Mountain Scholar :: Browsing by Author "McGrath, Dan, committee member"

Browsing by Author "McGrath, Dan, committee member"

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Open Access
Form and function: quantifying geomorphic heterogeneity and drivers in dryland non-perennial river corridors
(Colorado State University. Libraries, 2023) Scamardo, Julianne E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
Non-perennial rivers, including intermittent rivers and ephemeral streams, comprise the majority of drainage networks globally. However, ephemeral streams remain understudied compared to perennial counterparts, and the majority of extant studies focus on in-channel dynamics. Floodplains along perennial streams are known to host a high density of ecosystem functions, including the attenuation of downstream fluxes and provision of habitat to diverse flora and fauna. These functions are thought to be correlated to geomorphic heterogeneity, and studies of floodplain heterogeneity are emerging on perennial rivers. Here, I extend the conceptualization of floodplain function and heterogeneity commonly focused in perennial watersheds to dryland, ephemeral streams. Based on a synthesis of current literature identifying ephemeral stream floodplain characteristics in drylands, a set of floodplain styles emerge dependent on confinement and the presence of channelized flow. Functions related to attenuation and storage are typically concentrated in unconfined and channeled floodplains. The temporary storage of sediment and sub-surface water in ephemeral stream floodplains make them hotspots for biogeochemical cycling and hosts to richer, denser, and more diverse vegetation communities compared to surrounding uplands. Many functions of ephemeral stream floodplains are also found in perennial counterparts, but flashy flow regimes and high sediment loads in ephemeral streams can potentially impact rates and magnitudes of comparable processes and functions. Similar to perennial rivers, the diverse physical and ecological functions in ephemeral stream floodplains are thought to be related to spatial geomorphic heterogeneity. Although studies on the characteristics and drivers of geomorphic heterogeneity exist for perennial streams, similar studies in ephemeral streams are lacking. Geomorphic heterogeneity was therefore quantified along with potential drivers – including metrics related to geomorphic context and proxies for flood disturbance – to understand underlying processes in ephemeral river corridors. Geomorphic units were mapped in 30 unconfined river corridors within six non-perennial watersheds in Utah and Arizona, U.S. Landscape heterogeneity metrics – Shannon's Diversity Index, Shannon's Evenness Index, and patch density – were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, channel change and sediment storage times. Although heterogeneity positively correlated with metrics for morphology and disturbance (i.e., channel change and storage), statistical models suggest that morphologic context, particularly floodplain width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that non-perennial rivers with greater geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long-term temporal heterogeneity are needed. The lack of direct flux observations could also be restricting insight into how floods interact with large wood and vegetation, which are known to have complex relationships with geomorphic heterogeneity in perennial rivers. In the absence of flood observations, a hydro-morphodynamic model was developed to investigate changes to channel and floodplain morphology due to wood and vegetation in an ephemeral river corridor in southeastern Arizona, U.S. Three scenarios were modeled: the actual configuration of the river corridor; an experiment in which jams were removed; and an experiment in which vegetation was removed. Both large wood and vegetation effectively confined flow to the main, unvegetated channel, which became wider and deeper over the course of a single moderate flood. When isolating the impact of large wood, model results show that wood increases the magnitude of channel change created by vegetation, resulting in ±0.1 to 0.3 m of additional scour or aggradation. The simulated removal of vegetation resulted in more channel change than the removal of wood alone, partially because vegetation occupies a much greater area within the stream corridor than large wood. I propose a conceptual framework in which large wood could mediate sedimentation as well as the recruitment and growth of vegetation in ephemeral streams, contributing to the evolution of ephemeral stream morphology over time. Due to the ubiquity of dryland ephemeral streams, results of this research have the potential to influence watershed management globally. Wide, unconfined ephemeral stream floodplains and riparian forests could be targets for protection and restoration similar to current efforts in perennial rivers. Particularly in the context of future climate and land use changes, understanding the natural character, function, and heterogeneity of ephemeral stream floodplains highlights their physical and ecological importance in dryland landscapes.
Open Access
Islands in the stream: spatial and temporal patterns of logjam-induced river corridor dynamics
(Colorado State University. Libraries, 2024) Marshall, Anna E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
Spatial and temporal variations in water and sediment fluxes moving within the river corridor drive changes in the three-dimensional geometry of channels and floodplains. In forested river corridors, pieces of large wood (> 10 cm diameter and 1 m length) and logjams (≥ 3 pieces of large wood) become an integral part of the interactions among water, sediment, and the resulting river corridor form and function. The net effect of logjams stored at least temporarily in the river corridor is to increase spatial heterogeneity, or patchiness, via processes such as channel avulsion and formation/abandonment of secondary channels, increased channel-floodplain connectivity, and greater instream aggradation. The importance of spatial heterogeneity, logjams, and secondary channels/islands to river corridor function has been well documented, but a lack of existing quantitative underpinning creates knowledge gaps in the processes driving island formation and persistence, the role of wood in facilitating these processes, and the complex interactions between flow, sediment, and wood in dynamic river corridors. This dissertation addresses some of the existing knowledge gaps around how logjams interact in a river corridor to create heterogeneity at different spatial and temporal scales by characterizing the patterns, processes, and interactions occurring in a naturally dynamic system. The topics explored here focus on research primarily conducted along the Swan River in the Northern Rocky Mountains of Montana with mention of sites in the Southern Rocky Mountains of Colorado. These locations represent some of the few remaining river corridors in the contiguous U.S. with natural flow, sediment, and wood regimes, but represent former widespread conditions. In the work that follows, Chapter 2 explores the processes driving spatial patterns in bifurcations induced by logjams. I find that logjam-induced bifurcations exist as a continuum of different patterns and the position of a river segment along this continuum correlates with the ratio of erosive force to erosional resistance. Chapter 3 builds on this by investigating how accretionary and avulsive processes shape bifurcations over time, emphasizing a temporal progression of logjam-induced features using 14C and tree ring data. I find that islands tend to grow through upstream migration – the presence of buried logs with contemporary trees growing on them indicates this process – and lateral accretion. Chapter 4 dives deeper into the interactions between process and form, demonstrating the relationship between channel dynamism, logjam presence, and spatial heterogeneity at larger temporal and spatial scales. I find that logjams and channel movement through time interact in a cascade of processes and feedbacks that foster increased spatial heterogeneity. Wood preferentially accumulates in more geomorphically heterogeneous portions of the river corridor that provide sites capable of trapping and retaining wood. Logjams can then drive greater total sinuosity and the formation of secondary channels that result in further wood trapping, greater heterogeneity of floodplain vegetation, and ideal habitat for beaver that further modify river corridor heterogeneity. I also find that bifurcations and spatial heterogeneity persist even after logjam is no longer present. These results have implications for river management. If sections of the river corridor with more logjams and more beaver meadows display higher spatial heterogeneity, creating and protecting wood-rich heterogeneous retention zones within a river corridor is an important component to emphasize for river resilience. If physical effects persist even after a logjam is no longer present, than wood reintroduced to the river corridor as individual pieces or engineered logjams does not have to be anchored in place to facilitate formation of geomorphic heterogeneity within the river corridor. By dissecting the complexities of processes governing naturally dynamic river corridors, this work adds quantitative insight to the diverse functionality of heterogenous river systems in forested or historically forested regions and provides a launching point for future river management aimed at fostering river corridor function and resilience.
Open Access
Monitoring heterogeneity and carbon sequestration of restored river-wetland corridors
(Colorado State University. Libraries, 2022) Hinshaw, Sarah Kathleen, author; Wohl, Ellen, advisor; Rathburn, Sarah, committee member; Morrison, Ryan, committee member; McGrath, Dan, committee member
Innovation of new stream restoration strategies over the past three decades has added much-needed geomorphic complexity and ecological consideration to the practice of stream restoration. With the modernization of stream restoration to include biologically driven feedbacks, methods in monitoring must be simultaneously created to match the current progress. In addition, modern stream restoration practices offer significant opportunity to store carbon in restored river-wetland corridors by increasing carbon sequestration potential of the affected landscapes via rewetting valley bottoms and enhancing fluvial deposition. To address the need for monitoring techniques that capture complex river corridor restoration and carbon sequestration, I present in this dissertation: 1) the development of a geomorphic monitoring strategy that is applied to a valley-scale floodplain enhancement project that involved regrading of the valley bottom and abundant large wood placement, 2) a conceptual framework and protocol for estimating carbon sequestration potential in restored river-wetland corridors, and 3) the application of the latter protocol across multiple restoration projects in the western USA. With the monitoring protocol, which is based on plots rather than river cross-sections, I found finer substrate grain sizes, reduced canopy cover, spatial patterns in particulate organic matter, and the initial signatures of expected changes in heterogeneity at the valley-scale floodplain enhancement project over two years after project implementation. In the context of carbon sequestration among eight sites combined, I found the majority of pre-restoration or degraded condition sites to have significantly fewer carbon stocks than restoration projects or reference conditions, and the highest carbon stocks in reference condition sites. The chapters of this dissertation are not only intended to provide context and methods to measure stream restoration projects, but also to examine the state and trends of stream restoration in general and to contribute to the understanding of rivers in the global carbon cycle.
Open Access
Shifting sands: drivers and mobilization of fine sediment on the Cache la Poudre River following a wildfire
(Colorado State University. Libraries, 2024) Katz, Aaron, author; Wohl, Ellen, advisor; Bestgen, Kevin, committee member; McGrath, Dan, committee member
The Cameron Peak wildfire (840 km2) of 2020 led to widespread but varied detrimental effects throughout the Poudre River watershed (4,895 km2). The Poudre River flows through a high gradient canyon section with a somewhat unimpaired flow regime before it reaches a low gradient transition zone with several human impacts including channel modifications and flow reduction. Burned tributaries contributed elevated levels of fine sediment (< 2 mm) to the mainstem Poudre within the canyon, and in 2021 a large debris flow in Black Hollow Creek, a canyon tributary, delivered substantial amounts of material, including fine sediment, directly into the mainstem Poudre River. This led to a major fish kill and the transportation and deposition of fine sediment for at least 100 km downstream. In the transition zone, extensive fine sediment deposits either partially blocked or filled several channel margin backwaters and side channels, which are important habitats for native fish, and fine sediment filled interstices of coarse substrate grains in the mainstem river, which impacts benthic macroinvertebrate and fish spawning habitat. I quantify the degree of fine sediment retention along 100 km of the Poudre River by measuring reach-averaged fine sediment volumes and embeddedness and use model selection of multiple linear regression models to determine whether distance downstream from the sediment source or reach-scale geometric variables are the primary drivers of fine sediment retention. I also conduct a flushing flows study using 2-dimensional hydraulic modelling to determine the discharge required to mobilize the substrate at four sites along the longitudinal gradient of the study area representing different geomorphic settings and hydrologic regimes. Results from model selection using Akaike's Information Criterion corrected for small sample size (AICc) show that for both metrics of fine sediment retention, reach location (canyon vs. transition zone) is the primary driver of sediment retention, but some reach-scale variables are significant predictors of fine sediment retention. Both fine sediment retention metrics (site-averaged volumes and embeddedness) are greater in the transition zone. At the reach scale, gradient and cross-sectional area are both significant predictors of embeddedness, and models with gradient as a predictor variable have substantial support in explaining site-averaged fine sediment volumes. A mixed model of embeddedness at the transect level with bedform as the fixed effect nested with site as the random effect indicates that fine sediment is preferentially retained in pools and that all backwaters are 100% embedded. Although there are only three sites upstream of Black Hollow, fine sediment retention is either greater or not statistically different than canyon sites downstream of Black Hollow. Hydraulic modelling of the 2-, 5-, and 10-year recurrence interval flows at four sites indicates major differences in the magnitude and frequency of bed substrate mobilization between the canyon and transition zone. At a high gradient canyon site, the 2-year flow mobilizes a substantial portion of the bed, while at transition zone sites, bed mobilization occurs only at the 5- or 10-year flow level. I posit that artificially reduced flows in the transition zone are responsible for the lack of bed mobilization and will lead to longer residence times of fine sediment and prolonged impacts to aquatic ecosystems. This study adds to the literature by investigating post-fire fluvial responses at a greater spatial scale than most previous studies of the matter. By quantifying spatial distribution, physical drivers, and mobilization potential of fine sediment following a large wildfire on a major river, we can better understand how large rivers with varied human impacts respond to major disturbances and make informed management and restoration decisions going forward.
Open Access
Snow depth measurement via automated image recognition
(Colorado State University. Libraries, 2019) Brown, Kevin S. J., author; Fassnacht, Steven, advisor; Ham, Jay, committee member; McGrath, Dan, committee member; Ross, Matt, committee member
Seasonal snow is a significant contributor to the water supply of nearly 2 billion people in semi-arid regions around the world. Quantification of this resource is critical to planning sustainable water and food supplies in these regions. While Snow Water Equivalent (SWE) is the most common parameter used to estimate snow water storage, snow depth has often been used as a proxy since it is much simpler to measure and can be converted to SWE if density can be estimated. Depth of snow varies greatly at the regional, watershed, and plot scales and better quantification of this variability can improve water storage estimates. Installation and maintenance of new snow measurement sites is typically expensive and time consuming, so a technology that could produce high temporal resolution snow depth data for a low cost would be useful. Manual reading of snow depth from graduated staffs driven into the ground has been used by the Natural Resources Conservation Service (NRCS) for operational and research purposes. The amount of data available from this method has traditionally been limited by the time-consuming step of manually reading snow depths in images. The central objective of this research was to automate this process in order to reduce the time requirement and allow this technology to be deployed more widely. Five sites were established with time lapse cameras and a set of snow depth staffs around the state of Colorado. Several image recognition methods were considered, and the Aggregate Channel Features technique was used to detect snow depths based on images of the depth staffs. At the most successful sites, absolute error was close to 20 cm, while at less successful sites consistent errors as high as 100 cm made the data unusable. The variety of site configurations examined allowed factors which increased error such as forested backgrounds, close staff placement, and poor camera mounting, to be identified. Additional studies could take advantage of new, cloud-based image recognition technologies in order to allow anyone with a camera and an internet connection to measure snow depth automatically from pictures taken at specific locations.
Open Access
The dynamic nature of snow surface roughness
(Colorado State University. Libraries, 2022) Sanow, Jessica, author; Fassnacht, Steven, advisor; Sexstone, Graham, committee member; McGrath, Dan, committee member; Bauerle, William L., committee member
Throughout the winter season, the snowpack becomes the surface-atmosphere boundary for the energy balance within the hydrologic cycle and is key for understanding and modeling meltwater availability, streamflow, and groundwater recharge. The aerodynamic roughness length, z0, is one metric to quantify the roughness characteristics of the snowpack surface. Roughness is a key component when analyzing the snowpack surface energy exchange because it exerts a strong influence on turbulent energy exchanges between the snowpack and atmosphere. Snow surface roughness fluctuates throughout the winter season due to snowpack accumulation and melt, redistribution, ecological, and meteorological influences. However, current hydrologic and energy balance models use a static z0 value despite the snowpack surface, and resulting z0 value, being spatially and temporally dynamic throughout the winter. Inclusion of a site specific, spatially, and temporally variable z0 is expected to improve hydrologic and energy balance models. Therefore, the following research investigates 1) comparing the anemometric and geometric methods of measuring z0, 2) the correlation between z0 and snow depth, 3) spatial and temporal variability of z0, 4) post-processing effects on z0 measurements, and 5) application of a variable z0 within the SNOWPACK model. Results of this study indicate a strong correlation when comparing geometric versus anemometrical methods of calculation. 30 wind profiles were compared to 30 corresponding geometrically calculated surface measurements using a terrestrial based LiDAR. These combined profiles had a Nash-Sutcliffe Coefficient of Efficiency of 0.75, an r2 of 0.96, a best fit slope of 0.98, and a Root Mean Square Error of 8.9 millimeters. The correlation between snow depth and z0 is variable depending on periods of melt, accumulation, and the initial snow-free roughness. The z0 was shown to be spatially and temporally variable across study sites. Interpolation resolution during post processing of z0 was found to modify z0 by several orders of magnitude. Variable z0 values were found to alter SNOWPACK model results within several of the output variables. The most sensitive output variables were sublimation, latent, and sensible heat due to the direct use of z0 within the calculations. These key findings highlight the importance of a variable z0. Inclusion of a variable z0 parameterization within models should be site specific, spatially and temporally dynamic, with special attention to post-processing steps.
Open Access
Vegetation and lithologic influences on channel morphology in the southwestern U.S.
(Colorado State University. Libraries, 2024) Wieting, Celeste, author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; McGrath, Dan, committee member; Morrison, Ryan, committee member; Friedman, Jonathan, committee member
Vegetation and lithology play critical roles in shaping landscapes, creating diverse river and gully morphologies. Vegetation stabilizes banks and alters flow dynamics. In the Southwestern United States, non-native, invasive plant species contributed to regional trends of river channel narrowing and simplification and degraded diverse riparian habitats throughout the 20th century. More recently, efforts to remove invasive riparian vegetation (IRV) have been widespread, especially since 1990. Restoration practitioners who perform IRV treatments often focus on wildlife or vegetation response; however, geomorphic processes should be considered in restoration planning because they drive flow, sediment transport, and aquatic habitat and vegetation dynamics, and because of the potential for damage to downstream people and infrastructure. Depending on the restoration goal, management practices can be used to enhance or minimize the increase in channel dynamism caused by IRV removal. At the river reach scale, I investigated biogeomorphic feedbacks at one of the 15 previously analyzed study sites, the Rio Grande in Texas. Along the Rio Grande in Big Bend National Park (BIBE), restoration goals to remove invasive giant cane (Arundo donax) include decreasing channel narrowing and increasing water and sediment conveyance. Recent work has indicated that removal of giant cane has successfully reduced its extent, but the geomorphic effects of giant cane treatment and subsequent revegetation are still not well understood. A general lack of reach-scale studies of riparian plant pronation during flow inundation and the biogeomorphic feedbacks between plants, flow, and sediment transport contribute to this knowledge gap. I quantified morphological-effect plant traits for three common riparian plant species: invasive giant cane, native baccharis (Baccharis salicifolia), and native phragmites (Phragmites australis). I collected data at the plant, plot, and reach scales and created upright and flexible frontal area and vegetation roughness curves using photographs of plants and stem counts of plots. Then, I used these data in a reach-scale 2D hydraulic model to simulate species-specific effects and the effects of giant cane removal on channel hydraulics. Results indicate that the mean vegetation roughness is similar for all three species at the plant scale, but at the plot scale, vegetation roughness is higher for giant cane and phragmites due to higher stem densities. Hydraulic modeling results suggest that vegetation increased velocities in the center of the channel and decreased velocities on the channel margins. When all the vegetation was represented as giant cane, reach-scale water surface elevations were the highest and reach-scale velocities the lowest. Removing giant cane decreased water surface elevations, indicating increased conveyance. To determine the effects of IRV removal on a regional scale across the Southwest U.S., treated and untreated reaches at 15 sites along 13 rivers were compared before and after IRV treatment using repeat aerial imagery to assess long-term (~10 year) channel change. Resolving observations of channel change into separate measures of floodplain destruction and formation provided more information on underlying processes than simple measurements of channel width and centerline migration rate. IRV treatment significantly increased channel width and floodplain destruction. Treated reaches had higher floodplain destruction than untreated reaches at 14 of 15 sites, and IRV treatment increased floodplain destruction by a median factor of 1.9. The effect of treatment increased with the stream power of the largest flow over the study period. From the results, I suggest that restoration managers consider the system's susceptibility to change, downstream threats, and desired process changes when defining their geomorphic restoration goal because treatment of a dominant species over a large area can be expected to have major fluvial geomorphic consequences. In addition to vegetation, the lithology and surficial sediment properties influence hydrological processes, sediment transport, and gully and channel morphology. In semi-arid environments where vegetation is lacking, and precipitation is sufficient to drive erosion, sediment yields tend to be greatest. Increased landscape erosion is predicted as more extreme weather causes frequent or intense rainfall, and flooding. In Wupatki National Monument (WUPA), heavy rainstorms over the past decade, lack of vegetation, and presence of unconsolidated volcanic-derived cinders expose archaeological sites to erosion, a concern to cultural resource managers. To identify archaeological sites of highest vulnerability to erosion, I analyzed gully morphologic change over a 5-year period. I found that 35 measured gullies are actively eroding, with statistically significant changes in gully depth from 2016 to 2021. Up to 0.5 m of incision was documented over a five-year period. A structure-from-motion analysis at the hillslope scale confirmed gully morphological changes and supports the applicability of conducting similar analyses on a larger scale. More erosion occurred in gullies with catchments predominantly covered with cinders because of cinder mobility. A weak relationship was noted between gully catchment area and gully head slope, likely related to runoff processes from outcrops of resistant sedimentary rocks forming cliffs and characteristics of cinders that maximize infiltration and transport. Based on assessment of gully morphologic change and substrate characteristics, 22 archaeological sites along Wupatki Wash were identified as having a high vulnerability to erosion.

Browsing by Author "McGrath, Dan, committee member"

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