Mountain Scholar :: Browsing by Author "Rathburn, Sara, committee member"

Browsing by Author "Rathburn, Sara, committee member"

Now showing 1 - 20 of 47

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
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.
Open Access
Channel initiation in the semiarid Colorado Front Range
(Colorado State University. Libraries, 2010) Henkle, Jameson E., author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Bledsoe, Brian, committee member
The channel head, defined as the upstream boundary of concentrated water flow and sediment transport between definable banks, represents the transition from hillslope processes to fluvial processes. The ability to delineate the location along a slope at which channels initiate is important for understanding hydrologic and geomorphic processes governing headwater streams. Studies demonstrating an inverse relationship between either contributing drainage area (A) and local valley slope {6) or basin length (L) and 6 for channel heads come primarily from regions with humid climates. Seventy-eight channel heads were mapped in the headwaters of the Cache la Poudre River and the North St. Vrain Creek in the semiarid Colorado Front Range. Multiple field sites were chosen along both rivers to account for variability due to aspect and elevation. Surface topographic parameters were measured in the field and analyzed to test the hypothesis that surface processes control channel initiation in this region. Although simple linear regressions indicate a poor inverse relationship between A and L and no relationship between L and (9, multiple regressions indicate that surface topographic parameters explain over half the variability in the location of channel heads. This suggests that surface processes exert an influence on channel initiation, but do not explain as much of the variability as observed in previous studies from wetter regions. A threshold of erosion necessary to initiate a channel was observed at approximately 10,000 for .4, although values as high as 600,000 were mapped for some channel heads. Variation within the study area correlated with elevation, which is a proxy for differences in volume and type of precipitation; sites at lower elevation with less precipitation, but more intense convective rainfall, tend to have smaller contributing area and basin length. Aspect did not influence surface topographic parameters. Field-mapped channel head locations plot at or downslope from the inflection point of a regional slope-area curve generated from 10 m DEMs, although some extend well downslope. Most actual drainage areas for channel initiation are thus an order of magnitude larger, and plot in a significantly different portion of the slope-area graph, than would result from the widespread practice of assuming channel heads are located at the gradient reversal in such curves.
Open Access
Controls on and trends in sediment and particulate organic matter storage by instream wood in north Saint Vrain Creek, Colorado
(Colorado State University. Libraries, 2017) Pfeiffer, Andrew, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Baker, Daniel, committee member
Sediment and particulate organic matter (POM) retained by wood within the bankfull channel were evaluated for 58 stream reaches at the headwaters of North Saint Vrain Creek, Colorado. Wood-induced storage in headwater regions is hypothesized to be important in buffering downstream transport of material. However, the magnitude of storage has not been thoroughly investigated in relation to different potential control variables (e.g., wood volume, channel gradient, channel confinement, and riparian basal area) and spatial scales (jam, reach, and drainage basin) of control. Multiple and single variable linear regressions informed results. On the jam scale, no relationship was observed between storage and visually estimated jam porosity and permeability. In contrast, the reach-scale volume of stored coarse sediment (gravel, cobble) responds strongly to reach-scale wood volume. Reach-scale fine sediment (sand and finer) volume responds most strongly to wood piece characteristics (average piece length/average channel width and longitudinal spacing) and reach-scale coarse sediment storage. POM storage was most strongly related to riparian controls (channel confinement and riparian forest basal area). These results were translated into a drainage basin-scale analysis in ArcGIS. Despite comprising 14% of the stream network, third-order reaches were found to store 41% of total estimated coarse sediment, 34% of total wood, and 23% of total fine sediment. Large logjams likely exert a high cumulative storage effect in a relatively small portion of the watershed. In contrast, 60% of estimated total POM storage occurs in first-order streams (47% of network stream length). Low transport capacity in these small streams retains highly mobile POM and lateral roots from the nearby riparian forest may serve as retention structures. These results indicate that wood exerts different geomorphic effects depending on its location within the stream network. From a management perspective, road building and campsite development should avoid impacts to first-order streams, as they are important to overall drainage basin POM retention. Third-order streams are hotspots of wood, coarse sediment, and fine sediment; promoting or allowing wood recruitment processes in these areas can facilitate high sediment retention and buffering of downstream transport.
Open Access
Crossing a threshold: the legacy of 19th century logging on log jams and carbon storage in Front Range headwater streams
(Colorado State University. Libraries, 2013) Beckman, Natalie, author; Wohl, Ellen, advisor; Kampf, Stephanie, committee member; Niemann, Jeffrey, committee member; Rathburn, Sara, committee member
Instream wood has an important effect on the geomorphic and ecological function of streams, but human impacts have altered both the forests that supply wood and the streams themselves. These changes may have pushed many stream systems over a threshold past which the stream morphology and ecology do not return to their pre-disturbance state, but instead settle into a "new normal." This dissertation addresses the question of whether logging which took place in the 19th century has had lasting and significant effects on the instream wood and carbon storage of headwater streams in Colorado's Front Range. The distribution of logs within the headwaters of the Big Thompson River, North Saint Vrain Creek and Cache la Poudre River in northern Colorado were assessed to quantify the ways in which logs and forest characteristics relate to carbon storage within a stream. Characteristics of jams (size, number per kilometer) and carbon storage correlate most closely with reach-scale variables, implying that management would be most effective at the reach scale. Increased total wood load and decreased spacing between key pieces are the most important changes that can be made to promote the formation of jams within a reach. Old growth forest creates significantly different total carbon storage and partitioning of carbon storage, which extends previous work on the effects of old growth forest on terrestrial carbon to riverine environments.
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
Developing a physical effectiveness monitoring protocol for aquatic organism passage restoration at road-stream crossings
(Colorado State University. Libraries, 2014) Klingel, Heidi M., author; Wohl, Ellen, advisor; Bledsoe, Brian, committee member; Cenderelli, Daniel, committee member; Rathburn, Sara, committee member
To view the abstract, please see the full text of the document.
Open Access
Downstream effects of diversion dams on riparian vegetation communities in the Routt National Forest, Colorado
(Colorado State University. Libraries, 2013) Caskey, Simeon Tadgerson, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Bledsoe, Brian, committee member; Merritt, David, committee member
Diversions are ubiquitous throughout the American west, with over 68000 known in Colorado alone. Diversions vary greatly in their structure and ability to extract water, but overall they can alter important components of the flow regime, affecting the magnitude and duration of baseflows and flooding. Riparian plant communities have adapted to unique hydrologic and geomorphic conditions existing in the areas subject to fluvial processes. My study used vegetation and geomorphic data from low-gradient (≤3%) streams, in the Rocky Mountains of north-central Colorado, above 2440 m. Data were collected at 32 reaches, totaling 16 paired upstream and downstream sites, to infer the impact of diversion-induced flow alteration on riparian vegetation communities. Vegetation data were collected using the line-point intercept method along transects oriented perpendicular to the channel, from bankfull to 5-10 meters away, totaling 100 sampling points per reach. Topographic data were associated with each sampling point, to analyze differences in lateral and vertical zonation of communities between upstream and downstream reaches. Vegetation data were analyzed using traditional biological diversity metrics, richness, evenness and diversity, as well as multivariate community analysis using ANOSIM, MRPP, and permanova. Across all data points, field observations indicate evenness increased downstream from diversions, through decreased frequency of hydrophytic, wetland indicator functional species groupings, and increase in frequency of several upland indicator species. Regarding elevation, immediately above the channel no differences were observed between communities, but at 1 m above the channel increase in upland species and decrease in wetland species downstream of diversion became apparent. Logistic regression supports this, indicating probability of occurrence for upland species downstream of diversion increases at a greater rate beginning around 0.5 m above active channel. Related to distance, nearest the channel no compositional differences were observed, but with increasing distance from channel decreased wetland and increased upland species relative frequency were observed downstream of diversion. Fluvial surface analyses, which are related to distinct hydrologic and geomorphic processes, also indicated composition shift as a function of diversion. Floodplains had significantly lower relative frequency of wetland species grouping, whereas low terraces had both increased upland and decreased wetland species relative frequency downstream of diversion. The findings of my study imply that riparian plant communities along low-gradient reaches in montane environments in the Rocky Mountains of Colorado are being impacted by diversion-induced flow alteration, in general having a reduced frequency of hydrophytic, wetland species, and encroachment of non-hydrophytic, upland species.
Open Access
Evaluating spatial and temporal controls on recharge fluxes in a stream-alluvial-bedrock aquifer system
(Colorado State University. Libraries, 2023) Cognac, Kristen, author; Ronayne, Michael, advisor; Bailey, Ryan, committee member; Rathburn, Sara, committee member; Stright, Lisa, committee member
The dynamics and timescales associated with natural and induced recharge to aquifers dictate whether and for how long groundwater resources are sustainable. This dissertation contains three studies which apply groundwater flow and geostatistical modeling to evaluate spatial and temporal controls on recharge fluxes in a stream-alluvial-bedrock system. Each study is based on a recharge mechanism that occurs within the Denver Basin aquifer system, a regionally significant water supply for which long-term pumping and active aquifer depletion call for improved characterizations of recharge. While recharge is the theme of this dissertation, I don't attempt to directly estimate recharge for the Denver Basin, but rather to investigate and expose dynamics of recharge that are essential for accurate conceptualizations and estimates of recharge. The first study investigates controls and timescales associated with streambed fluxes which are an important component of seepage recharge along mountain-front streams. Streambed fluxes are highly variable through time and space, having a range of implications for stream-aquifer processes. While spatial variations in streambed flux have been heavily characterized, temporal variability has been limited to short-term or low-frequency measurements. This study calculates high-frequency time series of Darcy-based streambed fluxes over a three-year period using water level and temperature inputs from shallow (<1.5m) nested streambed piezometers installed in two mountain-front streams in Colorado, USA. Results reveal important conclusions about controls and patterns of temporal variability. Three predominant temporal scales of variability, sub-daily (<1day), daily (>1d; <1y), and interannual (>1y), are quantified through statistical measures. Sub-daily variability was related to ET, temperature-induced changes in hydraulic conductivity, and variable stream stage while daily variability was highly seasonal and related to specific events on the channel (e.g., beaver dams). The magnitude of sub-daily variability was significant compared to daily variability (ratio 0.03 to 0.7). Annual median fluxes at each site varied across years, but typically remained consistent in order of magnitude and direction. A strong linear correlation characterizes the relationship between the daily variability and the median annual flux at individual sites, highlighting how sites with greater fluxes also exhibit greater temporal variability. The temporal flux variations documented in this study have important implications for calculations and interpretations of hyporheic exchange and groundwater recharge. Results provide a basis for quantifying temporal variations in streambed fluxes and highlight the extent to which fluxes vary over multiple timescales. Chapters 3 and 4 are organized to progress vertically downward within the system to investigate controls for inter-aquifer exchange between the alluvial and bedrock aquifer, an important component of recharge to the underlying bedrock aquifer system. In Chapter 3, the potential for and controls of hydraulic disconnection between the alluvial and bedrock aquifer are investigated. Hydraulic disconnection occurs when unsaturated conditions develop between a stream and water table causing seepage rates to stabilize with additional water table drawdown. In this study, I demonstrate that hydraulic disconnection can occur between an alluvial and bedrock aquifer when unsaturated conditions develop between the two water tables and inter-aquifer flow rates stabilize with subsequent drawdown. Variably saturated flow modeling is performed to simulate the effects of drawdown on alluvium to bedrock flow rates (A-B flow). Bedrock aquifer heterogeneity is represented through object-based geostatistical models that are conditioned to wellbore data from the Denver Basin aquifer system. The Monte Carlo framework includes 200 heterogeneity realizations across a range of sandstone fractions. Results document the formation of unsaturated regions beneath the alluvium in all models, particularly where sandstone channels underlie thinner low-permeability mudstones. Three-dimensional heterogeneity creates complex saturation patterns that result in localized flow paths, spatially varying disconnection, and a gradual transition to hydraulic disconnection as the regional water table is lowered. Successive changes in A-B flow decrease over the course of simulations by 80% to 99% and final rates approach stability as indicated by changes of <1% between successive stress periods. Of the 200 models, 190 reach full hydraulic disconnection and 10 conclude with a transitional flow regime. Dynamic connectivity metrics developed within the study strongly explain flow results. I also evaluate the aspects of heterogeneity that are most likely to produce disconnection, highlighting several factors that influence disconnection potential. Chapter 4 evaluates the potential for a beaver dam to drive flow across the alluvial-bedrock contact. Beavers construct dams which promote a range of surface and near-surface hydrologic processes, however, the potential for beavers to influence deeper aquifer dynamics is less often, if ever, considered. In this study I consider the potential for a beaver dam, specifically increased stream stage and width upstream of a dam, to drive deeper flow from an alluvial to bedrock aquifer. I utilize a numerical groundwater flow model to simulate the effects of the beaver dam on inter-aquifer exchange rates. The base case model is parameterized based on observations from a beaver dam constructed on Cherry Creek in 2020 and the stream-alluvial-bedrock aquifer sequence in the Denver Basin in previous chapters. I also test whether the influence of the beaver dam is sensitive to the alluvial-bedrock contact depth, beaver pond depth, and hydraulic properties by simulating flow across a range of sensitivity scenarios. Model results document an increase in alluvial to bedrock flow on the order of 0.5% to 4%, depending on the contact depth, beaver pond depth, and hydraulic properties. Changes in hydraulic head due to the dam propagate deep into the aquifer (>30m), highlighting the potential for deeper aquifer impacts. The effect of the beaver dam is greatest for shallow alluvial-bedrock contact depths, deeper pond depths, and lower hydraulic conductivity contrasts between the alluvial and bedrock aquifer. Overall, results document the potential for beavers to influence deeper aquifer fluxes where regional hydraulic gradients are downward, highlighting broader potential for beaver dams to enhance aquifer recharge in deeper aquifer settings.
Open Access
Evaluating subalpine lake delta carbon storage in the Colorado Front Range and Washington Central Cascades
(Colorado State University. Libraries, 2015) Scott, Daniel, author; Wohl, Ellen, advisor; Bledsoe, Brian, committee member; Rathburn, Sara, committee member
Mountainous regions are important contributors to the terrestrial organic carbon (OC) sink that affect global climate through the regulation of carbon-based greenhouse gases. However, mountain OC dynamics are poorly quantified. I sought to explore OC storage in subalpine lake deltas in the Washington Central Cascades and Colorado Front Range with the objectives of determining the magnitude of carbon storage and understanding the differences in storage between the two ranges. I used field, laboratory, and GIS techniques to determine the magnitude of and controls on the subaerial portion of the subalpine lake delta OC sink in 26 subalpine lake deltas, 14 in the Front Range and 12 in the Cascades. Soil moisture, texture, and delta valley confinement are significantly correlated with soil carbon on deltas. Average soil OC content on subalpine lake deltas ranges from 3 to 41%, and 140 to 1256 MgC/ha. Surprisingly, the carbon stocks of subalpine lake deltas are not significantly different between regions. I present a conceptual model that invokes basin-scale carbon dynamics to offer an explanation for how two regions with very different climate and tectonics have unexpectedly similar carbon stocks in their subalpine lake deltas. This conceptual model suggests that carbon is more likely to reach subalpine lake deltas from the upstream basin in the Colorado Front Range compared to the Washington Central Cascades. This points to a complex interaction among carbon production, transport, and stability in each region, and supports the idea that mountainous regions are complex carbon reactors.
Open Access
Examining geomorphic effects of flow diversions on low-gradient mountain streams in the Routt National Forest, Colorado
(Colorado State University. Libraries, 2012) Blaschak, Tyanna Schlom, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Bledsoe, Brian, committee member
The western United States is faced with an increasing human demand for water, coupled with a decreasing supply. Resource managers are looking for ways to meet the demands of both anthropogenic use and the needs of instream flows to maintain channel characteristics for water quality as well as riparian and aquatic ecosystems. In the Routt National Forest in northern Colorado, ditches typically divert flows from headwater streams to supply the land below the mountains for agricultural purposes. Many studies have focused on the biotic response to streamflow diversions, but relatively little research has been done to quantify the physical effects of ditch diversions. The purpose of this study was to contribute to the understanding of geomorphic effects of flow diversions in the Routt National Forest, and to inform management decisions related to water on the Routt by supplying localized data. Thirteen streams were surveyed during the summer of 2011, yielding 11 control reaches, located upstream of a diversion point, and 11 diverted reaches, which were downstream of a diversion point. Reach lengths were spaced approximately 20 times bankfull width. Four cross sections per reach were surveyed to collect width and depth information using reference discharge indicators approximating bankfull flow. Pebble counts of 100 clasts per reach were evenly spaced between riffles, and pools were avoided. Riparian vegetation, lithology, and valley characteristics were qualitatively and quantitatively assessed at the reach sites and using US Forest Service geospatial data. Statistical analyses conducted using the collected data included both t-tests and non-parametric Wilcoxon tests, as the small sample size limited the ability to reject assumptions of normality and conduct multivariate analyses. Univariate mixed-effects models were developed to compare reach response variables between diverted and control reaches while including the effects of unevenly-paired reaches, valley characteristics, lithology, and riparian vegetation. T-tests and Wilcoxon tests found only sinuosity to be significant, with the possibility of riparian vegetation types (willow or grass/sedge) having an effect on variables related to bank stability (width, depth). The mixed effects models found width, width-to-depth ratio, sinuosity, and cross-sectional area to be significant. Because the mixed models included the effects of valley characteristics, riparian vegetation types, lithology, and drainage basin size, these are considered to be more representative of the downstream response to flow diversions than the t-tests and Wilcoxon tests. This study provides some evidence for the downstream alteration of channels due to diversions. Two channels were noted to have been completely dewatered at the time of surveying in late July-to-August, and several variables were significantly different in statistical tests. For management purposes, it is recommended that high flows periodically enter diverted reaches to help offset the morphology and water quality effects of diversions during dry years. This study stresses the importance of further research to more accurately constrain and quantify physical effects of diversions.
Open Access
Experimental flume and numerical studies into the influence of floodplain vegetation on river-corridor hydrodynamic processes
(Colorado State University. Libraries, 2023) White, Daniel C., author; Morrison, Ryan, advisor; Nelson, Peter, advisor; Thornton, Chris, committee member; Rathburn, Sara, committee member
The active channel has historically been the primary focus of river hydrodynamic process studies and river engineering. However, increased global flood risk and awareness of ecosystem services provided by floodplains has encouraged managers to broaden their perspective beyond the banks. As water exits and reenters the channel during floods, water, nutrients, and sediment are exchanged with the floodplain. This flux is heavily influenced by both channel-floodplain hydrologic connectivity, or the ability of water to access the floodplain, and by floodplain land cover types. River and hydrologic modifications that result in disconnected floodplains include channel planform and cross-section geometry alterations, diversions and dams, levees, land cover change, and river sediment mining. As river managers, land-use managers, and landowners acknowledge the benefits of functional, laterally connected river corridors, more river restoration projects are undertaken with a primary goal of reconnecting a river channel to the adjacent floodplain. A major component of large river restoration and river engineering projects includes designing for and predicting future flow scenarios using hydraulic models and other analytical methods. Developing a hydraulic model for river restoration design relies on the theory and science of fluvial morphodynamic processes as well as the parameterization of hydraulic roughness coefficients. Because of the historical emphasis on in-channel processes, the scientific literature related to channel-floodplain hydrodynamics and floodplain roughness parameterization is sparse. Specifically, there are limited studies investigating the influence of vegetation on channel-floodplain exchange flow, lateral connectivity, and resulting channel topography. To address this knowledge gap, I conducted a series of physical and numerical modeling experiments where floodplain vegetation and flow parameters were varied. In Chapters 2 and 3, I present the results of flume experiments where I measured bedform topography and the flow field under varied floodplain vegetation conditions at two overbank flow depths. The experiments were conducted in a 1-m wide meandering compound channel inset in a 15.4-m long, 4.9-m wide basin. The channel bed was a mobile sand-and-gravel mixture with a median sediment size of 3.3 mm, and sediment transport occurred only within the channel. I tested bare and vegetated floodplain conditions with 2.7-cm diameter rigid emergent vegetation elements at spacings of 3.0 units m-2 and 12.1 units m-2. My observations of the flow field indicate that high density vegetation enhances secondary circular flow through the meander bend and reduces momentum exchange at the channel-floodplain interface. At a low relative depth, flow through high density vegetation was deflected away from the down-valley direction and forced to reenter the channel at a steep angle with respect to the channel centerline. However, at a high relative depth, dense vegetation steered in-channel surface flows more closely following the channel centerline. These observations shed light on the hydrodynamic processes leading to flood wave attenuation, enhanced nutrient cycling, and channel altering stresses, and these results may inform river restoration riparian management best practices. To investigate bedform response, I performed a moving-window analysis of topographic surface metrics including skewness, coefficient of variation, and standard deviation, as well as topographic patch analysis of area and contagion to measure changes in bedform heterogeneity as flow depth and vegetation density were varied. My results show that both greater density vegetation and larger flows can increase bedform topographic heterogeneity. These findings suggest that floodplain vegetation and natural hydrologic regimes that include overbank flows can enhance stream habitat complexity. Designing for the effects of established vegetation conditions and prioritizing floodplain vegetation planting may be useful for river managers striving to achieve successful biomic river restoration. Expanding on the observations made in the flume, I explored the ability of a 2D hydraulic model to predict the effects of vegetation on meandering channel flow dynamics. I used the TreeLS point cloud processing tool to automatically extract woody floodplain vegetation characteristics and estimate Manning's roughness coefficients for vegetation from aerial LiDAR. I investigated the influence of varied vegetation densities on channel-floodplain exchange flows in HEC-RAS 2D. I developed hydraulic models for three reaches along the Butokamabetsu River in the Hokkaido University Uryu Experimental Forest in Northern Japan where each reach had distinct biogeomorphic characteristics including channel width, slope, sinuosity, and floodplain vegetation density. I found that in the lower gradient, higher sinuosity reaches, floodplain vegetation density had more influence on channel-floodplain exchange flow attenuation. These results highlight the importance of planning for the presence and density of vegetation in river restoration projects particularly in lower gradient, more sinuous stretches of river. The results and analysis presented in this dissertation suggest that biological drivers such as rigid emergent floodplain vegetation play an important role in river form and function particularly in conjunction with floods that occasionally access the floodplain. These detailed observations of flow, sediment, and resulting bed morphology as well as analysis of innovative remote sensing techniques provide a basis for an improved understanding of morphodynamic processes in meandering rivers.
Open Access
Exploring new approaches to understanding channel width and erosion rates in bedrock rivers, Puerto Rico, USA
(Colorado State University. Libraries, 2022) Eidmann, Johanna Sophie, author; Gallen, Sean, advisor; Rathburn, Sara, committee member; Hughes, Kenneth Stephen, committee member; Ham, Jay, committee member
Earth system dynamics produce constant adjustments to sea level, tectonics, and climate. Bedrock rivers communicate these changes throughout mountains by driving landscape and erosional responses that facilitate topographic change. It follows that an improved understanding of bedrock rivers can help us better model and reconstruct the interplay of changes to base level, uplift, and climate from landscapes. Although bedrock channel width plays a first-order role in river stream power and stream power-based landscape evolution models, because of the physical challenges associated with acquiring these data, channel width is often estimated and introduces uncertainty. In addition, the lack of bedrock channel width data has limited our understanding of what factors control channel width. In this dissertation (Chapter 2), I leverage high-resolution topographic data, Mean Annual Precipitation information, and use the HEC-RAS river modeling software to remotely derive bedrock channel width at desired flow scenarios. The accuracy of modeling results is verified for rivers in Puerto Rico using USGS gauging station field measurements, as well as my own channel width field measurements associated with 1-year recurrence interval discharges. As a next step, (Chapter 3) I implement the bedrock width modeling method derived in Chapter 2 to obtain >4,000 channel width measurements from reaches across Puerto Rico. I then compare these bedrock river width values to various factors (e.g. rock type and rock strength, drainage area, Ecozone, and grain size) that have been identified in the literature to scale with or influence channel width. My analyses indicate that, in Puerto Rico, rock type is a dominant control of bedrock channel width in small (≤6-10 km2) drainage areas. Contrary to patterns of rock strength and bedrock width documented in the literature (e.g. Montgomery and Gran, 2001), I find that width doesn't appear to correlate with proxies for bedrock channel strength. Strong granodiorites have the widest low-order channels and the strong volcaniclastics and weak serpentinites have comparably narrow low-order channels. Analysis of limited grain size measurements shows a discernable difference in the coarse grain size distribution between the three rock types, with the volcaniclastic and serpentinite draining rivers having coarser sediment than granodiorite draining streams. These findings suggest that bedrock channel width may be influenced by unmeasured lithological parameters that impact the size of grains delivered to river channels from adjacent hillslopes (i.e. rock fracture density and spacing, as well as weathering). Lastly, (Chapter 4) I spatially analyze in-situ cosmogenic nuclide (10Be in quartz and 36Cl in magnetite) concentrations and find that bedrock erosion rates are higher in the central part of Puerto Rico than toward the east. Analysis of erosion rates compared to other parameters reveals that channel steepness, rather than precipitation or rock type, is positively associated with erosion rates. I further apply these erosion rate data to test the accuracy of four incision models of varying complexity. Model comparisons reveal that drainage area is a better predictor of incision rates in Puerto Rico than a precipitation-weighted drainage area parameter. In addition, whereas an increase in model complexity slightly improves model performance, the model only explains ~35% of the variability in erosion rates. It follows that current incision models are still missing many controlling factors of river incision rates in Puerto Rico.
Open Access
Field delineation of geomorphic process domains along river networks in the Colorado Front Range
(Colorado State University. Libraries, 2013) Livers, Bridget, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Bledsoe, Brian, committee member
Many of the conceptual models developed for river networks emphasize progressive downstream trends in morphology and processes. Such models are well-suited for larger, low-gradient rivers, but fall short in describing the extreme variability associated with headwater streams, which occupy the majority of length of stream networks, provide unique biological productivity and habitat, and can be sites of great sediment production. A more thorough understanding of the influence of local variability of process and form in headwater stream channels is required to remotely and accurately predict channel geometry characteristics for management purposes. Local variability of valley types and sediment production, or local process domains defined as glacial versus non-glacial valleys and levels of valley confinement, was evaluated for the Colorado Front Range by systematically following stream channels, categorizing them into stream type and process domain, and evaluating a number of channel geometry characteristics. The 111 reaches were then evaluated for significant differences in channel geometry among stream types and process domains, location and clustering of stream types on a slope-drainage area (S-A) plot, and downstream hydraulic geometry relationships. Statistical analyses revealed significant correlations between channel type and channel gradient, and channel type and substrate size. Although downstream hydraulic geometry relationships are well-defined using all reaches in the study area, reaches in glacial valleys display much more variability in channel geometry characteristics than reaches in fluvial valleys, as evidenced in larger ranges of channel geometry characteristics, greater difficulty in efficiently classifying stream types, less pronounced downstream hydraulic geometry relationships, and greater scatter of reaches on an S-A plot. Streams flowing through inherited terrain in glacial valleys continue to adjust to sediment and water dynamics, and level of confinement influences locations of certain stream types. Thus, local spatial variability associated with process domains at the reach scale (101-102 m) overrides progressive downstream relationships in mountain headwaters, and field calibration of relations between reach-scale channel gradient and channel characteristics is necessary to predict process and form of headwater streams in the Colorado Front Range.
Open Access
Floodplain organic carbon storage in the central Yukon River Basin, interior Alaska
(Colorado State University. Libraries, 2018) Lininger, Katherine Blom, author; Wohl, Ellen, advisor; Covino, Tim, committee member; Leisz, Stephen, committee member; Rathburn, Sara, committee member
River channels and floodplains transport, transform, deposit, and store organic carbon (OC) as active participants in the carbon cycle. Two of the largest stocks of OC in floodplains include soil and downed large wood (LW). This dissertation investigates floodplain OC stocks in LW and soil, and the geomorphic controls on soil OC stocks in the central Yukon River Basin in the Yukon Flats region of interior Alaska. The Yukon Flats region contains discontinuous permafrost, has a semiarid boreal climate, and has experienced little human modification. Almost all studies of floodplain OC have occurred in the temperate regions, despite permafrost regions storing large amounts of OC in the subsurface due to cold and wet conditions. In addition, relatively little is known about the geomorphic processes that control soil OC distribution on the landscape, particularly over large regions. Wood has been removed for navigation and infrastructure protection in many river corridors, and thus knowledge of natural wood loads, particularly on floodplains, is limited. I first present floodplain downed large wood measurements for the Yukon Flats region, and compare those measurements to downed wood loads in unaltered floodplains in two additional biomes, the subtropical lowlands and the semiarid temperate mountains. Average volumes of downed LW are 42 m3ha-1, 50 m3ha-1, and 116 m3ha-1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. I find patterns in LW loads reflect climatic controls, such as decay rate and primary productivity, as well as increases in floodplain downed wood loads with recent disturbances such as fire. Next, I assess the geomorphic controls on floodplain soil OC concentrations along the Yukon River and four of its tributaries using a large dataset of floodplain soil samples, finding that river basin characteristics and geomorphic unit characteristics likely influence the spatial distribution of soil OC on the landscape. Average OC concentration within floodplain soil is 2.8% (median = 2.2%). Most floodplain soil OC likely comes from riparian vegetation, which is influenced by channel migration rates and the development of geomorphic units within the floodplain. Greater variability in OC concentrations among geomorphic units compared to among river basins indicates that a bottom-up approach to estimating OC on the landscape (scaling up from small-scale landscape units) may be necessary. Finally, I estimate the soil OC stock in the floodplains of the Yukon Flats and find that my estimate results in approximately an 80% increase in OC stock when compared to a previously published database. The residence time of floodplain sediment is constrained using radiocarbon dates taken from cutbanks, and indicates that OC may be stored in floodplains for over 7000 years before being eroded by the channel. This dissertation provides much needed information on the geomorphic controls on floodplain OC storage in permafrost regions, which are undergoing relatively rapid warming due to anthropogenic climate change. In addition, it highlights the importance of accounting for floodplains as unique landscape units and mediators of OC fluxes, water, and nutrients.
Open Access
Floodwave and sediment transport assessment along the Doce River after the Fundão Tailings Dam collapse (Brazil)
(Colorado State University. Libraries, 2019) Palu, Marcos Cristiano, author; Julien, Pierre, advisor; Thornton, Christopher, committee member; Ettema, Robert, committee member; Rathburn, Sara, committee member
The collapse of the Fundão Tailings Dam in November 2015 spilled 32 Mm3 of mine waste, causing a substantial socio-economic and environmental damage within the Doce River basin in Brazil. Approximately 90% of the spilled volume deposited over 118 km downstream of Fundão Dam on floodplains. Nevertheless, high concentration of suspended sediment (≈ 400,000 mg/l) reached the Doce River, where the floodwave and sediment wave traveled at different velocities over 550 km to the Atlantic Ocean. The one-dimensional advection-dispersion equation with sediment settling was solved to determine, for tailing sediment, the longitudinal dispersion coefficient and the settling rate along the river and in the reservoirs (Baguari, Aimorés and Mascarenhas). The values found for the longitudinal dispersion coefficient ranged from 30 to 120 m2/s, which are consistent with those in the literature. Moreover, the sediment settling rate along the whole extension of the river corresponds to the deposition of finer material stored in Fundão Dam, which particle size ranged from 1.1 to 2 μm. The simulation of the flashy hydrographs on the Doce River after the dam collapse was initially carried out with several widespread one-dimensional flood routing methods, including the Modified Puls, Muskingum-Cunge, Preissmann, Crank Nicolson and QUICKEST. All of these methods presented unsatisfactory results, with prediction errors in peak discharge up to 44%, and differences in timing to peak up to 5 hours. A new and more accurate one-dimensional flood routing approach was then used, solving the full dynamic equation into an equivalent diffusive wave format and reformulating the hydraulic diffusion coefficient in terms of the Froude number and floodwave celerity. The numerical solution to this new approach was implemented using Crank Nicolson and QUICKEST schemes. The error in predicted peak discharge along the Doce River was reduced to 2%, and the maximum difference found in time to peak was about 1 hour. Regarding sediment transport, a comprehensive one-dimensional numerical model is developed, coupling the new floodwave propagation algorithm with the numerical solution for advective sediment transport and settling. One of the main features of this model is the ability to simulate the propagation of the floodwave and sediment through the entire Doce River extension with or without reservoirs. A sensitivity analysis showed that a hypothetical decrease in water temperature from 30°C to 5°C would have resulted in a concentration 13 times higher at the outlet. In addition, without the presence of hydropower reservoirs on the Doce River, the sediment concentration at the basin outlet would have been 70,000 mg/l instead of the observed 1,600 mg/l. Finally, a simplified numerical model based on the Doce River measurements can simulate the hypothetical collapse of 56 tailings dams in the Doce River basin to estimate the potential impact on the water supply for the towns along the river. Those simulation results show that tailings dams located in the Piracicaba basin, a Doce River sub-basin, have the highest potential to adversely impact the water supply of the downstream towns due the volume stored and proximity with populated towns. Ultimately, the collapse of the biggest dams in this sub-basin could affect approximately 1,000,000 people for several days.
Open Access
Flow resistance corrections for physical models using unit flowrates
(Colorado State University. Libraries, 2024) Cote, Cassidy B., author; Thornton, Christopher, advisor; Ettema, Robert, committee member; Rathburn, Sara, committee member
Flow resistance is an essential aspect of evaluating flow behavior in open-channel hydraulic models. Flow resistance in open channels is commonly characterized by Manning's resistance equation, where a value of Manning's roughness coefficient n, indicates the magnitude of flow resistance. Physical hydraulic models are one method to estimate Manning's n values for prototype channel reaches. A physical hydraulic model evaluates prototype channel characteristics at the model scale. The scale for a given physical model may be characterized by length-scale factor, given by the relationship of prototype to model geometry. Models that have a large length-scale factor are known to introduce errors associated with instrumentation, measurement, and scale effects, therefore minimization of the length-scale factor is an important consideration in the development of hydraulic models. Evaluating physical models using a scaled unit flowrate provides a method by which the length-scale factor may be minimized. In this way, a scaled design discharge per unit width of channel is applied to a channel that is less wide than the prototype design. Using this approach greatly improves the ability of laboratories to utilize available facilities, without being constrained by prototype design width, which can otherwise be a driving factor increasing the length-scale factor for a given model. This thesis documents the construction and analysis of two physical models of a proposed rectangular canal along Rio Puerto Nuevo in San Juan, Puerto Rico. One model used a scaled unit flowrate and a reduced channel width at a lesser length-scale factor, and the other model accommodated the total scaled design flowrate and design channel width at a larger-scale factor. Tests were conducted for three sidewall conditions to identify the impact associated with applying a unit flowrate physical modeling approach for models with different Manning's n values specific to the sidewalls. The unit flowrate approach was found to result in larger estimates of flow depth and composite Manning's n compared to the model that accommodated the full prototype channel width. Insights regarding the variability of Manning's n as a function of channel width for each sidewall condition were identified by comparing results from the two models. A correction method was proposed for improving estimates of Manning's n derived from scaled unit flowrate models. Correction factors were identified as a function of two dimensionless parameters, relative prototype channel width (defined as the ratio of the width evaluated using a unit flowrate model to the design width of the channel), and relative flow resistance exerted by the individual boundary elements as determined from the unit flow rate model (defined as the ratio of Manning's n values between the sidewall and channel bed boundary elements). Findings indicate that it becomes increasingly important to apply correction factors to flow resistance estimates on unit flowrate models when wall boundary elements exert a larger contribution to flow resistance than that of the channel bed (large relative roughness), and when the scaled unit flowrate approach results in a prototype channel width that is significantly smaller than the proposed design channel width (small relative channel width). Correction factors were developed for a range of relative channel width values from approximately 0.4 to 1.0, and a range of relative roughness values from approximately 0.5 to 3.0. Future physical models using unit flowrates with relative channel widths and relative flow resistance within the range evaluated may use the presented correction methods to improve estimates of flow resistance.
Open Access
Flow, sediment transport, and bed topography in straight and curved gravel-bed channels
(Colorado State University. Libraries, 2016) Hanson, Tessa Catherine, author; Nelson, Peter A., advisor; Bledsoe, Brian, committee member; Rathburn, Sara, committee member
In recent years, many river restoration projects have aimed to restore natural channel stability and dynamism by re-establishing channel meanders lost to historical channelization. An understanding of meandering channel behavior is crucial to successful restoration of these rivers. Meandering and straight channels differ greatly in terms of sediment transport, velocity, and flow patterns under equilibrium conditions. The primary objective of this study was to investigate the mechanisms responsible for sorting patterns in mixed-grain straight and curved channels using flume experiments. After an absence of sorting was observed in the flume experiments, the study objective was modified to: 1) investigate the formation, behavior, and dynamics of free and forced bars within a straight channel with and without an upstream barrier and 2) explore the mechanism that accommodates for spatial boundary shear stress variations in curved gravel bed channels. The flume experiments involved detailed measurements of bed topography, velocity, and sediment transport in both a curved channel and straight channel with and without an upstream obstruction. It was expected that the gravel bed meandering river would compensate for spatial variability in boundary shear stress through surface grain size adjustment (sorting), as opposed to sediment transport convergence. Instead, the data reveal sediment transport divergence as the primary mechanism for balancing shear stress variability. The lack of sorting may likely be attributed to low excess shear stress and steady, rather than unsteady flow conditions. Regarding free and forced bar behavior, no stability was achieved in the straight channel without an obstruction. This can be attributed to a range of factors related to upstream boundary conditions, shear stress, and lack of forcing topography. It is suggested that future studies utilize both higher excess shear stress and unsteady flow conditions in investigating shear stress variability in curved gravel-bed channels.
Open Access
Form and function: quantifying geomorphic heterogeneity and drivers in dryland non-perennial river corridors
(Colorado State University. Libraries, 2023) Scamardo, Julianne E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
Non-perennial rivers, including intermittent rivers and ephemeral streams, comprise the majority of drainage networks globally. However, ephemeral streams remain understudied compared to perennial counterparts, and the majority of extant studies focus on in-channel dynamics. Floodplains along perennial streams are known to host a high density of ecosystem functions, including the attenuation of downstream fluxes and provision of habitat to diverse flora and fauna. These functions are thought to be correlated to geomorphic heterogeneity, and studies of floodplain heterogeneity are emerging on perennial rivers. Here, I extend the conceptualization of floodplain function and heterogeneity commonly focused in perennial watersheds to dryland, ephemeral streams. Based on a synthesis of current literature identifying ephemeral stream floodplain characteristics in drylands, a set of floodplain styles emerge dependent on confinement and the presence of channelized flow. Functions related to attenuation and storage are typically concentrated in unconfined and channeled floodplains. The temporary storage of sediment and sub-surface water in ephemeral stream floodplains make them hotspots for biogeochemical cycling and hosts to richer, denser, and more diverse vegetation communities compared to surrounding uplands. Many functions of ephemeral stream floodplains are also found in perennial counterparts, but flashy flow regimes and high sediment loads in ephemeral streams can potentially impact rates and magnitudes of comparable processes and functions. Similar to perennial rivers, the diverse physical and ecological functions in ephemeral stream floodplains are thought to be related to spatial geomorphic heterogeneity. Although studies on the characteristics and drivers of geomorphic heterogeneity exist for perennial streams, similar studies in ephemeral streams are lacking. Geomorphic heterogeneity was therefore quantified along with potential drivers – including metrics related to geomorphic context and proxies for flood disturbance – to understand underlying processes in ephemeral river corridors. Geomorphic units were mapped in 30 unconfined river corridors within six non-perennial watersheds in Utah and Arizona, U.S. Landscape heterogeneity metrics – Shannon's Diversity Index, Shannon's Evenness Index, and patch density – were used to quantify geomorphic heterogeneity within each reach. Additionally, variables that potentially constrain or drive heterogeneity were quantified, including floodplain shape, grain size, large wood abundance, channel change and sediment storage times. Although heterogeneity positively correlated with metrics for morphology and disturbance (i.e., channel change and storage), statistical models suggest that morphologic context, particularly floodplain width, was a more important predictor for estimating geomorphic heterogeneity. Still, geomorphic units reflected aggradation processes indicative of a range of flood energies, suggesting a strong tie between heterogeneity and disturbance. Results suggest that non-perennial rivers with greater geomorphic heterogeneity may be resilient to changes in flood disturbance frequency or magnitude, but future studies investigating long-term temporal heterogeneity are needed. The lack of direct flux observations could also be restricting insight into how floods interact with large wood and vegetation, which are known to have complex relationships with geomorphic heterogeneity in perennial rivers. In the absence of flood observations, a hydro-morphodynamic model was developed to investigate changes to channel and floodplain morphology due to wood and vegetation in an ephemeral river corridor in southeastern Arizona, U.S. Three scenarios were modeled: the actual configuration of the river corridor; an experiment in which jams were removed; and an experiment in which vegetation was removed. Both large wood and vegetation effectively confined flow to the main, unvegetated channel, which became wider and deeper over the course of a single moderate flood. When isolating the impact of large wood, model results show that wood increases the magnitude of channel change created by vegetation, resulting in ±0.1 to 0.3 m of additional scour or aggradation. The simulated removal of vegetation resulted in more channel change than the removal of wood alone, partially because vegetation occupies a much greater area within the stream corridor than large wood. I propose a conceptual framework in which large wood could mediate sedimentation as well as the recruitment and growth of vegetation in ephemeral streams, contributing to the evolution of ephemeral stream morphology over time. Due to the ubiquity of dryland ephemeral streams, results of this research have the potential to influence watershed management globally. Wide, unconfined ephemeral stream floodplains and riparian forests could be targets for protection and restoration similar to current efforts in perennial rivers. Particularly in the context of future climate and land use changes, understanding the natural character, function, and heterogeneity of ephemeral stream floodplains highlights their physical and ecological importance in dryland landscapes.
Open Access
Geomorphic analysis of the Middle Rio Grande - Elephant Butte Reach, New Mexico
(Colorado State University. Libraries, 2012) Owen, Tracy Elizabeth, author; Julien, Pierre, advisor; Thornton, Christopher, committee member; Rathburn, Sara, committee member
The Elephant Butte Reach spans about 30 miles, beginning from the South Boundary of the Bosque del Apache National Wildlife Refuge (River Mile 73.9) to the "narrows" of the Elephant Butte Reservoir (River Mile 44.65), in central New Mexico. Sediment plugs occasionally form along the Middle Rio Grande, completely blocking the main channel of the river. In 1991, 1995, and 2005, the Tiffany Plug was initiated at the upstream end of the Elephant Butte Reach. In 2008, the Bosque del Apache Plug formed just upstream of the Elephant Butte Reach. Sediment plugs occur at the location of a constriction or channel aggradation (Burroughs 2011). As aggradation within the Elephant Butte Reach is known to contribute to a decrease in channel capacity (Reclamation 2007), it is important to understand the influences of Elephant Butte Reservoir levels on channel aggradation/degradation in order to decrease the potential for future sediment plug formation. Further understanding of the historical and spatial changes within Elephant Butte Reach, along with a better understanding of the influences of Elephant Butte Reservoir levels on channel aggradation/degradation, are essential for improvement in future river management practices along the Middle Rio Grande. Using aerial photographs, survey data, reservoir water surface elevation data, and bed material data, the following objectives are addressed in this study: 1. Quantify temporal changes in channel widths and sinuosity from 1935 to 2010. 2. Quantify change in channel slope temporally. 3. Quantify rate of aggradation/degradation in response to a change in base-level (i.e., change in reservoir water surface elevation). 4. Quantify aggradation/degradation wave propagation upstream. 5. Quantify spatial and temporal trends in bed material grain size. From 1935 to 2010, channel widths and sinuosity decrease over time. The majority of the Reach's channel slope decreases from 1935 to 2010; the downstream-most stretch of the channel, closest to Elephant Butte Reservoir, alternates between increasing and decreasing channel slopes. As the Elephant Butte Reservoir level (base-level) increases, the channel aggrades in response. As the base-level decreases, the channel degrades. The rates of aggradation and degradation vary between different periods of base-level changes, and are quantified within the report. When the base-level changes a wave of aggradation/degradation travels upstream. The rate of wave propagation upstream varies relative to the rate of base-level change, and is quantified within the report for four sets of aggradation/degradation waves. Bed material samples obtained from cross-section surveys and at the San Acacia and San Marcial gauges showed a coarsening at a rate of about 0.03 mm/year. In the downstream direction, bed material became slightly finer. The median bed material grain size ranged from 0.11 mm to 0.26 mm.