwaters to assess whether the Grand Ditch has increased magnitude and frequency of debris flow occurrence on the west side of the Colorado River valley. Ten distinct sites of debris flow deposition were mapped using aerial photographs and field exploration, dated from tree cores and tree scars, and analyzed for magnitude using field-estimated volumes of deposition. Six of these ten depositional sites are on the west side of the valley, and several of them have evidence of multiple debris flows. Forty scarred survivor trees and 38 cores from even-aged stands were dated, with corresponding dates of debris flow occurrence ranging from 1923 to 2003. At least 19 debris flows have occurred in this catchment over the last century, but only those at the across-from-Specimen Creek, Lady Creek, Lulu Creek, and Little Yellow sites appear to have been large enough to affect the Colorado River. There is not a substantial difference in the frequency of total debris flows catalogued at the ten sites of deposition between the east (8) and west (11) sides of the Colorado River valley over the last century, but three of the four largest debris flows originated on the west side of the valley in association with the Grand Ditch, while the fourth is on a steep hillslope of hydrothermally altered rock on the east side of the valley. Although ability to interpret the debris flow record is limited by frequent disturbance and burial of older deposits, and estimates of magnitude have high uncertainty, these data suggest that the Grand Ditch has altered the natural regime of debris flow activity in the Colorado River headwaters by increasing the frequency of debris flows large enough to reach the Colorado River. Likelihood of debris flow occurrence is augmented by steep slopes and hydrothermally altered rock, which are both common in the vicinity of the Grand Ditch. This study demonstrates the applicability of dendrochronology for dating geomorphic events in Rocky Mountain National Park and provides context for restoration following debris flows.
Open Access
A geomorphic classification of ephemeral streams in arid regions
(Colorado State University. Libraries, 2013) Sutfin, Nicholas A., author; Wohl, Ellen, advisor; Bledsoe, Brian, committee member; Kampf, Stephanie, committee member
Current stream classifications do not adequately describe ephemeral streams in arid regions because these environments are characterized by high spatial and temporal variability of complex hydrologic interactions. To investigate the influence of channel form on riparian vegetation in the arid southwestern United States, I test a geomorphic classification for ephemeral streams based on the degree of confinement and the composition of confining material. I present five stream types: 1) bedrock channels entirely confined by exposed bedrock and void of persistent alluvium; 2) bedrock with alluvium channels at least partially confined by bedrock but containing enough alluvium to create bedforms that persist through time; 3) incised alluvium channels bound only by unconsolidated alluvial material into which they are incised; 4) braided washes that exhibit multi-thread, braided characteristics regardless of the degree and composition of confining material; and 5) piedmont headwater 0-2nd order streams confined only by unconsolidated alluvium and which initiate as secondary channels on piedmont surfaces. The objectives of this thesis were to i) validate distinct differences of channel geometry among the five stream types and ii) examine localized differences in geometry of the five stream types across watersheds with varying characteristics. Eighty-six study reaches were surveyed on the U.S. Yuma Army Proving Ground (YPG) and eighteen study reaches on Barry Goldwater Air Force Range (BMGR) in southwestern Arizona. Non-parametric permutational multivariate analysis of variance (PERMANOVA) for all 101 study reaches indicates significant differences (P<0.001) in channel geometry between the five stream types with regard to width-to-depth ratio, stream gradient, shear stress, and unit stream power. PERMANOVA results indicate no significant differences in channel geometry of individual stream types within watersheds of differing characteristics. A linear discriminant function of the four physical driving variables derived from 86 study reaches at YPG predict stream type with a 73% external hit rate for the 15 study reaches at BMGR. Classification and regression tree (CART) analysis identify thresholds for distinguishing stream types and indicates the relative importance of variables such that: width-to-depth ratio (W/D) correctly distinguishes 93.8% of braided channels (W/D > 91.2), shear stress (τ) correctly distinguishes 95.2% of bedrock channels (τ > 151.6 Pa), and unit stream power (ω) correctly distinguishes 68% of piedmont headwater channels (ω ≤ 35.63 W/m2). The resulting classification will provide a basis for examining relationships between channel characteristics, hydrologic process, riparian vegetation and ecosystem sensitivity of ephemeral streams in arid regions of the American Southwest.
Open Access
A method using drawdown derivatives to estimate aquifer properties near active groundwater production well fields
(Colorado State University. Libraries, 2014) Lewis, Alan, author; Ronayne, Michael, advisor; Sale, Tom, committee member; Sanford, William, committee member
This thesis describes the development of a new inverse modeling approach to estimate aquifer properties in the vicinity of continuously-pumped well fields. The specific emphasis is on deep bedrock aquifers where monitoring well installation is often not practicable due to high drilling costs. In these settings, water levels from groundwater production wells offer a transient dataset that can be used to estimate aquifer properties. Well interference effects, if detectable at neighboring production wells, allow for an interrogated aquifer volume that is larger (and therefore more representative at the well field scale) when compared to single well hydraulic tests. The parameter estimation method utilizes drawdown derivatives to estimate the aquifer transmissivity and storativity. The forward model consists of an initial water level (or a recoverable water level drift function), an analytical solution for aquifer drawdown, and a correction term for well loss. The aquifer drawdown component is based on superposition of the Theis solution, although other analytical solutions are also applicable. The observed dataset was judiciously trimmed to reduce computer run-time while retaining enough points to adequately characterize aquifer and well parameters. By limiting observation points to special domains, the calculated drawdown and observed well water level derivatives with respect to time are independent of well loss, and therefore the transmissivity and storativity can be estimated without knowledge of the recoverable water level or loss coefficient for individual pumping wells. Aquifer properties in the forward model were estimated by minimizing the difference between the modeled and observed drawdown derivatives. The parameter estimation method is tested using hourly water level and pumping data from municipal well fields producing groundwater from sandstone aquifers of the Denver Basin. Data collected over a seven-year period from two distinct well fields, one operating in the Denver aquifer and another operating in the Arapahoe aquifer, are considered. The estimated transmissivities are 30.0 m2/d and 46.5 m2/d for the Denver and Arapahoe aquifers, respectively, whereas the storativities are 4.7×10-4 and 2.0×10-4, respectively. These estimates are within the range of previously reported values, indicating that production well data can be used to derive reasonable aquifer properties. A separate synthetic aquifer test case was considered to further test the parameter estimation methodology, as well as to evaluate the appearance of Theis-like response behavior at the wells. Synthetic water levels were generated using a numerical model with geostatistically-simulated heterogeneity that is characteristic of the Denver Basin (sandstone bodies separated by less permeable inter-bedded siltstone and shale). Analysis of the synthetic water levels revealed meaningful hydraulic properties; the effective hydraulic conductivity (best-fit transmissivity divided by the modeled aquifer thickness) was slightly higher than the geometric mean hydraulic conductivity of the heterogeneous field. In addition to aquifer properties, observed water level data were used to estimate the well-loss coefficient and recoverable water level for individual pumping wells. Loss coefficients obtained for wells in the Denver Basin indicate that this mechanism (head losses due to turbulence around the well screen) may contribute between 20 and 150 m of the total drawdown (based on a pumping rate of 1500 m3/d) commonly observed in these wells. The recoverable water level at each well, when fit with a linear drift function, provides a means of investigating the prevailing trend in aquifer heads due to other regional influences outside the modeled well field.
Open Access
A methodology for estimating detectable change in water quality due to prescribed fire in northern Colorado
(Colorado State University. Libraries, 2001) Lange, Robert William, author
Increases in nutrients and metals in receiving waters have been documented after wildfire. However, water quality impacts from prescribed fire are not well known. This research investigated the design of a post-fire water quality monitoring program using a pre-fire dataset to detect water quality changes from prescribed fire. Since water quality changes due to land use practices are often difficult to detect due to high natural variability, a paired watershed approach was implemented. Two small watersheds were selected in the Cache la Poudre watershed in Northern Colorado and monitored for one year, resulting in 14 pre-fire water quality samples. A single station and paired approach, which consider statistical power are presented and the minimum detectable change is calculated for a range of post-fire sample sizes. Samples from the Bobcat Fire in the Big Thompson Watershed near Drake, Colorado are used to evaluate the results. These results show that with 16 post-fire samples a change of less than 1% of the difference between pre-firewater quality samples and samples from the Bobcat Fire can be detected for most parameters with a statistical power of 80%. The paired watershed approach is shown to reduce the minimum detectable change by half for parameters that are correlated between the two watersheds.
Open Access
A new method to test shear wave splitting: improving statistical assessment of splitting parameters
(Colorado State University. Libraries, 2016) Corbalán Castejón, Ana, author; Schutt, Derek, advisor; Breidt, Jay, committee member; Aster, Richard, committee member; Egenhoff, Sven, committee member
Shear wave splitting has proved to be a very useful technique to probe for seismic anisotropy in the earth’s interior, and measurements of seismic anisotropy are perhaps the best way to constrain the strain history of the lithosphere and asthenosphere. However, existent methods of shear wave splitting analysis do not estimate uncertainty correctly, and do not allow for careful statistical modeling of anisotropy and uncertainty in complex scenarios. Consequently, the interpretation of shear wave splitting measurements has an undesirable subjective component. This study illustrates a new method to characterize shear wave splitting and the associated uncertainty based on the cross-convolution method [Menke and Levin, 2003]. This new method has been tested on synthetic data and benchmarked with data from the Pasadena, California seismic station (PAS). Synthetic tests show that the method can successfully obtain the splitting parameters from observed split shear waves. PAS results are very reasonable and consistent with previous studies [Liu et al., 1995; Özalaybey and Savage, 1995; Polet and Kanamori, 2002]. As presented, the Menke and Levin [2003] method does not explicitly model the errors. Our method works on noisy data without any particular need for processing, it fully accounts for correlation structures on the noise, and it models the errors with a proper bootstrapping approach. Hence, the method presented here casts the analysis of shear wave splitting into a more formal statistical context, allowing for formal hypothesis testing and more nuanced interpretation of seismic anisotropy results.
Open Access
A paleohydrologic investigation in the vicinity of Harpers Ferry, West Virginia
(Colorado State University. Libraries, 1992) Fuertsch, Susan Jane, author; Wohl, Ellen E., 1962-, advisor; Salas, J. D. (Jose D.), committee member; Stednick, John, committee member
A paleohydrologic investigation of the Shenandoah River in the vicinity of Harpers Ferry, West Virginia, was conducted in response to the recent periodic floods that devastate the community. The study reach was approximately 7.5 km long and consisted of thirty-two surveyed cross-sections. Gaging stations established in 1895 at Millville, West Virginia and in 1882 at Harpers Ferry, West Virginia record flows ranging from a maximum of 6,509 m³s¯' , to a minimum of 2 m³s¯'. The average annual peak discharge for a seventy-year water record was 1,244 m³s¯'. Botanical flood evidence preserved as adventitious sprouts, tree scars and eccentric rings were documented in thirty-seven trees. A flood chronology established from these data extended from 1896 to 1955 after which no botanical indicators were found. Botanical indicators did not extend the systematic record, but they did provide an accurate, although not complete, flood chronology. The completeness of the botanical flood record is highly Sedimentological flood evidence was limited within the study area due to the influence of a humid-temperate climatic regime, which is not conducive to the stratigraphic preservation of individual flood depositional units. Human habitation of the area began in 1733; therefore, qualitative historical records were plentiful. Various historical records were cross-referenced to yield the most complete flood history. The correlation between the various sources was extremely high, demonstrating the comprehensiveness of the record. The historical flood record extends from 1748 to the beginning of the systematic record in 1896. The ability to determine accurate flood stages from paleoflood indicators varied highly. Botanical indicators were found to yield very inaccurate and inconsistent flood stages, and only minimum values of flood stage could be obtained from these data. Historical data did yield accurate stages; however, these stages did not necessarily yield accurate discharge values, depending upon the stationarity and hydraulic complexity of the area.
Open Access
A test of general shear versus folding for the origin of a cryptic structural feature, Nason terrane, Washington
(Colorado State University. Libraries, 2013) Jackl, Max Alexander, author; Magloughlin, Jerry, advisor; Ridley, John, committee member; Baird, Graham, committee member; Stednick, John, committee member
The Nason terrane of the Cascades Crystalline Core is a complex tectonostratigraphic terrane that has been the topic of much study due to its long deformational history and importance in the debate surrounding the Baja B.C. hypothesis. Structural patterns along Nason Ridge in the central part of the terrane have been interpreted as the result of either the presence of a major shear zone or the development of fold interference patterns. Distinguishing fold-controlled lineations from shear zone produced lineations can be a complex, but important problem. This study aims to provide a better understanding of high-temperature deformation which may be obscured due to extensive recrystallization. Outside of the cryptic zone the foliation is steeply dipping and striking to the NW with lineations which are subhorizontal. Previous observations revealed a structural zone ~1 km wide striking NW-SE. Within the zone, fold hinges and mineral lineations are typically subvertical with mineral lineations clustering tightly and trending NE, indicating that this zone has a fundamentally different structural fabric than the surrounding terrane. Fold hinge lines are more dispersed along the trend of the zone, possibly reflecting incomplete rotation of pre-existing fold hinges into a near vertical orientation. Anisotropy of magnetic susceptibility analysis indicates the magnetic fabric correlates well with lineations and foliations measured in the field, and lineations likewise shallow to near horizontal outside the zone. Quantification of the strength of the AMS fabric reveals an oblate spheroid, which is interpreted as a proxy for an oblate strain ellipsoid, indicating the presence of a component of pure shear. Asymmetrical microstructural features present include tailed porphyroclasts, mineral fish, and domino clasts. Sense of shear indicators agree with the hypothesized NE-side-up motion (Magloughlin, 1990). Electron backscatter diffraction analysis reveals biotite and amphibole possess a lattice preferred orientation (LPO), whereas quartz and plagioclase have a nearly random crystallographic fabric. The biotite and amphibole LPO was used to determine NE-side-up sense of shear in thin sections from the cryptic zone. It is clear from the agreement between outcrop scale structures, magnetic fabric, microstructures, and crystallographic fabric that a significantly different structural fabric is tightly localized on the zone. The highly focused nature of the zone and the lack of structures typically created by fold interference indicates that superposed folding is unlikely. These data, coupled with geothermobarometric constraints, point to a zone of displacement best characterized by northeast-side-up general shear.
Open Access
Alteration of organic matter and copper mineralization in the Midcontinent Rift, USA
(Colorado State University. Libraries, 2017) Schultz, Max, author; Ridley, John, advisor; Sutton, Sally, committee member; Kennan, John, committee member
To view the abstract, please see the full text of the document.
Open Access
Ambient and teleseismic elastic wavefields of the Ross Ice Shelf, Antarctica, and their application to crustal scale seismic imaging
(Colorado State University. Libraries, 2020) Baker, Michael G., author; Aster, Richard C., advisor; Schutt, Derek L., committee member; Krueger, David A., committee member; Dueker, Ken G., committee member
Laterally extensive floating tabular ice, such as the Ross Ice Shelf in Antarctica, is mechanically equivalent to a floating elastic plate and thus supports a variety of vibrational modes, including short-period (< 20 s) elastic waves, intermediate-to-long period (20–100 s) buoyancy-coupled elastic plate waves, and ultra long period (>100 s) gravity waves. Vibrational energy may be excited by near-field sources such as intra-shelf crevassing or the impingement of ocean gravity waves at the shelf ice front, and also by far-field sources such as teleseismic earthquake waves incident at the sub-shelf seafloor and the shelf grounded margins. Broadband seismometers deployed on an ice shelf readily observe these signals and facilitate large scale studies of ice shelf properties (via, e.g., travel-time tomography or velocity dispersion analysis) and near-field environment processes (via remote signal detection and analysis). Using two years of continuous data from a 34-station broadband seismic array deployed to the Ross Ice Shelf, Antarctica, I analyzed spatial and temporal variations in the short-to-intermediate period (0.4–25 s) ambient and teleseismic vibrational wavefields. I show that the ambient, ocean-wave-coupled wavefields are strongly modulated by sea ice concentrations in the adjacent Ross Sea, and identify three separate source processes operating in distinct period bands. Next, I show that body wave and surface wave arrivals from teleseismic earthquakes (>3000 km distant) are observed on the vertical components of ice shelf-sited seismometers with signal-to-noise ratios generally sufficient for crustal and mantle scale tomographic studies. I also show that teleseismic S-waves incident at the grounded margins routinely generate symmetric mode Lamb waves which propagate a minimum of 250 km into the ice shelf interior; this phenomenon occurs throughout the year, with broad azimuthal distribution, and may be exploited for travel-time tomography of the ice shelf. Finally, I present an algorithm for processing vertical channel autocorrelations of teleseismic P-wave coda, as recorded by floating-ice-sited seismometers, to illuminate crustal-scale features such as the Mohorovičić discontinuity. I present Markov Chain Monte Carlo inversions of these autocorrelations which yield crustal thickness estimates that are compatible with previous studies of crustal thicknesses for the Ross Sea Embayment and West Antarctica.
Open Access
An investigation into beaver-induced Holocene sedimentation using ground penetrating radar and seismic refraction: Beaver Meadows, Rocky Mountain National Park
(Colorado State University. Libraries, 2011) Kramer, Natalie, author; Wohl, Ellen, advisor; Harry, Dennis, advisor; Meiman, Paul, committee member
This study used ~1 km of near-surface seismic refraction (SSR) data and ~6 km of ground penetrating radar (GPR) data to infer the impact of Holocene beaver activity on sediment aggradation in Beaver Meadows, Rocky Mountain National Park. GPR data were used to uniquely identify radar packages of genetically related strata corresponding to glacial, non-glacial, and beaver-induced sedimentation. The radar package relating to glacial deposition was wedge-shaped and predominantly composed of a diffraction-rich, chaotic facie. The radar package relating to alluvium was draped over the glacial deposits and was characterized by multiple facies, but in general contained complex, slightly continuous reflectors interfingered with continuous, horizontal to subhorizontal reflectors. The radar package related to beaver-induced sedimentation was characterized by a laterally continuous parallel facie, interpreted to be ponded sediment, that abruptly truncated into a zone of chaotic reflectors, interpreted to be a beaver dam. In order to determine the relative magnitude of post-glacial deposition, the bedrock contact was determined using seismic refraction, GPR, auger data, and previous seismic and drilling studies. This study concludes that beaver damming is an important aggradational process trapping sediments within the Holocene, but did not cause significant amounts of aggradation. Beaver-induced sediments constituted ~50 percent of the alluvium, but the alluvium only constituted ~15 percent of the total valley fill, with a mean depth of ~1.3 m. The thin veneer of Holocene sedimentation implies that Beaver Meadows is not a site of continuous sediment deposition, but rather one characterized by episodic temporal and spatial aggradation, punctuated by incision, re-working and exhumation.
Open Access
Analysis of flexural evolution of the lithosphere over the past 4.6 Ma around Ross Island, West Antarctica
(Colorado State University. Libraries, 2019) Jha, Sumant, author; Harry, Dennis L., advisor; Aster, Rick C., committee member; Schutt, Derek L., committee member; Amberg, Gregory C., committee member
Ross Island is in the southern Victoria Land Basin along the western margin of the West Antarctic Rift System. Episodic volcanism since ca. 4.6 Ma produced a discontinuous sedimentary moat around the island coeval with ongoing extension. The moat is a composite of four smaller flexural sub-basins created during four distinct phases of volcanism on Ross Island. In this research we determine the flexural rigidity of lithosphere under Ross Island, to understand the load partitioning between surface (relief) and subsurface (e.g. the density anomaly in crust/mantle) and to test the hypothesis that the strength of the lithosphere around the island varied with time over last 4.6 Ma. An interactive toolbox called Toolbox for Analysis of Flexural Isostasy (TAFI) was developed in MATLAB to model the flexure around Ross Island. TAFI supports two-dimensional (2-D) and three-dimensional (3-D) modeling of flexural subsidence and uplift of the lithosphere in response to vertical tectonic loading. Flexural deformation is approximated as bending of a thin elastic plate overlying an inviscid fluid asthenosphere. The associated gravity anomaly is calculated by summing the anomalies produced by flexure of each density interface within the lithosphere, using Parker's algorithm. TAFI includes MATLAB functions provided as m-files (also called script files) to calculate the Green's functions for flexure of an elastic plate subjected to point or line loads, and functions to calculate the analytical solution for harmonic loads. Numerical solutions for flexure due to non-impulsive two-dimensional (2-D) or three-dimensional (3-D) loads are computed by convolving the appropriate Green's function with a spatially discretized load function read from a user-supplied file. TAFI uses MATLAB's intrinsic functions for all computations and does not require any other specialized toolbox, functions, or libraries except those distributed with TAFI. The modeling functions within TAFI can be called from the MATLAB command line, from within user-written programs, or from a graphical user interface (GUI) provided with TAFI. The GUI facilitates interactive flexural modeling and easy comparison of the model to gravity observations and to data constraining flexural subsidence and uplift. Flexural subsidence within each of the four flexural sub-basins around Ross Island is modeled by using a continuous elastic plate in TAFI. Models of subsidence are constrained by thickness of strata that accumulated in each sub-basin during the time interval in which the associated volcanic center was active. Flexure models were created along profiles trending radially away from volcanic centers. The load due to each volcanic center is represented as a point load at the location of volcano. These models yield best-fit flexural rigidities ranging from 6-36 x 10¹⁸ N-m, with the lowest values on the south and southwest sides of the island and associated with the youngest volcanoes. This systematic variation in flexural rigidity may be attributed to progressive weakening of the plate with time, or to spatial variations in plate strength. A second group of models constrained by the thickness of the entire stratigraphic interval filling the moat since volcanism began on Ross Island yield a flexural rigidity up to twenty times greater than models that consider only strata deposited within each sub-basin when the associated volcano was active. The second group of models overestimates the strength of the lithosphere due to the inclusion of strata deposited during periods in which regional extension rather than local flexure was the dominant control on subsidence in each sub-basin. In addition to plate flexure, the models indicate a small buoyant load with a magnitude of 7-10 x 10¹⁶ N beneath Ross Island, equivalent to a volume of 1.4 – 6.9 x 10¹³ m³ assuming a density contrast range of 3010 – 3160 kg/m³, is needed to maintain isostatic equilibrium with a moderately low-density upper mantle. 3-D flexural models are constructed to determine whether variations in the shape of the flexural moat around Ross Island are a result of variations in the strength of the lithosphere with space or with time. These models were constrained by the width and depth of flexure sub-basins around associated loading centers derived from the thickness of entire stratigraphic interval filling the moat since volcanism began on Ross Island. The models with constant flexural rigidity through time are unable to fit the width and depth of flexural subsidence in all parts of flexural moats around Ross Island. Models with time-varying flexural rigidities resulted in an improved fit for different parts of the moat with flexural rigidities varying between 3.0 x 10¹⁹ N-m to 2.6 x 10¹⁹ N-m, which is not a resolvable difference.
Open Access
Analyzing post-flood recovery after an extreme flood: North St. Vrain Creek, CO
(Colorado State University. Libraries, 2018) Eidmann, Johanna S., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
Assessing the ongoing sediment remobilization and deposition following an extreme flood is important for understanding disturbance response and recovery, and for addressing the challenges to water resource management. From September 9-15, 2013, a tropical storm generated over 350 mm of precipitation across the Colorado Front Range. The resulting 200-year flood triggered landslides and extreme channel erosion along North St. Vrain Creek, which feeds Ralph Price Reservoir, water supply for the Cities of Lyons and Longmont, CO. The flood resulted in 10 m of aggradation upstream of the reservoir, transforming the reservoir inlet into an approach channel. 4 years after the flood, downstream transport of flood sediment and deposition in the reservoir continues. This research tracks the fate of flood-derived sediment to understand the evolution of the approach channel and delta to assess post-flood response processes and controls and to quantify sediment remobilization. Photographic analysis and DEM differencing of the approach channel indicates that the majority of channel response to the flood occurred within 1 year following the flood. Evolution of the channel from an initial plane bed occurred through channel incision of up to 2.5 m and widening of up to 10 m, forming a trapezoidal cross section. Channel geometry changes in years 2-5 post-flood are limited in spatial extent, largely dependent on sediment discharge and local variations in channel confinement. Bathymetric DEM differencing from 2014 and 2016 (years 1 and 3 post-flood) indicates a minimum sediment accumulation of 68,000 m3 on the delta plain, and progradation of 170 m of the delta front since the 2013 flood. Between fall 2016 and spring 2017, the reservoir level was dropped approximately 10 m during construction at the spillway, creating a base level drop, delta incision, and causing over 15,000 m3 of sediment to be transported further into the reservoir. Based on bathymetry and reservoir core analyses, a total of 74,000 m3 of sediment was deposited in the delta from 2014 through 2017, producing an estimated loss of 0.4% in reservoir storage capacity. Approximately 184,000 m3 (equivalent to another 1% of reservoir storage capacity) is estimated to remain in storage upstream of the reservoir. Although the approach channel appears to be adjusted to a typical snowmelt runoff, stored sediment remaining upstream of the reservoir indicates that complete recovery of the approach channel may not occur on a management time scale. The remaining large volume of sediment still in storage upstream highlights the potential for future disturbances to trigger additional sediment inputs.
Open Access
Application of ground penetrating radar to sub-alpine hydrogeology, Snowy Range, Wyoming
(Colorado State University. Libraries, 2011) Page, Nathan Richard, author; Harry, Dennis L., advisor; John, Stednick D., advisor; Gregory, Butters, committee member; Michael, Ronayne J., committee member
A ground penetrating radar (GPR) survey was conducted at the Glacier Lakes Ecosystem Experiment Site, in the Snowy Range of Wyoming with the objective of determining the potential groundwater storage in the sediment overburden and determining the bedrock fracture density. The survey was completed along 12 transects with 100 MHz antennas using a Sensors and Software® PulseEKKO® 100 system. The step size between shots was 0.25 meters and the separation between the antennas was 1 meter. Two common midpoint (CMP) surveys were conducted and the subsurface GPR velocity was determined to be 0.06 m/ns. The survey results indicate that sediment overburden thickness ranges from 0 m at bedrock outcrops to > 9 m near East Glacier Lake. The volume of sediment at the study site estimated at 83,800 ± 8,380 m3. Based on a grainsize analysis of representative soil pedons the average specific yield at the study site is estimated to be approximately 10%. Given these values for soil volume and specific yield, the potential groundwater storage is 8,380 ± 838 m3 for the study site, which extrapolates to 61,300 ± 6,130 m3 in groundwater storage in the East Glacier Lake watershed. The two dimensional bedrock fracture density was determined to be 1 fracture per 4,810 m2. The bedrock groundwater storage is undetermined.
Open Access
Assessing channel change and bank stability downstream of a dam, Wyoming
(Colorado State University. Libraries, 2011) Gilliam, Elizabeth Ann, author; Wohl, Ellen, advisor; Cenderelli, Dan, committee member; Bledsoe, Brian, committee member
The Hog Park Creek watershed, in south-central Wyoming, has experienced several anthropogenic influences through time, the most notable in contemporary times being a reservoir in the upper extent of the watershed that was initially constructed in 1965 (Stage 1) and then later enlarged in 1985 (Stage 2). Flows released from the reservoir augment flows in Hog Park Creek. The existence of the channel-spanning dam creates a direct and identifiable disruption in the function of the two main drivers of geomorphic process: the water discharge, which has nearly doubled annually, and the concomitant disruption in the sediment transport regime. In order to assess channel responses, multiple analyses across a range of spatial and temporal scales were conducted. These include: a covariate hydrologic analysis relating three operational time periods using the Index of Hydrologic Alteration (IHA) software; an examination of the channel planform change from historical aerial photographs; analyses of annually repeated cross section survey data; and a study of bank erosion dynamics using the Bank Stability and Toe Erosion Model (BSTEM). The timing, magnitude and duration of flows have been altered since the Stage 1 implementation of the reservoir in 1965. Following a Stage 2 enlargement in 1985, the snowmelt-dominated hydrograph has most notably experienced a shift to bimodal high flows (an early spring, low-magnitude flow release from the reservoir and a late spring, high-magnitude flow release from the reservoir), a 550% increase in seasonal low flows, and a 10% reduction in peak discharges. The discharge historically corresponding to a 5-year recurrence interval now occurs annually under Stage 2 reservoir operations. Hence, formational flows for channel morphology have increased in both frequency and duration. The reduction in flow variability has ultimately altered the sediment transport regime, which is the base of the productivity and disturbance regimes that influence food web interactions, the composition of riparian vegetation and other ecological attributes of the pre-dam river ecosystem. Aerial photographic analysis of 29 years prior to and 36 years following the construction of the dam indicates an adjustment of channel width both temporally and spatially through the system. Statistical analyses suggest that the overall rate of change corresponds significantly to both location in the watershed (distance downstream of the reservoir) and the operation of the reservoir (volume, timing, and duration of water released). Most notably, the channel has shifted to a single-thread channel with reduced morphologic heterogeneity. Responses are most abrupt immediately downstream of the dam following its construction in 1965, whereas responses are more muted and delayed in the furthest downstream study reach. Cross section analyses indicate that each of the four study sites has experienced net erosion over the past five years. However, variation exists in erosion rates on the reach and site scales. Modeled erosion rates in BSTEM, corroborated with field data from bank erosion pins and repeated cross section surveys, suggest that the altered flow regime enhances bank erosion. The enhanced duration of high flows directly lead to increased amounts of toe scour. Flow regulation has changed the forces acting on the banks, including subsurface flow fluxes related to water level fluctuations and increased shear forces. This in turn has created hydraulic conditions that increase preferential erosion of the finer bank materials. However, this response is partially offset as channel geometry changes with width increase relative to depth, which alters the shear stress acting on the banks.
Open Access
Automated event detectors utilized for continental intraplate earthquakes: applications to tectonic, induced, and magmatic sequences
(Colorado State University. Libraries, 2018) McMahon, Nicole D., author; Aster, Richard C., advisor; Schutt, Derek L., committee member; Cheney, Margaret, committee member; Benz, Harley M., committee member
Event detection is a crucial part of the data-driven science of seismology. With decades of continuous seismic data recorded across thousands of networks and tens of thousands of stations, and an ever-accelerating rate of data acquisition, automated methods of event detection, as opposed to manual/visual inspection, allow scientists to rapidly sift through enormous data sets extracting event information from background noise for further analysis. Automation naturally increases the numbers of detected events and lowers the minimum magnitude of detectable events. Increasing numbers and decreasing magnitudes of detected events, particularly with respect to earthquakes, enables the construction of more complete event catalogs and more detailed analysis of spatiotemporal trends in earthquake sequences. These more complete catalogs allow for enhanced knowledge of Earth structure, earthquake processes, and have potential for informing hazard mitigation. This study utilizes automated event detection techniques, namely matched filter and subspace detection, and applies them to three different types of continental intraplate earthquake sequences: a tectonic aftershock sequences in Montana, an induced aftershock sequence in Oklahoma, and a magmatic swarm sequence in Antarctica. In Montana, the combination of matched filtering and multiple-event relocation techniques provided a more complete picture of the spatiotemporal evolution of the aftershock sequence of the large intraplate earthquake that occurred near Lincoln, Montana in 2017. The study reveals movement along an unmapped fault that is antithetical to the main fault system trend in the region and demonstrates the hazards associated with a highly faulted and seismically active region encompassing complex and hidden structures. In Oklahoma, subspace detection methodology is used in combination with multiple-event relocation techniques to reveal movement along three different faults associated with the 2011 Prague, Oklahoma induced earthquake sequence. The study identifies earthquakes located in both the sedimentary zone of wastewater injection as well as the underlying crystalline basement indicating that faults traverse the unconformity. Injecting fluid into the overlying sediment can easily penetrate to the basement where larger earthquakes nucleate. In Antarctica, subspace detection is again used in a very remote intraplate region with sparse station coverage to detail the sustained and ongoing magmatic deep, long-period earthquake swarm occurring beneath the West Antarctic Ice Sheet and Executive Committee Range in Marie Byrd Land, Antarctica. These earthquakes indicate the present-day location of magmatic activity, which appears appear to have increased in intensity over the last few years. This dissertation contributes to the growing bodies of literature around three distinctly interesting types of seismicity that are not associated to the first order with plate tectonic boundaries. Large tectonic intraplate earthquakes are relatively uncommon. Induced seismicity has only drastically increased in the central US during the last decade and created new insights into this process. Deep, long-period, magmatic earthquakes are still a poorly understood type of seismicity in volcanic settings.
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
Biotic controls on post-glacial floodplain dynamics in the Colorado Front Range
(Colorado State University. Libraries, 2011) Pilgrim, Lina Eleonor Polvi, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Merritt, David, committee member; Bledsoe, Brian, committee member
A recent surge in ecogeomorphic research has shed light on the numerous feedbacks and couplings between physical and biotic processes in developing geomorphic and ecologic process and form. Recent work has shown the critical importance of vegetation in altering overall channel form and developing meandering channel systems. This dissertation expands on planform classifications and the understanding of biotic-physical couplings through examining two components of post-glacial floodplain evolution in broad headwater valleys in the Colorado Front Range. First, I evaluate the role of beaver in Holocene floodplain evolution in low-gradient, broad headwater valleys to understand the historical range of variability of sedimentation processes and to determine the role of beaver in altering channel complexity and how that contributes to spatial heterogeneity of sedimentation processes. These objectives were carried out in Beaver Meadows and Moraine Park in Rocky Mountain National Park through analysis of subsurface sediment, geomorphic mapping, and aerial photography analyses. Second, I examine the role of various riparian species in stabilizing streambanks in order to determine the relative importance of bank versus root characteristics in stabilizing streambanks and to develop a functional classification of riparian vegetation in stabilizing streambanks. Data for this portion of the project were collected in three study sites along an elevation gradient in the Colorado Front Range: Phantom Canyon on the North Fork Poudre River (1920 m), North Joe Wright Creek (3000 m), and Corral Creek (3100 m), all of which are located in the Cache la Poudre drainage. For fourteen species (4 trees, 3 shrubs, 3 graminoids, and 4 herbs), root tensile strength, root size distribution, and root morphology were characterized. Streambank geometry and stratigraphy from Moraine Park were combined with vegetation characteristics in a physically-based bank stability model to determine the role of various physical bank characteristics and root characteristics in stabilizing streambanks. Examination of Holocene sedimentation processes in these broad, low-gradient headwater valleys, which are fairly disconnected from their hillslopes, lends support to the beaver-meadow complex hypothesis that uses beaver dams as the mechanism to explain the accumulation of fine sediment in glacial valleys. In the study valleys, sediment associated with beaver dams account for a significant (30-50%) portion of the relatively thin alluvium overlaying glacial till and outwash. Sedimentation rates were temporally and spatially heterogeneous across the floodplain, with higher rates associated with beaver pond sedimentation. Fluvial complexity, in terms of multi-thread channels, islands, and channel bifurcations, increases with beaver populations and number of ponds, and magnifies the potential for beaver damming because of increased channel length, which accelerates the development of fluvial complexity and valley sedimentation. Bank stability modeling determined that although bank and root characteristics are interrelated, physical bank characteristics play a larger role in determining bank stability than root characteristics. However, within similar streambank types, vegetation type is a strong predictor of overall streambank stability, and streambanks without vegetation were consistently the least stable. The presence of rhizomes, the maximum root diameter, the root tensile strength, and the lateral root extent of each species are the most important root characteristics in determining streambank stability. Riparian shrubs (willows) and riparian trees are the best streambank stabilizers. Upland trees and graminoids are mid-level bank stabilizers, and herbaceous species are mid/low-level bank stabilizers. In addition to sediment and flow regimes, the two biotic processes studied interact to form the overall channel planforms that dominate these broad headwater valleys. Assuming a relatively snowmelt-dominated flow regime and a gravel-bed channel system in the headwaters, four planform regimes are identified based on low to high beaver populations and the abundance and presence of xeric or riparian vegetation. Without beaver or bank-stabilizing vegetation, a braided channel planform will likely develop. With bank stabilizing vegetation but without a sustainable beaver population, a single-thread meandering channel will form, which only has a thin riparian vegetation strip and small fluvial influence on the overall valley ecological and geomorphic processes. With a sustainable beaver population and riparian vegetation along the streambank, a stable multi-thread channel system will form which has implications for the ecological and physical form and process of the valley. A valley with abundant beaver but little to no bank-stabilizing vegetation is impossible under natural conditions, because riparian vegetation is necessary to sustain a beaver population and their dam-building. However, a narrow, incised channel may be observed as a legacy effect from beaver removal. The probable planform regimes can be inferred over the range of Holocene climate conditions in the Colorado Front Range, and understanding of these biotic-physical interactions should be a crucial component of any management decisions for geomorphic or ecologic conditions.