Browsing by Author "McGrath, Daniel, advisor"

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Open Access
Cryo-geohazards in a warming climate: geophysical, hydrological, and remotely sensed investigations of glacial lakes, outburst floods, and rock glaciers
(Colorado State University. Libraries, 2022) Rick, Brianna, author; McGrath, Daniel, advisor; Rathburn, Sara, committee member; McCoy, Scott, committee member; Klein, Julia, committee member
Changes to the cryosphere impact both societal and ecological communities, and understanding where changes have occurred in the past allow us to predict changes in the future, and help in creating plans to minimize or alleviate potential societal stressors. The overarching goal of this dissertation is to explore changes to the cryosphere at varying spatial and temporal scales, utilizing a range of methods from in situ measurements to large-scale remote sensing, exploring seasonal to annual to decadal scale changes. I investigate ice-marginal lake changes in Alaska (Chapter 2), document ice-dammed lake drainages in Alaska (Chapter 3), and explore the hydrological influence of the Lake Agnes rock glacier in Colorado (Chapter 4). Ice-marginal lakes impact glacier mass balance, water resources, and ecosystem dynamics, and can produce catastrophic glacial lake outburst floods (GLOFs). Multitemporal inventories of ice-marginal lakes are a critical first step in understanding the drivers of historic change, predicting future lake evolution, and assessing GLOF hazards. In Chapter 2, I use Landsat satellite imagery and supervised classification to semi-automatically delineate lake outlines for four, ~5 year time periods between 1984 and 2019 in Alaska and northwest Canada. Overall, ice-marginal lakes in the region have grown in total number (+183 lakes, 38% increase) and area (+483 km2, 59% increase) between the time periods of 1984–1988 and 2016–2019, though 56% of inventoried lakes did not experience detectable change. Changes in lake numbers and area were notably unsteady and nonuniform. I demonstrate that lake area changes are connected to dam type (moraine, bedrock, ice, or supraglacial) and the spatial relationship to their source glacier (proglacial, detached, unconnected, ice, or supraglacial), with important differences in lake behavior between the sub-groups. In strong contrast to all other dam types, ice-dammed lakes decreased in number (–6, 9% decrease) and area (–51 km2, 40% decrease), while moraine-dammed lakes increased (+56, 26% and +479 km2, 87% for number and area, respectively) at a faster rate than the average when considering all dam types together. Proglacial lakes experienced the largest area changes and rate of change out of any lake position throughout the period of study, and moraine-dammed lakes experienced the largest increases. Moraine-dammed lakes with large growth are also associated with clean-ice glaciers (<19% debris cover). By tracking individual lakes through time and categorizing lakes by dam type, subregion, and location, I detect trends that would otherwise be obscured if these characteristics were not considered. Chapter 2 highlights the importance of including lake characteristics when performing ice-marginal lake inventories, and provides insight into the physical processes driving recent ice-marginal lake evolution. Chapter 3 focuses specifically on ice-dammed lakes, as the glacial lake outburst flood record is dominated by these types of lakes, yet as I found in Chapter 2, ice-dammed lakes are decreasing in number and area. Rapid lake drainage (on the order of hours to days) can produce devastating outburst floods leading many to propose that hazards from glacial lakes are increasing. Outburst flood compilations do show an increase in number of events over time, however, recent studies attribute such trends to observational bias. This leaves large uncertainty about current and future glacial-lake hazards. Using multitemporal satellite imagery, I documented 1150 drainages from 106 lakes between 1985–2020, with an apparent increase in event frequency from 5 in 1985 to 70 in 2020. However, accounting for the increasing number of satellite images throughout the record, I find no temporal trend in drainage frequency. Furthermore, I document a loss of >75% of ice-dammed lakes since the 1960s. This suggests a decrease in regional flood hazard and motivates an unbiased look at other regions. As the world deglaciates, rock glaciers are important headwater features that have a delayed response to warming. Over 10,000 rock glaciers have been mapped in the contiguous United States, and 38% of these rock glaciers are found in Colorado. North American rock glaciers are estimated to have the third largest water volume equivalent by region, though these features are an often-disregarded component of the water budget in alpine basins. In this study, I incorporate geophysical, hydrochemical, and remotely sensed data to investigate the ice presence, movement, and hydrologic influence of the Lake Agnes rock glacier in the northern Front Range, Colorado. I observe an average horizontal velocity of 17 ± 5 cm yr-1 between 2019 and 2021 for the active lobe. Rock glacier streams remained below 2.5 °C throughout the summer, mixed-source streams remained below 3.5 °C, and the non-rock glacier stream reached 13.5 °C. The geophysical surveys suggest an internal rock glacier structure of an active layer ~3 m thick, underlain by an ice-poor layer up to 10 m thick, underlain by an ice-rich layer up to 18 m thick, with total rock glacier thickness between 20–30 m. This study confirms the presence of ice within the Lake Agnes rock glacier and documents its influence on basin hydrochemistry, elevating ion concentrations, pH, and maintaining low stream temperatures. In basins such as the Lake Agnes basin, the reduced climate sensitivity of rock glaciers and their sustained cold-water input to mountain streams will likely provide a refuge for cold-water species in a warming climate.
Open Access
Distributed seasonal and annual mass balance measurements of Wolverine Glacier, Alaska, using geodetic surveys and emergence velocities
(Colorado State University. Libraries, 2021) Zeller, Lucas R., author; McGrath, Daniel, advisor; Aster, Richard C., committee member; Leisz, Stephen J., committee member
Glaciers are key components of human-environmental systems worldwide. They are a source of fresh water for human consumption, crop irrigation, and hydroelectric power even during times of drought. Glaciers promote environmental and ecological heterogeneity by modulating stream temperatures and providing key nutrient, geochemical, and sediment fluxes, are popular tourism destinations, and introduce risks from natural hazards such as glacier-lake outburst floods. Glaciers have undergone dramatic retreat and thinning over the past 50 years, and these trends are predicted to accelerate through the 21st century. Short term (seasonal to annual) measurements of glacier mass balance provide valuable insight on how glaciers respond to climatological forcings and the processes that drive those changes. However, in-situ measurements are prohibitively time consuming, logistically difficult, and prone to uncertainty, rendering them insufficient for global-scale analyses. The increasing availability of high-resolution geodetic products offers promising opportunities for measuring mass balance from a remote platform if the confounding effects of ice emergence velocities and firn compaction on surface elevation can be correctly constrained. In this study, I present spatially and temporally distributed measurements of emergence velocities on Wolverine Glacier, Alaska, derived from three methods: 1) repeat Global Navigation Satellite System (GNSS) measurements of mass balance stakes, 2) modelled from annual mass balance measurements and glacier thinning rates, and 3) a novel approach of differencing geodetic surveys and snow depths derived from ground penetrating radar surveys. These emergence velocities, in conjunction with estimates of firn compaction, were used to measure distributed mass balances of Wolverine Glacier over three winter seasons, one summer season, and two annual time periods via geodetic surveys. The three approaches to measuring emergence velocity showed overall agreement but had important spatiotemporal differences. Comparison of geodetic mass balances with in-situ point and glacier-wide average mass balances had root mean square errors of 0.42 and 0.46 meters water equivalent. These results indicate that if emergence velocities and firn compaction are carefully considered, geodetic methods can provide accurate measurements of distributed mass balances over seasonal and annual time frames, yielding an improved understanding of glacier response and trend over these time scales. Such an understanding will facilitate improvements in model physics and parameterizations, thus improving projections for the magnitude and timing of future glacier losses and their effects on downstream communities and ecosystems.
Open Access
Evaluating L-band radar for the future of snow remote sensing
(Colorado State University. Libraries, 2024) Bonnell, Randall, author; McGrath, Daniel, advisor; Fassnacht, Steven, committee member; Kampf, Stephanie, committee member; Marshall, Hans-Peter, committee member; Ronayne, Michael, committee member
Snowpack monitoring is essential because seasonal snowpacks provide water for billions of people, support streamflow and ecosystems, and are a fundamental component of the Earth's energy system. However, no current snowpack monitoring system is capable of measuring snow water equivalent (SWE), the most important snowpack hydrologic variable, accurately and at high spatiotemporal (<500 m, 20 km of nearly continuous relative permittivity estimates, and thereby bulk density, from combined near-coincident measurements of GPR two-way travel times and lidar snow depths at three different field sites and in both dry and wet snow conditions. Variogram analyses were conducted and revealed a 19 m median correlation length for relative permittivity and density in dry snow. For wet snow, the correlation length increased to >30 m. I then leveraged the derived densities to evaluate six snow density models to better understand the limitations of these models within lidar and radar remote sensing methods. Two models yielded densities that estimated SWE within ±10% when SWE exceeded 400 mm, but model uncertainty increased to >20% when SWE was less than 300 mm. Thus, the refinement of these density models and the development of future density models is a high priority to fully realize the potential of SWE remote sensing methods. The L-band (1–2 GHz) InSAR technique for measuring changes in SWE (ΔSWE) is a promising method for SWE retrievals because the longer wavelength (~0.25 m) has minimal interaction with the snowpack microstructure and has increased canopy penetrative capabilities. In Chapter 3, I evaluated 10 L-band InSAR pairs collected by NASA UAVSAR near Cameron Pass, Colorado with GPR and terrestrial lidar measurements of ΔSWE in open meadows and burned forests. For single InSAR pairs, UAVSAR ΔSWE retrievals yielded an overall Pearson's correlation coefficient of 0.72–0.79, with a RMSE of 19–22 mm. I expanded the analysis beyond the locations of GPR and lidar surveys to evaluate the time series of UAVSAR SWE retrievals by including measurements of SWE from seven automated stations and found a RMSE of 42 mm. These findings support the use of this technique in unforested areas with dry snow conditions for the upcoming L-band NISAR satellite mission Given the findings of Chapter 3 and the canopy penetration capabilities of L-band radar, I designed Chapter 4 to evaluate the influence of forest cover on the UAVSAR signal. In Chapter 4, I evaluated eight L-band InSAR pairs collected by UAVSAR over the montane forests of Fraser Experimental Forest, Colorado with manually surveyed snow depths and snow pits and a pair of airborne lidar surveys. Compared with in situ measurements, I found that forest cover fractions <40% yielded RMSEs of ~15 mm, whereas RMSE more than doubled for forest cover fractions >50%. Further, normalized cumulative UAVSAR SWE and normalized lidar snow depths yielded identical statistical distributions for forest cover fractions <50% across the full study area, but these distributions diverged as forest cover fraction increased. Thus, forest cover fraction is a significant source of uncertainty for L-band InSAR retrievals of SWE, but this technique may be the first space-borne technique capable of retrieving SWE below non-dense forest canopy without any a priori information.
Open Access
Post-glacial alluvial valley dynamics of the South Fork Cache la Poudre River Valley at the Colorado State University Mountain Campus
(Colorado State University. Libraries, 2022) Suhr, Jens Christoph, author; Rathburn, Sara, advisor; McGrath, Daniel, advisor; Morrison, Ryan, committee member
Wide valley bottoms are physically important sediment storage sites where alluvial records of past landscape dynamics may be preserved. Following deglaciation after the Last Glacial Maximum (LGM), unconfined valleys in the Colorado Front Range experienced periods of fluvial aggradation and incision, creating distinctive valley bottom morphologies and the substrates which influence present-day hydrological and ecological characteristics. The objectives of this study are to investigate the processes and chronology of post-glacial geomorphic evolution of an unconfined portion of the South Fork Cache la Poudre River (South Fork) Valley, Colorado Front Range, to identify the dominant processes and temporal patterns of valley alluviation and incision following LGM retreat at the Colorado State University Mountain Campus (Mountain Campus). Methods used include geologic mapping, ground-penetrating radar (GPR) surveys, coring of valley bottom sediments, radiocarbon geochronology, and analysis of historical aerial images. Mapping of the Quaternary sediments indicates a variety of glacial and fluvial deposits occur in the South Fork Valley, including moraines, two distinct outwash terraces (approximately 8 m and 6 m above the present-day channel, respectively), fluvial terraces 1–2 m high, and an extensive floodplain. Well logs indicate over 10 m of glaciofluvial outwash sediment was deposited upstream of the LGM terminal moraine, and GPR reflections suggest that lateral bar migration, channel filling, and vertical accretion of sediments were important processes of outwash aggradation in the valley. The South Fork River has since incised into the outwash. A fluvial terrace and the modern floodplain are inset within the outwash sediments and are composed of overbank-deposited silt-to-sand sized sediments. Radiocarbon samples of valley sediments indicate that outwash was deposited at least 16.8 ka, with 8–10 m incision occurring after 16.8 ka and prior to 7.8 ka. Fine-grained sedimentation occurred on the fluvial terrace and floodplain from at least 2.1 ka to 1.3 ka. The modern floodplain has been vertically accreting for at least the last 500 years. Historical aerial images show that the South Fork channel was relatively stable from 1938 to the present; the channel planform area changed by no more than 2.5% per year during this period. Additionally, in the last ~80 years, the channel has largely occupied the center of the unconfined valley, reducing connectivity between the channel, terraces, and the valley sides. My results highlight the complex patterns of sediment storage and removal in unconfined valleys, with at least two phases of aggradation and one phase of incision following deglaciation. In addition, the South Fork Valley is relatively geomorphically stable: large volumes of Quaternary sediments have been stored for over 16.8 ka years, and the modern fluvial system has not responded drastically to local disturbances because of low connectivity between hillslopes and the valley bottom. The South Fork Valley is an effective site of fluvial sediment storage following deglaciation despite a long-term trend of sediment removal from the valley in the Holocene. Broader implications of assessing valley bottom stability and long-term sediment storage in mountains include managing unconfined valleys where development pressures, proposed water diversions, and climatically forced changes to the hydrology are occurring. Findings presented herein may provide insights for maintaining riparian biodiversity and surface-subsurface water exchange in formerly glaciated environments.
Open 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.
Open Access
Spatiotemporal variations in liquid water content in a seasonal snowpack: implications for radar remote sensing
(Colorado State University. Libraries, 2020) Bonnell, Randall Ray, author; McGrath, Daniel, advisor; Fassnacht, Steven, committee member; Rasmussen, Kristen, committee member
Mountain snowpacks act as seasonal reservoirs, providing a critical water resource to ~1.2 billion people globally. Regions with persistent snowpacks (e.g., mountain and polar environments) are responding quickly to climate change and are warming at faster rates than low-elevation temperate and equatorial regions. Since 1915, snow water equivalent (SWE) in the western U.S. snowpack has declined by 21% and snow covered area is contracting in the Rocky Mountains. Despite the clear importance of this resource and the identification of changes affecting it, no current remote sensing approach can accurately measure SWE at high spactiotemporal resolution. L-band (1-2 GHz) Interferometric Synthetic Aperture Radar (InSAR) is a promising approach for detecting changes in SWE at high spatiotemporal resolution in complex topography, but there are uncertainties regarding its performance, particularly when liquid water content (LWC) is present in the snowpack. LWC exhibits high spatial variability, causing spatially varying radar velocity that introduces significant uncertainty in SWE-retrievals. The objectives of this thesis include: (1) examine the importance of slope, aspect, canopy cover, and air temperature in the development of LWC in a continental seasonal snowpack using 1 GHz ground-penetrating radar (GPR), a proxy for L-band InSAR, and (2) quantify the uncertainty in L-band radar SWE-retrievals in wet-snow. This research was performed at Cameron Pass, a high elevation pass (3120 m) located in north-central Colorado, over the course of multiple survey dates during the melt season of 2019. Transects were chosen which represent a range in slope, aspect and canopy cover. Slope and aspect were simplified using the northness index (NI). Canopy cover was quantified using the leaf area index (LAI). Positive degree days (PDD) was used to represent available melt-energy from air temperature. The spatiotemporal development of LWC was studied along the transects using GPR, probed depths, and snowpit measured density. A subset of this project substituted Terrestrial LiDAR Scans (TLS) for probed depths. Surveys (17 in total, up to 3 surveys per date) were performed on seven dates which began on5 April 2019, where LWC values were ~0 vol. %, and ended on 19 June 2019 where LWC values exceeded 10 vol. %. Point measurements of LWC were observed to change (ΔLWC) by +9 vol. % or -8 vol. % over the course of a single day, but median ΔLWC were ~0 vol. % or slightly negative. LAI was negatively correlated with LWC for 13 out of the 17 surveys. NI was negatively correlated with LWC for 10 out of the 17 surveys. Multi-variable linear regressions to estimate ΔLWC identified several statistically significant variables (p-value < 0.10): LAI, NI, ΔPDD, and NI x ΔPDD. Snow-on Terrestrial LiDAR Scans (TLS) were conducted twice during the melt season, and a snow-off scan was conducted in late summer. Snow-on scans were differenced from the snow-off scan to produce distributed snow depth maps. TLS-derived snow depths compared poorly with probe-derived depths, which is attributed to poor LiDAR penetration through the thick vegetation present during the snow-off scan. Finally, radar measurements of SWE (SWE-retrievals), if coupled with velocities derived from dry-snow densities, overestimated the mean SWE along transects by as much as 40% during the melt season, highlighting a potential issue for water managers during the melt season. Future work to support the testing of L-band radar SWE-retrievals in wet-snow should test radar signal-power attenuation methods and the capabilities of snow models for estimating LWC.
Open Access
Understanding the daily to decadal evolution of mountain glaciers in Alaska and high mountain Asia from satellite remote sensing
(Colorado State University. Libraries, 2024) Zeller, Lucas R., author; McGrath, Daniel, advisor; Gallen, Sean, committee member; Ross, Matthew, committee member; Florentine, Caitlyn, committee member
Glaciers are important components of mountain ecosystems, mountain hydrological systems, and the global water cycle. Improving our scientific understanding of the spatial and temporal variability in glacier changes and the physical processes that drive those changes will allow better prediction of future glacier evolution. In this dissertation, I explore ways in which satellite-based remote sensing products can be used to study mountain glaciers across a wide range of spatial and temporal scales, with a specific focus on Alaska and High Mountain Asia. The accumulation area ratio (AAR) of a glacier reflects its current state of equilibrium, or disequilibrium, with climate and its vulnerability to future climate change. In Chapter 1, I present an inventory of glacier-specific annual accumulation areas and equilibrium line altitudes (ELAs) for over 3,000 glaciers in Alaska and northwest Canada (88% of the regional glacier area) over the 2018–2022 period derived from Sentinel-2 satellite imagery. I find that the five-year average AAR of the entire study area is 0.41, with an inter-annual range of 0.25–0.49. More than 1,000 glaciers, representing 8% of the investigated glacier area, were found to have effectively no accumulation area. Summer temperature and winter precipitation from ERA5-Land were found to be effective predictors of inter-annual ELA variability across the entire study area (R2=0.47). An analysis of future climate projections (SSP2-4.5) shows that ELAs will rise by 170 m on average by the end of the 21st century. Such changes would result in a loss of 25% of the modern accumulation area, leaving more than 1,900 glaciers (22% of the investigated area) with no accumulation area. These results highlight the current state of Alaska glacier disequilibrium with modern climate, as well as their vulnerability to projected future warming. In High Mountain Asia, many glaciers have thick debris cover over the majority of their ablation zones, earning them the name 'debris-covered glaciers'. Supraglacial lakes (SGLs) play an important role in debris-covered glacier (DCG) systems by enabling efficient interactions between the supraglacial, englacial, and subglacial environments. Developing a better understanding of the short-term and long-term development of these features is needed to constrain DCG evolution and the hazards posed to downstream communities, ecosystems, and infrastructure from rapid drainage. In Chapter 2, I present an analysis of supraglacial lakes on eight DCGs in the Khumbu region of Nepal by automating SGL identification in PlanetScope, Sentinel-2, and Landsat 5–9 satellite images. I identify a regular annual cycle in SGL area, with lakes covering approximately twice as much area during their maximum annual extent (in the pre-monsoon season) than their minimum annual extent (in the post-monsoon season). The high spatiotemporal resolution of PlanetScope imagery (∼ daily, 3 m) shows that this cycle is driven by the appearance and expansion of small lakes in the upper debris-covered regions of these glaciers throughout the winter. Decadal-scale expansion of large, near-terminus lakes was identified on four of the glaciers (Khumbu, Lhotse, Nuptse, and Ambulapcha), while the remaining four showed no significant increases over the study period. The seasonal variation in SGL area is of comparable or greater magnitude as decadal-scale changes, highlighting the importance of accounting for this seasonality when interpreting long-term records of SGL changes from sparse observations. The complex spatiotemporal patterns revealed in this analysis are not captured in existing regional-scale glacial lake databases, suggesting that more targeted efforts are needed to capture the true variability of SGLs on large scales. In Chapter 3, I expand these methods across a wider spatial extent by using the Landsat 5, 7, 8, and 9 archive to delineate SGLs on debris-covered glaciers across all of High Mountain Asia at near-annual cadence from 1988–2023. I find that SGL area has increased throughout the study period, rising to 17.2 km2 (0.7% of the investigated debris-covered area) in 2023, compared to ~8 km2 (0.3% of debris-covered area) in 1988. SGL growth is most concentrated in the Himalaya and Nyainqêntanglha regions, which have also experienced the greatest rates of 20th and 21st century mass loss. The 21st century SGL growth is concentrated almost entirely near the termini of these glaciers, indicating the possibility of continued growth and coalescence into large proglacial lakes. Areas of high SGL concentration are predominantly found in areas with lower surface gradients, low velocity, and thicker debris cover. Glaciers with high SGL concentrations are found to have steeper longitudinal gradients of thinning, with greater thinning rates further from the terminus resulting in lower surface slopes and more concave geometries throughout their entire debris-covered extents. However, the representative longitudinal thinning pattern of glaciers without substantial SGL formation have become more similar to this pattern in recent years, suggesting that more of these glaciers may be primed for SGL formation in the future.