Mountain Scholar :: Browsing by Author "Wohl, Ellen, advisor"

Browsing by Author "Wohl, Ellen, advisor"

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Open Access
A catchment is more than the sum of its reaches: post-fire resilience at multiple spatial scales
(Colorado State University. Libraries, 2024) Triantafillou, Shayla P., author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Morrison, Ryan, committee member
As wildfires are projected to increase in frequency and severity, there is a growing interest in understanding river resilience to the wildfire disturbance cascade. Numerous 3rd-order mountain catchments within the Cache la Poudre (Poudre) River basin in the Colorado Front Range, USA burned severely and extensively during the 2020 Cameron Peak fire. Many of these catchments experienced debris flows and flash floods triggered by convective storms after the fire. The downstream effects of the debris flow sediment varied along a continuum from attenuated and largely contained within the catchment, through contributing to a pre-existing debris fan at the catchment outlet, to releasing substantial volumes of water and sediment to the Poudre River. I conceptualize these catchments as exhibiting decreasing resilience to post-fire disturbance along the continuum described above based on the geomorphic evidence of relative sediment export. The characteristics affecting resilience and magnitude of response to disturbance span multiple spatial scales from the catchment to stream corridor reaches hundreds of meters in length. I conceptualize characteristics on different spatial scales as driving or resisting response to disturbance and therefore impacting the resilience outcome of the catchment. As the magnitude of resisting characteristics increases at the catchment, inter- and intra- reach scales, I hypothesize that a catchment will be more resilient to the wildfire disturbance cascade. At the catchment scale I consider geomorphic, burn, vegetation, and precipitation characteristics. I conducted longitudinally continuous surveys to measure reach-scale characteristics within each study catchment. I focus on the reach-scale geomorphic, vegetation, and burn characteristics, with a particular focus on elements that introduce inter- and intra-reach spatial heterogeneity including channel planform, beaver-modified topography, the distribution of channel and floodplain logjam distribution density, and the floodplain width/channel width ratio for the population of reaches within each catchment. The floods observed at the study catchments illustrate fire lifting the elevation above which rainfall-induced flooding occurs due to the efficient conveyance of water from hillslopes to channels after wildfire. Results suggest that inter- and intra-reach spatial heterogeneity are better descriptors of resilience than catchment-scale characteristics: resilience is associated with greater longitudinal variations in floodplain/channel width and more reaches with wide floodplains, low channel gradients, beaver-modified topography, and multi-stem deciduous vegetation.
Open Access
A debris flow chronology and analysis of controls on debris flow occurrence in the Upper Colorado River valley, Rocky Mountain National Park, CO
(Colorado State University. Libraries, 2012) Grimsley, Kyle J., author; Rathburn, Sara, advisor; Wohl, Ellen, advisor; Bledsoe, Brian, committee member
The role of debris flows along the Upper Colorado River was recently highlighted when the Grand Ditch, a 19th-century water-conveyance ditch, overtopped from snowmelt in 2003 and triggered a large debris flow along Lulu Creek, a tributary of the Colorado. Historical aerial photographs indicate that at least two other debris flows have been triggered from the Grand Ditch over the last century. This study examines the natural regime of debris flows in the Colorado River headwaters to assess whether the Grand Ditch has increased magnitude and frequency of debris flow occurrence on the west side of the Colorado River valley. Ten distinct sites of debris flow deposition were mapped using aerial photographs and field exploration, dated from tree cores and tree scars, and analyzed for magnitude using field-estimated volumes of deposition. Six of these ten depositional sites are on the west side of the valley, and several of them have evidence of multiple debris flows. Forty scarred survivor trees and 38 cores from even-aged stands were dated, with corresponding dates of debris flow occurrence ranging from 1923 to 2003. At least 19 debris flows have occurred in this catchment over the last century, but only those at the across-from-Specimen Creek, Lady Creek, Lulu Creek, and Little Yellow sites appear to have been large enough to affect the Colorado River. There is not a substantial difference in the frequency of total debris flows catalogued at the ten sites of deposition between the east (8) and west (11) sides of the Colorado River valley over the last century, but three of the four largest debris flows originated on the west side of the valley in association with the Grand Ditch, while the fourth is on a steep hillslope of hydrothermally altered rock on the east side of the valley. Although ability to interpret the debris flow record is limited by frequent disturbance and burial of older deposits, and estimates of magnitude have high uncertainty, these data suggest that the Grand Ditch has altered the natural regime of debris flow activity in the Colorado River headwaters by increasing the frequency of debris flows large enough to reach the Colorado River. Likelihood of debris flow occurrence is augmented by steep slopes and hydrothermally altered rock, which are both common in the vicinity of the Grand Ditch. This study demonstrates the applicability of dendrochronology for dating geomorphic events in Rocky Mountain National Park and provides context for restoration following debris flows.
Open Access
A geomorphic classification of ephemeral streams in arid regions
(Colorado State University. Libraries, 2013) Sutfin, Nicholas A., author; Wohl, Ellen, advisor; Bledsoe, Brian, committee member; Kampf, Stephanie, committee member
Current stream classifications do not adequately describe ephemeral streams in arid regions because these environments are characterized by high spatial and temporal variability of complex hydrologic interactions. To investigate the influence of channel form on riparian vegetation in the arid southwestern United States, I test a geomorphic classification for ephemeral streams based on the degree of confinement and the composition of confining material. I present five stream types: 1) bedrock channels entirely confined by exposed bedrock and void of persistent alluvium; 2) bedrock with alluvium channels at least partially confined by bedrock but containing enough alluvium to create bedforms that persist through time; 3) incised alluvium channels bound only by unconsolidated alluvial material into which they are incised; 4) braided washes that exhibit multi-thread, braided characteristics regardless of the degree and composition of confining material; and 5) piedmont headwater 0-2nd order streams confined only by unconsolidated alluvium and which initiate as secondary channels on piedmont surfaces. The objectives of this thesis were to i) validate distinct differences of channel geometry among the five stream types and ii) examine localized differences in geometry of the five stream types across watersheds with varying characteristics. Eighty-six study reaches were surveyed on the U.S. Yuma Army Proving Ground (YPG) and eighteen study reaches on Barry Goldwater Air Force Range (BMGR) in southwestern Arizona. Non-parametric permutational multivariate analysis of variance (PERMANOVA) for all 101 study reaches indicates significant differences (P<0.001) in channel geometry between the five stream types with regard to width-to-depth ratio, stream gradient, shear stress, and unit stream power. PERMANOVA results indicate no significant differences in channel geometry of individual stream types within watersheds of differing characteristics. A linear discriminant function of the four physical driving variables derived from 86 study reaches at YPG predict stream type with a 73% external hit rate for the 15 study reaches at BMGR. Classification and regression tree (CART) analysis identify thresholds for distinguishing stream types and indicates the relative importance of variables such that: width-to-depth ratio (W/D) correctly distinguishes 93.8% of braided channels (W/D > 91.2), shear stress (τ) correctly distinguishes 95.2% of bedrock channels (τ > 151.6 Pa), and unit stream power (ω) correctly distinguishes 68% of piedmont headwater channels (ω ≤ 35.63 W/m2). The resulting classification will provide a basis for examining relationships between channel characteristics, hydrologic process, riparian vegetation and ecosystem sensitivity of ephemeral streams in arid regions of the American Southwest.
Open Access
An investigation into beaver-induced Holocene sedimentation using ground penetrating radar and seismic refraction: Beaver Meadows, Rocky Mountain National Park
(Colorado State University. Libraries, 2011) Kramer, Natalie, author; Wohl, Ellen, advisor; Harry, Dennis, advisor; Meiman, Paul, committee member
This study used ~1 km of near-surface seismic refraction (SSR) data and ~6 km of ground penetrating radar (GPR) data to infer the impact of Holocene beaver activity on sediment aggradation in Beaver Meadows, Rocky Mountain National Park. GPR data were used to uniquely identify radar packages of genetically related strata corresponding to glacial, non-glacial, and beaver-induced sedimentation. The radar package relating to glacial deposition was wedge-shaped and predominantly composed of a diffraction-rich, chaotic facie. The radar package relating to alluvium was draped over the glacial deposits and was characterized by multiple facies, but in general contained complex, slightly continuous reflectors interfingered with continuous, horizontal to subhorizontal reflectors. The radar package related to beaver-induced sedimentation was characterized by a laterally continuous parallel facie, interpreted to be ponded sediment, that abruptly truncated into a zone of chaotic reflectors, interpreted to be a beaver dam. In order to determine the relative magnitude of post-glacial deposition, the bedrock contact was determined using seismic refraction, GPR, auger data, and previous seismic and drilling studies. This study concludes that beaver damming is an important aggradational process trapping sediments within the Holocene, but did not cause significant amounts of aggradation. Beaver-induced sediments constituted ~50 percent of the alluvium, but the alluvium only constituted ~15 percent of the total valley fill, with a mean depth of ~1.3 m. The thin veneer of Holocene sedimentation implies that Beaver Meadows is not a site of continuous sediment deposition, but rather one characterized by episodic temporal and spatial aggradation, punctuated by incision, re-working and exhumation.
Open Access
Assessing channel change and bank stability downstream of a dam, Wyoming
(Colorado State University. Libraries, 2011) Gilliam, Elizabeth Ann, author; Wohl, Ellen, advisor; Cenderelli, Dan, committee member; Bledsoe, Brian, committee member
The Hog Park Creek watershed, in south-central Wyoming, has experienced several anthropogenic influences through time, the most notable in contemporary times being a reservoir in the upper extent of the watershed that was initially constructed in 1965 (Stage 1) and then later enlarged in 1985 (Stage 2). Flows released from the reservoir augment flows in Hog Park Creek. The existence of the channel-spanning dam creates a direct and identifiable disruption in the function of the two main drivers of geomorphic process: the water discharge, which has nearly doubled annually, and the concomitant disruption in the sediment transport regime. In order to assess channel responses, multiple analyses across a range of spatial and temporal scales were conducted. These include: a covariate hydrologic analysis relating three operational time periods using the Index of Hydrologic Alteration (IHA) software; an examination of the channel planform change from historical aerial photographs; analyses of annually repeated cross section survey data; and a study of bank erosion dynamics using the Bank Stability and Toe Erosion Model (BSTEM). The timing, magnitude and duration of flows have been altered since the Stage 1 implementation of the reservoir in 1965. Following a Stage 2 enlargement in 1985, the snowmelt-dominated hydrograph has most notably experienced a shift to bimodal high flows (an early spring, low-magnitude flow release from the reservoir and a late spring, high-magnitude flow release from the reservoir), a 550% increase in seasonal low flows, and a 10% reduction in peak discharges. The discharge historically corresponding to a 5-year recurrence interval now occurs annually under Stage 2 reservoir operations. Hence, formational flows for channel morphology have increased in both frequency and duration. The reduction in flow variability has ultimately altered the sediment transport regime, which is the base of the productivity and disturbance regimes that influence food web interactions, the composition of riparian vegetation and other ecological attributes of the pre-dam river ecosystem. Aerial photographic analysis of 29 years prior to and 36 years following the construction of the dam indicates an adjustment of channel width both temporally and spatially through the system. Statistical analyses suggest that the overall rate of change corresponds significantly to both location in the watershed (distance downstream of the reservoir) and the operation of the reservoir (volume, timing, and duration of water released). Most notably, the channel has shifted to a single-thread channel with reduced morphologic heterogeneity. Responses are most abrupt immediately downstream of the dam following its construction in 1965, whereas responses are more muted and delayed in the furthest downstream study reach. Cross section analyses indicate that each of the four study sites has experienced net erosion over the past five years. However, variation exists in erosion rates on the reach and site scales. Modeled erosion rates in BSTEM, corroborated with field data from bank erosion pins and repeated cross section surveys, suggest that the altered flow regime enhances bank erosion. The enhanced duration of high flows directly lead to increased amounts of toe scour. Flow regulation has changed the forces acting on the banks, including subsurface flow fluxes related to water level fluctuations and increased shear forces. This in turn has created hydraulic conditions that increase preferential erosion of the finer bank materials. However, this response is partially offset as channel geometry changes with width increase relative to depth, which alters the shear stress acting on the banks.
Open Access
Beyond the case study: characterizing natural floodplain heterogeneity in the United States
(Colorado State University. Libraries, 2023) Iskin, Emily Paige, author; Wohl, Ellen, advisor; Morrison, Ryan, committee member; McGrath, Daniel, committee member; Ronayne, Michael, committee member
With human degradation of natural river corridors, the number of natural, functional floodplains is rapidly decreasing due to dams, diversions, artificial levees, draining, development, agriculture, and invasive species. At the same time, small- to large-scale interest in and implementation of river restoration is expanding, with floodplain restoration soon to take a starring role. To properly manage and restore processes to floodplains, we first need a broad understanding of what they look like and why. A key component of natural river-floodplain systems is heterogeneity, defined as the spatial variation of geomorphic and vegetation classes and patches across a floodplain. Heterogeneity of floodplains both reflects and influences the fluvial processes acting on floodplains and can help shape our understanding of the form and function of floodplains. To begin characterizing floodplain spatial heterogeneity, I present in this dissertation: 1) the development of a method to combine field measurements and remote sensing data products to calculate integrative landscape-scale metrics of floodplain spatial heterogeneity, and the demonstration of which metrics from landscape ecology are likely to be useful for identifying qualities of natural floodplains at four case study sites; 2) a sensitivity analysis to determine whether and how the values of the heterogeneity metrics change when spatial and spectral resolution of the input data are increased, and the extraction of underlying data from the classification results to determine whether using higher resolution data allows identification of the resulting unsupervised classes in relation to field and remote data at four case study sites; and 3) quantification of floodplain spatial heterogeneity, evaluation of whether statistically significant patterns are present, and interpretation of the statistical analyses with respect to the influence of channel lateral mobility and valley-floor space available using a complete dataset of 15 sites representing diverse floodplains across the continental United States. I found that "stacking" Sentinel-2A multispectral satellite imagery and digital elevation model topographic data allows for unsupervised classification of floodplains, and that metrics from landscape ecology can differentiate between different floodplain types. I also found via a sensitivity analysis that increasing the spatial resolution of the topographic data to finer than 10 m and including band ratios related to vegetation improves the classification results. Comparison of the field classes with the remote sensing classes allows for general interpretation of the results, but it is the heterogeneity within the broad classes that I expect is most important to these ecosystems. Lastly, through classification of 15 diverse river corridors across the United States, calculation of five heterogeneity metrics, and completion of a comparative analysis, I found that these natural floodplains have moderate aggregation of classes (median aggregation index = 58.8%), high evenness (median Shannon's evenness index = 0.934) and intermixing of classes (median interspersion and juxtaposition index = 74.9%), and a wide range of patch densities (range of patch density = 491–1866 patches/100 ha). I also found that the river corridor characteristics of drainage area, floodplain width ratio (space available), and elevation, precipitation, total sinuosity, large wood volume, planform, and flow regime (channel mobility) emerge as important variables to understanding floodplain heterogeneity.
Open Access
Biotic controls on post-glacial floodplain dynamics in the Colorado Front Range
(Colorado State University. Libraries, 2011) Pilgrim, Lina Eleonor Polvi, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Merritt, David, committee member; Bledsoe, Brian, committee member
A recent surge in ecogeomorphic research has shed light on the numerous feedbacks and couplings between physical and biotic processes in developing geomorphic and ecologic process and form. Recent work has shown the critical importance of vegetation in altering overall channel form and developing meandering channel systems. This dissertation expands on planform classifications and the understanding of biotic-physical couplings through examining two components of post-glacial floodplain evolution in broad headwater valleys in the Colorado Front Range. First, I evaluate the role of beaver in Holocene floodplain evolution in low-gradient, broad headwater valleys to understand the historical range of variability of sedimentation processes and to determine the role of beaver in altering channel complexity and how that contributes to spatial heterogeneity of sedimentation processes. These objectives were carried out in Beaver Meadows and Moraine Park in Rocky Mountain National Park through analysis of subsurface sediment, geomorphic mapping, and aerial photography analyses. Second, I examine the role of various riparian species in stabilizing streambanks in order to determine the relative importance of bank versus root characteristics in stabilizing streambanks and to develop a functional classification of riparian vegetation in stabilizing streambanks. Data for this portion of the project were collected in three study sites along an elevation gradient in the Colorado Front Range: Phantom Canyon on the North Fork Poudre River (1920 m), North Joe Wright Creek (3000 m), and Corral Creek (3100 m), all of which are located in the Cache la Poudre drainage. For fourteen species (4 trees, 3 shrubs, 3 graminoids, and 4 herbs), root tensile strength, root size distribution, and root morphology were characterized. Streambank geometry and stratigraphy from Moraine Park were combined with vegetation characteristics in a physically-based bank stability model to determine the role of various physical bank characteristics and root characteristics in stabilizing streambanks. Examination of Holocene sedimentation processes in these broad, low-gradient headwater valleys, which are fairly disconnected from their hillslopes, lends support to the beaver-meadow complex hypothesis that uses beaver dams as the mechanism to explain the accumulation of fine sediment in glacial valleys. In the study valleys, sediment associated with beaver dams account for a significant (30-50%) portion of the relatively thin alluvium overlaying glacial till and outwash. Sedimentation rates were temporally and spatially heterogeneous across the floodplain, with higher rates associated with beaver pond sedimentation. Fluvial complexity, in terms of multi-thread channels, islands, and channel bifurcations, increases with beaver populations and number of ponds, and magnifies the potential for beaver damming because of increased channel length, which accelerates the development of fluvial complexity and valley sedimentation. Bank stability modeling determined that although bank and root characteristics are interrelated, physical bank characteristics play a larger role in determining bank stability than root characteristics. However, within similar streambank types, vegetation type is a strong predictor of overall streambank stability, and streambanks without vegetation were consistently the least stable. The presence of rhizomes, the maximum root diameter, the root tensile strength, and the lateral root extent of each species are the most important root characteristics in determining streambank stability. Riparian shrubs (willows) and riparian trees are the best streambank stabilizers. Upland trees and graminoids are mid-level bank stabilizers, and herbaceous species are mid/low-level bank stabilizers. In addition to sediment and flow regimes, the two biotic processes studied interact to form the overall channel planforms that dominate these broad headwater valleys. Assuming a relatively snowmelt-dominated flow regime and a gravel-bed channel system in the headwaters, four planform regimes are identified based on low to high beaver populations and the abundance and presence of xeric or riparian vegetation. Without beaver or bank-stabilizing vegetation, a braided channel planform will likely develop. With bank stabilizing vegetation but without a sustainable beaver population, a single-thread meandering channel will form, which only has a thin riparian vegetation strip and small fluvial influence on the overall valley ecological and geomorphic processes. With a sustainable beaver population and riparian vegetation along the streambank, a stable multi-thread channel system will form which has implications for the ecological and physical form and process of the valley. A valley with abundant beaver but little to no bank-stabilizing vegetation is impossible under natural conditions, because riparian vegetation is necessary to sustain a beaver population and their dam-building. However, a narrow, incised channel may be observed as a legacy effect from beaver removal. The probable planform regimes can be inferred over the range of Holocene climate conditions in the Colorado Front Range, and understanding of these biotic-physical interactions should be a crucial component of any management decisions for geomorphic or ecologic conditions.
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
Characterizing flow resistance in high gradient mountain streams, Fraser Experimental Forest, Colorado
(Colorado State University. Libraries, 2011) David, Gabrielle Catherine Leila, author; Wohl, Ellen, advisor; Cenderelli, Daniel, committee member; Ryan-Burkett, Sandra, committee member; Bledsoe, Brian, committee member
High gradient mountain streams dissipate energy when water flows over poorly sorted grains in the bed and banks and over bedforms such as steps and pools, creating a constant alternation between supercritical and subcritical flow and causing energy dissipation through hydraulic jumps. Mountain streams (bed slope ranging between 0.02 and 0.19) differ from their low gradient counterparts by having large boulders that are of the same order of magnitude as the depth of flow, low values of relative grain submergence (Rh/D 84, where Rh is hydraulic radius and D84 is the 84th percentile of the cumulative grain-size distribution), armored beds, and wood that commonly spans the entire width of the channel. The complex interaction of the different forms of flow resistance in steep mountain streams has made it particularly challenging to quantify flow resistance, usually represented by the dimensionless Darcy-Weisbach friction factor (ff). This research focuses on studying controls and interactions among different forms of resistance in step-pool, cascade, and plane-bed reaches on two different streams, where a reach is a length of channel 100-101 m in length with consistent channel morphology. The project is divided into three parts: 1) identify specific controls on the total flow resistance throughout the channel network using statistical analysis; 2) investigate specific variations and controls in relation to stage within each reach by analyzing at-a-station hydraulic geometry; and 3) quantify and evaluate interactions among the individual flow resistance components that contribute to total flow resistance. Detailed channel and water surface surveys were conducted on 15 mountain stream reaches (nine step-pool channels, five cascade channels, and one plane-bed channel) using a tripod-mounted Light Detection and Ranging (LiDAR) scanner and laser theodolite. Reach-average velocities were measured at varying discharges with dye tracers and fluorometers. Results indicate that gradient is a dominant control for both total ff and the individual components of ff, which were divided into grain (ff grain), form (ffstep), wood (ffwood), and spill resistance (ffspill). A second strong control on values of ff was discharge, with values of ff decreasing with increasing discharge. Spill and form resistance contributed the greatest amount towards total ff at low flows, whereas wood contributed a larger proportion at high discharges. The contribution of grain resistance was small at all flows, but generally decreased with increasing discharge. Methods for calculating the components of resistance were found to have large sources of error. Grain resistance was typically under-estimated at lower discharges, because methods assuming a semi-logarithmic velocity profile become invalid at base flows. A new method of calculating grain resistance is suggested for lower flows, by dividing the characteristic grain size between those elements that protrude above the water surface (D90) and those that are still submerged (D50). Methods for calculating wood resistance were also found to have high sources of error and cause the values of ffwood to be overestimated. An attempt is made to calculate form resistance created by adverse pressure gradients around the step bedforms at high flows. Commonly, this effect is ignored in favor of lumping the remaining component of resistance into spill resistance. Although spill resistance still made up the largest amount of the total at the lowest flows, ffstep made a significant contribution at bank-filling discharges and further work in the flume and field needs to be done to understand the contribution of form drag around steps. Interactions between components of resistance also indicate that an additive method of resistance partitioning is not appropriate in these higher gradient streams. Wood significantly affected the values of flow resistance throughout each channel type. The presence of wood increased resistance within each reach. Steps with wood are significantly wider and have greater drop heights than boulder steps. Wood also was significantly related to grain resistance, causing values of ffgrain to be smaller than in reaches without wood. The increase in resistance from wood, as well as the larger steps, caused reduced velocity, increased depth and therefore decreased ffgrain. The detailed analysis of these high gradient reaches shows the large amount of complexity inherent in these channel types, which makes developing predictive equations of ff difficult. This analysis was undertaken to better understand the complexity and to help in determining appropriate methods for calculating ff. The dominance of gradient as a control on both total ff and its components is useful to understand because this is a metric that can be used to remotely predict these characteristics, as the resolution of remote data improves with time.
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
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 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
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
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 effects of increased wood loading on hyporheic exchange flow
(Colorado State University. Libraries, 2019) Ader, Ethan, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Morrison, Ryan, committee member
Much of the recent scientific literature in the field of fluvial geomorphology has documented the benefits of the presence of large wood in rivers. One of these benefits is enhanced hyporheic exchange flow (HEF). Enhanced HEF has numerous benefits and therefore plays an important role in stream health. While the science of hyporheic exchange has progressed over the past few decades, studies thus far have focused on single pieces of wood or single jams. There have not yet been studies that examine whether multiple consecutive jams have an additive or nonlinear effect on HEF. This study focuses on the impacts of increased wood loading on geomorphic complexity and HEF. We examined relations among wood load, geomorphic complexity, and HEF by studying four different reaches along Little Beaver Creek, a 3rd order tributary to the Cache la Poudre River in the Colorado Front Range within the Arapaho and Roosevelt National Forest: 1) a single channel with no logjams, 2) a single channel with limited logjams, 3) an anabranching channel with limited logjams, and 4) an anabranching channel with abundant logjams. Pearson correlations were used to analyze the relationship between HEF, wood loading, and geomorphic complexity. We found that increased wood loading increases the volume of both pools and accumulated fine sediment at the reach level. Additionally, HEF positively correlates with geomorphic complexity and wood loading. The metrics that most strongly correlated with enhanced HEF all represent factors expected to increase connectivity from the channel to the hyporheic zone. These preliminary results suggest that it is through this mechanism of increasing hyporheic zone connectivity that HEF is enhanced.
Open Access
Great river wood dynamics in northern Canada
(Colorado State University. Libraries, 2016) Kramer, Natalie, author; Wohl, Ellen, advisor; Rathburn, Sara, committee member; Kampf, Stephanie, committee member; Leisz, Stephen, committee member
Downed wood is a resource easily utilized by plants and animals from the forests to the sea and is essential for many ecosystems. The diverse benefits that wood brings to streams and riparian corridors are well documented by river scientists and wood re-introduction is commonly used as a river restoration tool. However, much of the existing work investigates the short-term impact of wood rather than its variability through time and legacy on the landscape. In this dissertation, I use the Slave River (water discharge=2-7 x103 m3 s −1 , channel widths=300-2000 m, drainage area=6x105 km2 ), and its receiving sedimentary basin, the Great Slave Lake (surface area=273 km2 , depths 20-600 m, volume 1000-2000 km3 ), in northern Canada to better understand wood transport dynamics of a major river basin across varied timescales from minutes to centuries and the influence of driftwood on shoreline landscape evolution. The four primary contributions of this work are: a comprehensive literature review and synthesis of wood transport in rivers worldwide (Chapter 1), new methods for monitoring and quantifying wood flux with timelapse cameras (Chapter 2), description of processes among driftwood, sediment, and vegetation that result in shoreline features that I have coined "driftcretions" (Chapter 3), and expansion of wood transport research into multiple timescales with a focus on how flow history impacts magnitude of wood flux (Chapter 4). In Chapter 1, I: qualitatively summarize existing transport research around flow, wood and reach characteristics, quantitatively consolidate and analyze wood mobility field data in relation to increasing channel size, identify disconnects between driving processes and how mobility is measured, and constrain and conceptualize thresholds between wood dynamic ii regimes. In Chapter 2, I introduce a cheap, useful and fast way to monitor and estimate wood flux with timelapse photography through the use of the metric p, the probability of seeing wood within a timeframe, and I provide statistical methods to estimate appropriate sampling intervals to minimize bias and variance. In Chapter 3, I describe processes and rates by which pulsed driftwood export are delivered and accreted to shorelines and I discuss how these processes influence rates of carbon sequestration, sediment storage and habitat formation. In Chapter 4, I use a variety of methods centered around repeat photography and anecdotes to assess temporal variability of pulsed driftwood flux through the Slave River in the past century. Findings in this dissertation provide useful information for understanding pulsed wood flux, shoreline dynamics and landforms in marine and terrestrial water bodies before widespread historical deforestation, damming of rivers, and wood removal along major waterways. I not only synthesize and link existing work on wood mobilization, transport and deposition to an intriguing case study, but challenge existing wood transport premises, provide new conceptual models describing processes of wood transport through drainage networks, and present new approaches and methods for quantifying and analyzing the variability in wood flux and influence of wood deposits on landforms. My descriptions of wood transport and shoreline processes prior to development of river corridors will be an invaluable resource to groups who seek to identify environmental impacts of dams and to scientists who are investigating the impact that past and future development of river corridors has had or will have on ecosystems.