Browsing by Author "Rathburn, Sara, advisor"

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Open Access
A channel stability assessment and logistic regression model for a reach of muddy creek below Wolford Mountain Reservoir, in north-central Colorado
(Colorado State University. Libraries, 2014) Williams, Cory A., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Bledsoe, Brian, committee member
Water resource managers face increasing pressure to meet community water needs while responsibly managing resource infrastructure and preserving aquatic and riparian ecosystems. Management of many western rivers involves multiple uses for multiple stakeholders, especially rivers downstream from dams impounding water-supply reservoirs. These resource management issues arise in a reach of Muddy Creek near Kremmling, Colorado, which serves multiple functions including: (1) wetland mitigation, (2) private, recreational fishing, (3) and agriculture. Alteration of streamflow from reservoir operations and loss of upstream sediment supply, in conjunction with legacy management effects, have resulted in channel instability and increased streambank erosion in Muddy Creek below Wolford Mountain Reservoir. A typical response to channel instability on managed rivers includes installation of erosion-control structures. However, installation of these structures to protect property and infrastructure is expensive and can have unintended consequences at adjacent locations, highlighting the need for resource managers to better understand the underlying geomorphic processes controlling channel adjustment within the reach. To address these issues, field reconnaissance and channel surveying completed during base-flow conditions were used to (1) determine the dominant erosive and resistive processes within the reach that contribute to channel stability and response, and (2) assess the validity of using logistic regression techniques as an analytical framework and to estimate the probability or risk of localized streambank erosion. These findings can be used in conjunction with local management objectives to evaluate or gage acceptable risk to current infrastructure and to target and prioritize where monitoring or remediation should be conducted. Understanding the geomorphic processes and reach characteristics driving streambank erosion can be used to guide management and operational decisions within the reach to minimize impacts. A map of probability of erosion, for each streambank, is presented which shows risk (as a probability of streambank erosion, ranging from < 3 to 80 percent) based on significant explanatory variables from the logistic regression model. The study found that stream-induced scour and undercutting have differing effects within the reach due to changes in the erosive power of the stream and relative difference in streambank and riparian characteristics. Areas most susceptible to streambank erosion occurred in wider cross sections where fluvial energy was oriented into the streambanks, not necessarily in areas with the greatest fluvial energy and potential erosive power (i.e., areas with the steepest bed slope). This suggests additional, localized conditions within the reach need to be considered. Differences in streambank and riparian characteristics were shown to have varying levels of resistance to streambank erosion within the study reach. Larger streambank heights increased the probability of streambank erosion when these streambanks were not supported by bedrock outcrops of Pierre Shale or alluvial fans and talus slopes. Erosion-control structures decreased the probability of streambank erosion where structures retained original positions relative to flow. Where changes to flow orientation occurred, the probability of streambank erosion around these structures increased substantially. Riparian vegetation type also influenced streambank erosion as well as channel top-width. Streambanks covered in willows were found to decrease the risk of streambank erosion, whereas areas dominated by grasses increased streambank erosion potential as well as increased channel top-widths relative to areas dominated by willows. Additional effects from reach-scale characteristics were evaluated. Areas of greater sinuosity and wider valley widths show increased probability of streambank erosion, as well as in areas located downstream of an irrigation diversion structure. This may be due to a combination of effects from further confinement of the reach as valley widths decrease, increased streamflow from tributaries, and/or the occurrence of increased seepage along areas near an irrigation ditch. Streambanks with observed saturated soils were also found to have between 20-44 percent increased odds of streambank erosion. A linkage between proximity of streambank seeps and unlined irrigation ditches and irrigation turn-outs was highly significant, with potential effects extending to distances up to 250 m from the irrigation water sources.
Open Access
A 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
Analyzing post-flood recovery after an extreme flood: North St. Vrain Creek, CO
(Colorado State University. Libraries, 2018) Eidmann, Johanna S., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
Assessing the ongoing sediment remobilization and deposition following an extreme flood is important for understanding disturbance response and recovery, and for addressing the challenges to water resource management. From September 9-15, 2013, a tropical storm generated over 350 mm of precipitation across the Colorado Front Range. The resulting 200-year flood triggered landslides and extreme channel erosion along North St. Vrain Creek, which feeds Ralph Price Reservoir, water supply for the Cities of Lyons and Longmont, CO. The flood resulted in 10 m of aggradation upstream of the reservoir, transforming the reservoir inlet into an approach channel. 4 years after the flood, downstream transport of flood sediment and deposition in the reservoir continues. This research tracks the fate of flood-derived sediment to understand the evolution of the approach channel and delta to assess post-flood response processes and controls and to quantify sediment remobilization. Photographic analysis and DEM differencing of the approach channel indicates that the majority of channel response to the flood occurred within 1 year following the flood. Evolution of the channel from an initial plane bed occurred through channel incision of up to 2.5 m and widening of up to 10 m, forming a trapezoidal cross section. Channel geometry changes in years 2-5 post-flood are limited in spatial extent, largely dependent on sediment discharge and local variations in channel confinement. Bathymetric DEM differencing from 2014 and 2016 (years 1 and 3 post-flood) indicates a minimum sediment accumulation of 68,000 m3 on the delta plain, and progradation of 170 m of the delta front since the 2013 flood. Between fall 2016 and spring 2017, the reservoir level was dropped approximately 10 m during construction at the spillway, creating a base level drop, delta incision, and causing over 15,000 m3 of sediment to be transported further into the reservoir. Based on bathymetry and reservoir core analyses, a total of 74,000 m3 of sediment was deposited in the delta from 2014 through 2017, producing an estimated loss of 0.4% in reservoir storage capacity. Approximately 184,000 m3 (equivalent to another 1% of reservoir storage capacity) is estimated to remain in storage upstream of the reservoir. Although the approach channel appears to be adjusted to a typical snowmelt runoff, stored sediment remaining upstream of the reservoir indicates that complete recovery of the approach channel may not occur on a management time scale. The remaining large volume of sediment still in storage upstream highlights the potential for future disturbances to trigger additional sediment inputs.
Open Access
Braided river response to eight decades of human disturbance, Denali National Park and Preserve, AK
(Colorado State University. Libraries, 2016) Richards, Mariah, author; Rathburn, Sara, advisor; Booth, Derek, committee member; Nelson, Peter, committee member; Wohl, Ellen, committee member
The spatial complexity and stochastic nature of braided rivers complicate our ability to quantify natural rates of sediment transport and limit our understanding of braided river response to human disturbance. The Toklat River in Denali National Park and Preserve, a braided tributary of the Kantishna River draining the north-facing slopes of the Alaska Range, exemplifies these challenges. Eight decades of localized channel confinement due to installation of a causeway in the 1930's and over three decades of gravel extraction since the 1980's have occurred on the Toklat River adjacent to the Denali Park Road. A unique, multi-scalar and temporally diverse dataset records the responses of the river over a 10-km reach. I evaluated trends in short-term sediment storage through LiDAR differencing and analyzed long-term planform change using braiding index, braiding beltwidth and topographic ruggedness derived from aerial photographs. Two reference reaches along comparable adjacent braided rivers, with varying levels of confinement and no gravel extraction, illuminate the relative influence of these human disturbances on channel and planform change. Comparisons of 2009 and 2011 LiDAR-derived DEMs showed a statistically significant volumetric loss of -30,300 ± 27,600 m3 over 4 km of active braidplain within the study reach. Braidplain sediment loss adjacent to the channel-confining Denali Park Road bridge crossing was comparable to that removed biennially through gravel extraction downstream (17,100 m3). Upstream of both the gravel extraction site and the bridge crossing, the braiding beltwidth decreased by 400 m and the braiding index lowered from eight to one between 1988 and 2011. The reference reaches did not display such noticeable morphologic adjustments, implying upstream migration of gravel extraction and confinement impacts, which can significantly alter flow character, leading to increased localized stream power, degradation and infrastructure damage. These results are relevant to assessing the variety and spatial extent of human disturbance on braided river systems in general.
Open Access
Controls on post-High Park Fire channel response, South Fork Cache la Poudre Basin, Colorado
(Colorado State University. Libraries, 2015) Shahverdian, Scott M., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Nelson, Peter, committee member
Post-fire basin sediment yield is the product of multiple erosional processes operating at multiple spatial scales and in different process domains. Most post-fire erosion response studies have focused on the hillslope scale, yet land management decisions and post-fire treatments are addressed at the watershed scale. The goal of this study was to evaluate how the channel network contributes to the production, transport, and storage of sediment by monitoring post-fire channel response. A better understanding of channel production, transport, and storage of sediment post-fire is required in order to predict basin scale sediment yields and make informed management decisions. Two perennial headwater streams and two ephemeral tributaries of the South Fork Cache la Poudre River were monitored in two severely burned basins in the 2012 High Park Fire burn area of northern Colorado. The basins were either completely or partially mulched with agricultural straw and wood mulch during June 2013. Repeat cross section and longitudinal profile surveys were performed to evaluate event-driven changes. The dominant response in both basins post-fire was net degradation. Steep channel slopes promoted channel incision with no significant overbank deposition, indicating that the channel network was a substantial source of sediment and an efficient transporter of hillslope sediment. In 2013, six storms exceeded the 30 minute maximum intensity 10 mm hr⁻¹ associated with hillslope sediment production while in 2014 two storms exceeded this threshold. Perennial channel response in 2014, measured by mean bed elevation change at cross sections, ranged from -20 to +17 cm, but most cross sections experienced changes between 0-3 cm. Channel response was uncorrelated with channel slope, channel slope*contributing area product, or width to depth ratio. Ephemeral channels showed an alternating cycle of aggradation and degradation on the order of 0-3 cm per event, as well as a scour and fill response during storm events. Scour and fill often resulted in minimal net changes to channel geometry, suggesting that the channel was an important temporary source and sink of sediment and that post-fire peak flow calculations must account for event-based scour. In 2013, suspended sediment concentrations in the South Fork Cache la Poudre exceeded 2500 mg L⁻¹ 12 times, and exhibited a threshold response when MI₃₀ exceeded 10 mm hr⁻¹. In 2014, suspended sediment concentrations exceeded 1500 mg L⁻¹ once, and a MI₃₀ of 40 mm hr⁻¹ was insufficient to cause values to exceed 1500 mg L⁻¹. Post-fire suspended sediment concentrations from the South Fork Cache la Poudre River indicate that hillslopes were the primary source of suspended sediment. Where the straw mulch was retained on hillslopes, it was effective at limiting erosion. The channel network was largely resistant to change during the second year of post-fire monitoring due to the influence of a >200 year storm that occurred in September 2013. Following this storm, the channel network acted primarily to transport sediment rather than produce sediment. Sediment connectivity within the channel network was high in each basin due to steep channel slopes, but the development of an alluvial fan at each basin outlet as well as the morphology of the South Fork Cache la Poudre at each confluence suggest differences in the sediment delivery from each basin to downstream reaches. Sediment connectivity from the hillslopes to the channel network and along the channel network must be addressed in post-fire studies when predicting or interpreting post-fire basin sediment yields. Furthermore, assessing sediment connectivity is a useful tool for land managers making post-fire erosion mitigation decisions.
Open Access
Dammed ponds! A study of post-fire sediment and carbon dynamics in beaver ponds and their contributions to watershed resilience
(Colorado State University. Libraries, 2023) Dunn, Sarah B., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Morrison, Ryan, committee member
Excess sediment generated by wildfires threatens stream water quality, riparian habitat, and infrastructure. Beavers construct dams that pool water and capture sediment. Beaver ponds may bolster watershed resilience by providing sediment and carbon storage following wildfire. I tested the hypotheses that (1) burned ponds store greater relative volumes of sediment compared to unburned ponds, (2) post-fire sedimentation rates exceed pre-fire and unburned rates, and (3) post-fire sediment stored in beaver ponds is coarser and has a higher abundance of organic carbon relative to pre-fire sediment. I surveyed 48 beaver ponds in the Colorado Rocky Mountains. Approximately half of the ponds are in areas that burned in 2020 wildfires, whereas the other half remain unburned. Sites also spanned a range of geomorphic, vegetation, and individual pond characteristics. I conducted sediment probe surveys and collected sediment cores to quantify pond sediment storage and characterize sediment composition. Stratigraphic units present in sediment cores were analyzed for grain size and total organic carbon (TOC). Results indicate that beaver ponds in the Rocky Mountains store high volumes of sediment (mean = 796 m3). Burned ponds contain statistically significantly more relative sediment storage and have higher sedimentation rates than unburned ponds. Beaver ponds recorded high post-fire sedimentation rates (median = 19.8 cm/yr). Moreover, post-fire sedimentation rates are an order of magnitude higher than pre-fire rates in ponds with both pre- and post-fire sediments. Total sediment volume, sedimentation rates, grain size, and TOC content did not vary significantly between burned and unburned ponds. Geomorphology, vegetation, and pond characteristics exert additional influences on pond sediment dynamics. Pond characteristics determine the sediment trapping efficiency of ponds. Larger ponds store greater volumes of sediment, as do off-channel and older ponds. Ponds abandoned by beaver store greater volumes of sediment than actively maintained or human- constructed dams. Beaver activity and dam maintenance is critical for maintaining storage availability in ponds. Additionally, sedimentation rates are higher in ponds that are on-channel and recently constructed compared to off-channel and older ponds. These findings indicate that beaver-based restoration can be implemented prior to fire to provide critical post-fire sediment storage, thus enhancing watershed resilience and recovery.
Open Access
Evaluating channel morphologic changes and bed-material transport using airborne lidar, upper Colorado River, Rocky Mountain National Park, Colorado
(Colorado State University. Libraries, 2014) Mangano, Joseph F., author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Bledsoe, Brian, committee member
A debris flow associated with the 2003 breach of Grand Ditch in Rocky Mountain National Park, Colorado provided an opportunity to determine controls on channel geomorphic responses following a large sedimentation event. Due to the remote site location and high spatial and temporal variability of processes controlling channel response, repeat airborne lidar surveys in 2004 and 2012 were used to capture conditions along the upper Colorado River and tributary Lulu Creek i) one year following the initial debris flow, and ii) following two bankfull flows (2009 and 2010) and a record-breaking long duration, high intensity snowmelt runoff season (2011). Locations and volumes of aggradation and degradation were determined using lidar differencing. Channel and valley metrics measured from the lidar surveys included water surface slope, valley slope, changes in bankfull width, sinuosity, braiding index, channel migration, valley confinement, height above the water surface along the floodplain, and longitudinal profiles. Reaches of aggradation and degradation along the upper Colorado River are influenced by valley confinement and local controls. Aggradational reaches occurred predominantly in locations where the valley was unconfined and valley slope remained constant through the length of the reach. Channel avulsions, migration, and changes in sinuosity were common in all unconfined reaches, whether aggradational or degradational. Bankfull width in both aggradational and degradational reaches showed greater changes closer to the sediment source, with the magnitude of change decreasing downstream. Local variations in channel morphology, site specific channel conditions, and the distance from the sediment source influence the balance of transport supply and capacity and, therefore, locations of aggradation, degradation, and associated morphologic changes. Additionally, a complex response initially seen in repeat cross-sections is broadly supported by lidar differencing, although the differencing captures only the net change over eight years and not annual changes. Lidar differencing shows great promise because it reveals vertical and horizontal trends in morphologic changes at a high resolution over a large area. Repeat lidar surveys were also used to create a sediment budget along the upper Colorado River by means of the morphologic inverse method. In addition to the geomorphic changes detected by lidar, several levels of attrition of the weak clasts within debris flow sediment were applied to the sediment budget to reduce gaps in expected inputs and outputs. Bed-material estimates using the morphologic inverse method were greater than field-measured transport estimates, but the two were within an order of magnitude. Field measurements and observations are critical for robust interpretation of the lidar-based analyses because applying lidar differencing without field control may not identify local controls on valley and channel geometry and sediment characteristics. The final sediment budget helps define variability in bed-material transport and constrain transport rates through the site, which will be beneficial for restoration planning. The morphologic inverse method approach using repeat lidar surveys appears promising, especially if lidar resolution is similar between sequential surveys.
Open Access
Landslide response to climate change in Denali National Park, Alaska, and other permafrost regions
(Colorado State University. Libraries, 2019) Patton, Annette, author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Singleton, John, committee member; Niemann, Jeffrey, committee member
To view the abstract, please see the full text of the document.
Open Access
Monitoring the effectiveness of river realignment on the Upper Colorado River, Rocky Mountain National Park
(Colorado State University. Libraries, 2017) Sparacino, Matthew, author; Rathburn, Sara, advisor; Covino, Tim, committee member; Nelson, Peter, committee member; Ronayne, Mike, committee member
A 2003 debris flow introduced 36,000 m3 of sediment into a high-elevation wetland on the Upper Colorado River in Rocky Mountain National Park. In September 2015, Park staff built an earthen diversion dam and realigned a 190 m reach of the Colorado River into its historic thalweg through the center of Lulu City wetland. Initial dimensions of the constructed channel were 1.6 m wide and 0.4 m deep with an average bed slope of 1.9%. Pre- and post-restoration measurements are compared to assess the hydro-geomorphic response to the channel realignment within the adjacent wetland. The constructed diversion berm redistributed at least 48% of river discharge from a pre-realignment, west-side channel, to a central channel, which decreased surface-water groundwater exchange as well as the size of near-stream hyporheic zones and altered sediment transport capacity. A sodium chloride tracer was injected during base-flow and electrical resistivity was used to monitor changes in near-channel hyporheic exchange across the realigned channel for approximately 24 hours following the injection. Pre-and post-realignment electrical resistivity analyses indicate a loss of hyporheic exchange in the northern wetland, likely a result of decreased river complexity. Tracer mass balances derived from concurrent surface conductivity measurements indicate increases in solute retention throughout Lulu City wetland, possibly due to increased overbank flow. These results imply that solute retention can increase without an equal increase in hyporheic exchange. Furthermore, local incision greater than 0.5 m, widening of 0.2 to 1 m, and upstream knickpoint migration within the realigned channel during 2016 runoff indicate increases in erosion and local sediment transport. The growth of gravel bars upstream of the diversion berm indicate increased sediment deposition at the head of Lulu City wetland. Results from one year of post-realignment monitoring suggest that the channel realignment has had small-scale effects on hyporheic exchange, solute retention, and sediment transport capacity, with potentially negative consequences for the ecosystem services provided by river-wetland systems. Long-term monitoring and increased instrumentation are required to predict how these changes may be amplified in a larger restoration attempt.
Open Access
Post-glacial alluvial valley dynamics of the South Fork Cache la Poudre River Valley at the Colorado State University Mountain Campus
(Colorado State University. Libraries, 2022) Suhr, Jens Christoph, author; Rathburn, Sara, advisor; McGrath, Daniel, advisor; Morrison, Ryan, committee member
Wide valley bottoms are physically important sediment storage sites where alluvial records of past landscape dynamics may be preserved. Following deglaciation after the Last Glacial Maximum (LGM), unconfined valleys in the Colorado Front Range experienced periods of fluvial aggradation and incision, creating distinctive valley bottom morphologies and the substrates which influence present-day hydrological and ecological characteristics. The objectives of this study are to investigate the processes and chronology of post-glacial geomorphic evolution of an unconfined portion of the South Fork Cache la Poudre River (South Fork) Valley, Colorado Front Range, to identify the dominant processes and temporal patterns of valley alluviation and incision following LGM retreat at the Colorado State University Mountain Campus (Mountain Campus). Methods used include geologic mapping, ground-penetrating radar (GPR) surveys, coring of valley bottom sediments, radiocarbon geochronology, and analysis of historical aerial images. Mapping of the Quaternary sediments indicates a variety of glacial and fluvial deposits occur in the South Fork Valley, including moraines, two distinct outwash terraces (approximately 8 m and 6 m above the present-day channel, respectively), fluvial terraces 1–2 m high, and an extensive floodplain. Well logs indicate over 10 m of glaciofluvial outwash sediment was deposited upstream of the LGM terminal moraine, and GPR reflections suggest that lateral bar migration, channel filling, and vertical accretion of sediments were important processes of outwash aggradation in the valley. The South Fork River has since incised into the outwash. A fluvial terrace and the modern floodplain are inset within the outwash sediments and are composed of overbank-deposited silt-to-sand sized sediments. Radiocarbon samples of valley sediments indicate that outwash was deposited at least 16.8 ka, with 8–10 m incision occurring after 16.8 ka and prior to 7.8 ka. Fine-grained sedimentation occurred on the fluvial terrace and floodplain from at least 2.1 ka to 1.3 ka. The modern floodplain has been vertically accreting for at least the last 500 years. Historical aerial images show that the South Fork channel was relatively stable from 1938 to the present; the channel planform area changed by no more than 2.5% per year during this period. Additionally, in the last ~80 years, the channel has largely occupied the center of the unconfined valley, reducing connectivity between the channel, terraces, and the valley sides. My results highlight the complex patterns of sediment storage and removal in unconfined valleys, with at least two phases of aggradation and one phase of incision following deglaciation. In addition, the South Fork Valley is relatively geomorphically stable: large volumes of Quaternary sediments have been stored for over 16.8 ka years, and the modern fluvial system has not responded drastically to local disturbances because of low connectivity between hillslopes and the valley bottom. The South Fork Valley is an effective site of fluvial sediment storage following deglaciation despite a long-term trend of sediment removal from the valley in the Holocene. Broader implications of assessing valley bottom stability and long-term sediment storage in mountains include managing unconfined valleys where development pressures, proposed water diversions, and climatically forced changes to the hydrology are occurring. Findings presented herein may provide insights for maintaining riparian biodiversity and surface-subsurface water exchange in formerly glaciated environments.
Open Access
Process linkages in large watersheds: connecting tributary erosion to downstream channel change and floodplain forest establishment in the Yampa and Green River Basin
(Colorado State University. Libraries, 2022) Kemper, John Trusal, author; Rathburn, Sara, advisor; Friedman, Jonathan, committee member; Wohl, Ellen, committee member; Leisz, Stephen, committee member; Redmond, Miranda, committee member
It is well-understood that the physical state of a river is a combination and culmination of present processes and past trajectories. Similarly, conceptualizations of fluvial connection hold that various aspects of a given river reach – ecologic, geomorphic, hydrologic – do not operate in isolation, but rather as components within a linked system, both influencing and influenced by upstream and downstream conditions. To expand understanding of the river system as an intrinsically linked network of both process and form, here I establish connections between the processes of historical tributary erosion and distal downstream channel migration and floodplain forest establishment in the Yampa and Green River Basin. I then additionally summarize the extensive body of literature concerning the geomorphic response to sediment supply increases in low-gradient, alluvial rivers to further emphasize that the translation of sediment through the landscape can catalyze myriad responses that manifest across a continuum of scales. Concentrating initially on the investigation of historical erosion, examination of historical documents and aerial photos suggests that three key sediment contributing tributaries of the Yampa River – Sand Creek, Muddy Creek, and Sand Wash – underwent substantial historical erosion from 1880-1940. Using field investigation to determine historical channel location and field surveys of present-day dimensions, I then calculate that historical arroyo incision within the latter two tributary watersheds injected 30 x 106 tons of sediment into the mainstem Little Snake and Yampa Rivers during this time. Taking present-day annual sediment loads as an approximate background for the pre-erosion sediment regime, this represented a sizable increase in the sediment load of the Yampa River during the period of historical erosion. Moving downstream, results of dendrochronologic analysis of tree cores from three separate forest locations – Deerlodge Park on the Yampa River, Island Park and Tuxedo Bottom on the Green River – indicate that major portions of these forests established during the same time period of elevated historical erosion. Moreover, channel change analysis suggests that the channel at this time was relatively more dynamic than it has been since, and the area of forest dating to the historical period is much greater than can be explained by high flows alone. Viewed collectively, these findings suggest tributary erosion played a vital role in successful downstream forest establishment. Additional sediment fingerprinting analysis further supports this process link between geomorphic and ecologic process. Using sediment samples taken at the rooting surface of the cottonwood forest in Deerlodge Park, geochemical analysis indicates that the majority of this sediment was sourced from those tributaries – Muddy Creek and Sand Wash – that were undergoing enhanced erosion via arroyo incision during the historical period. Overall, the temporal overlap between the timing of historical tributary erosion and the establishment of substantial portions of downstream floodplain forest, in conjunction with the fact that floodplain sediment is dominantly sourced from watersheds that experienced enhanced historical erosion, together indicates a demonstrable link between the geomorphic process of historical erosion and the ecologic process of downstream floodplain forest establishment. From a summary of existing studies concerning the geomorphic adjustment of low-gradient, alluvial rivers to increased sediment supply, it is additionally clear that tributary erosion that injects substantial amounts of sediment into a river system can result in the requisite channel change necessary for successful forest establishment. The fluvial system is thus best understood as not just a physically coupled network, but a collectively connected web of processes that together regulate and are regulated by one another. Such an understanding emphasizes that management of large watersheds must be holistic and undertaken at the basin scale in order to ensure that vital riverine ecosystems endure.
Open Access
Seeing the river through the trees: using cottonwood dendrochronology to reconstruct river dynamics in the Upper Missouri River Basin
(Colorado State University. Libraries, 2017) Schook, Derek Michael, author; Rathburn, Sara, advisor; Friedman, Jonathan, committee member; Wohl, Ellen, committee member; Covino, Tim, committee member; Denning, Scott, committee member
Understanding the past is critical to preparing for the future, especially regarding rivers where extreme events and gradual changes underlie modern forms and processes. Both biological and human communities rely on the abundant resources provided by rivers and floodplains, particularly in dry regions of the western U.S. where water limits growth. To expand temporal perspectives on river processes, I reconstructed flow, channel migration, and riparian forest growth patterns in the Upper Missouri River Basin. Flow reconstructions typically use tree rings from montane conifers. However, I used riparian plains cottonwoods (Populus deltoides ssp. monilifera) directly connected to the alluvial water table to reconstruct flow on the Yellowstone (n = 389 tree cores), Powder (n = 408), and Little Missouri Rivers (n = 643). A two-curve Regional Curve Standardization approach was used to remove age-related growth trends from tree rings at each site. The flow reconstructions explained 57-58% of the variance in historical discharge and extended back to 1742, 1729, and 1643, respectively. Low-frequency flow patterns revealed wet conditions from 1870 to 1980, a period that includes the majority of the historical record. Two 19th century droughts (1816-1823 and 1861-1865) and one pluvial (1826-1829) were more severe than any recorded, revealing that risks are underestimated when using the instrumental period alone. These are the first flow reconstructions for the Lower Yellowstone and Powder Rivers, and they are the farthest downstream among Rocky Mountain rivers east of the Continental Divide. Cottonwood-based flow reconstructions were possible because the trees used river-connected groundwater, and tree ring width strongly correlated with March-June flow magnitude at the Yellowstone River (r = 0.69). Beyond the site-level growth patterns typically used to reconstruct flow, I found that biological and spatial characteristics affected how individual trees responded to flow and climate. Older trees contained stronger signals of non-growing season flow, precipitation, and temperature, which challenges the common dendrochronological assumption of stable tree ring-climate relationships through time. Although trees both near and far from the channel were better correlated to spring flow than precipitation, more distant trees had a stronger relative connection to precipitation, suggesting that greater distance decreases the ability of river water to fulfill transpirative demands. Like annual growth, cottonwood establishment is related to river flows, and tree age indicated fluvial processes including channel migration. I quantified nearly two centuries of channel migration on the Powder River by integrating measured channel cross-sections (1975-2014), air photos (1939-2013), and transects of aged cottonwoods (1830-2014). The combined data revealed that channel migration rates were lower (0.81 m/yr) in the recent and intensively studied cross-section period compared to the longer air photo (1.52 m/yr) and cottonwood (1.62 m/yr) periods. On the Powder River, extreme floods such as those in 1923 and 1978 increase subsequent channel migration rates and initiate decades of channel morphological adjustments. Across the study rivers, data indicate that fundamental fluvial processes have responded to climatic and watershed pressures. By identifying and quantifying past events, diverse research approaches improve understanding of the river, floodplain, and riparian forest processes that are essential to the persistence of these valuable ecosystems.
Open Access
Upland processes and controls on September 2013 debris flows, Rocky Mountain National Park, Colorado
(Colorado State University. Libraries, 2016) Patton, Annette, author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; Niemann, Jeffrey, committee member; Bilderback, Eric, committee member
More than 10 large debris flows occurred in and near Rocky Mountain National Park (RMNP) following the spatially extensive September 2013 rainstorm in the Colorado Front Range. These debris flows delivered sediment to valley bottoms and had the potential to damage infrastructure and endanger park visitors and staff. To characterize conditions of debris flow initiation when known thresholds of elevation, slope angle, and rainfall intensity are met, 11 debris flow sites in RMNP were surveyed. Slope variables including soil depth, soil texture, and slope morphology were compared between 11 failed and 30 undisturbed hillslopes (control sites) that were exposed to similar cumulative rainfall during the 2013 storm. Analysis of measured slope variables indicates that slope morphology is strongly related to debris flow occurrence. Four of the 11 surveyed debris flows initiated in or immediately below a colluvial hollow (a topographic concavity on a hillslope), while only 1/30 control sites were located near a colluvial hollow. Furthermore, 8/11 surveyed debris flows initiated in areas of convergent topography (including colluvial hollows and areas of broader convergence), while only 3/30 control slopes were convergent. The differences in these proportions suggest that hillslopes characterized by a colluvial hollow or other convergent topography accumulate surface and groundwater flow and collect unconsolidated material. Convergent hillslopes are therefore more susceptible to slope failure during rain events of sufficient intensity and/or duration. The other geomorphic hillslope variables evaluated in this study did not demonstrate statistically significant differences between debris flow sites and control sites. In some cases, small sample size or other data constraints may provide limited ability to discern geomorphic differences between debris flows and control sites. The Bighorn site, a large debris flow near an historic structure in RMNP, was selected for detailed geochronologic study to determine the ages of old debris deposits and evaluate debris flow frequency. Several numeric and relative dating techniques were applied to determine the age of pre-2013 debris deposits at this site. Numeric dating techniques included radiocarbon analysis of organic material and 10Be radionuclide analysis of boulders collected from four debris flow levees. 10Be exposure dating has not previously been applied to debris flow surfaces. Mapping of debris flow levees and stratigraphic study at the Bighorn debris flow site confirm that at least 2-3 and possibly more debris flows have occurred at this site within the last 102-103 years. The recurrence of debris flows at this site indicates that it may experience similar mass movements in the future. A boulder sample collected from the 2013 debris flow levee returned an 18.1 ka 10Be exposure age. Ranges of exposure ages for three older debris deposits are 54.2, 143, and 121 ka. The falsely old age of the 2013 sample and the wide range of ages determined for each of the older landforms indicate that exposure histories of debris deposits are complicated by inherited atmospheric exposure prior to debris flows. Radiocarbon ages of material collected from this site are on the order of 100-102 years old and do not cluster according to the landform sampled. The results of this study demonstrate that debris flow initiation is governed by a complex interaction of geomorphic and climate variables across a range of spatial and temporal scales. Additionally, the scatter of ages established for old debris flow deposits at the Bighorn debris flow site suggests that debris deposits continue to be modified by secondary processes after the initial event. The downslope pathway of debris flow material reflects highly individual histories from source to sink to deposition near the valley bottom. The numerous debris flows that initiated in the 2013 storm exemplify ongoing debris flow hazards in the Colorado Front Range and highlight the need for awareness of hillslope hazards in this region.
Open Access
Vegetation and lithologic influences on channel morphology in the southwestern U.S.
(Colorado State University. Libraries, 2024) Wieting, Celeste, author; Rathburn, Sara, advisor; Wohl, Ellen, committee member; McGrath, Dan, committee member; Morrison, Ryan, committee member; Friedman, Jonathan, committee member
Vegetation and lithology play critical roles in shaping landscapes, creating diverse river and gully morphologies. Vegetation stabilizes banks and alters flow dynamics. In the Southwestern United States, non-native, invasive plant species contributed to regional trends of river channel narrowing and simplification and degraded diverse riparian habitats throughout the 20th century. More recently, efforts to remove invasive riparian vegetation (IRV) have been widespread, especially since 1990. Restoration practitioners who perform IRV treatments often focus on wildlife or vegetation response; however, geomorphic processes should be considered in restoration planning because they drive flow, sediment transport, and aquatic habitat and vegetation dynamics, and because of the potential for damage to downstream people and infrastructure. Depending on the restoration goal, management practices can be used to enhance or minimize the increase in channel dynamism caused by IRV removal. At the river reach scale, I investigated biogeomorphic feedbacks at one of the 15 previously analyzed study sites, the Rio Grande in Texas. Along the Rio Grande in Big Bend National Park (BIBE), restoration goals to remove invasive giant cane (Arundo donax) include decreasing channel narrowing and increasing water and sediment conveyance. Recent work has indicated that removal of giant cane has successfully reduced its extent, but the geomorphic effects of giant cane treatment and subsequent revegetation are still not well understood. A general lack of reach-scale studies of riparian plant pronation during flow inundation and the biogeomorphic feedbacks between plants, flow, and sediment transport contribute to this knowledge gap. I quantified morphological-effect plant traits for three common riparian plant species: invasive giant cane, native baccharis (Baccharis salicifolia), and native phragmites (Phragmites australis). I collected data at the plant, plot, and reach scales and created upright and flexible frontal area and vegetation roughness curves using photographs of plants and stem counts of plots. Then, I used these data in a reach-scale 2D hydraulic model to simulate species-specific effects and the effects of giant cane removal on channel hydraulics. Results indicate that the mean vegetation roughness is similar for all three species at the plant scale, but at the plot scale, vegetation roughness is higher for giant cane and phragmites due to higher stem densities. Hydraulic modeling results suggest that vegetation increased velocities in the center of the channel and decreased velocities on the channel margins. When all the vegetation was represented as giant cane, reach-scale water surface elevations were the highest and reach-scale velocities the lowest. Removing giant cane decreased water surface elevations, indicating increased conveyance. To determine the effects of IRV removal on a regional scale across the Southwest U.S., treated and untreated reaches at 15 sites along 13 rivers were compared before and after IRV treatment using repeat aerial imagery to assess long-term (~10 year) channel change. Resolving observations of channel change into separate measures of floodplain destruction and formation provided more information on underlying processes than simple measurements of channel width and centerline migration rate. IRV treatment significantly increased channel width and floodplain destruction. Treated reaches had higher floodplain destruction than untreated reaches at 14 of 15 sites, and IRV treatment increased floodplain destruction by a median factor of 1.9. The effect of treatment increased with the stream power of the largest flow over the study period. From the results, I suggest that restoration managers consider the system's susceptibility to change, downstream threats, and desired process changes when defining their geomorphic restoration goal because treatment of a dominant species over a large area can be expected to have major fluvial geomorphic consequences. In addition to vegetation, the lithology and surficial sediment properties influence hydrological processes, sediment transport, and gully and channel morphology. In semi-arid environments where vegetation is lacking, and precipitation is sufficient to drive erosion, sediment yields tend to be greatest. Increased landscape erosion is predicted as more extreme weather causes frequent or intense rainfall, and flooding. In Wupatki National Monument (WUPA), heavy rainstorms over the past decade, lack of vegetation, and presence of unconsolidated volcanic-derived cinders expose archaeological sites to erosion, a concern to cultural resource managers. To identify archaeological sites of highest vulnerability to erosion, I analyzed gully morphologic change over a 5-year period. I found that 35 measured gullies are actively eroding, with statistically significant changes in gully depth from 2016 to 2021. Up to 0.5 m of incision was documented over a five-year period. A structure-from-motion analysis at the hillslope scale confirmed gully morphological changes and supports the applicability of conducting similar analyses on a larger scale. More erosion occurred in gullies with catchments predominantly covered with cinders because of cinder mobility. A weak relationship was noted between gully catchment area and gully head slope, likely related to runoff processes from outcrops of resistant sedimentary rocks forming cliffs and characteristics of cinders that maximize infiltration and transport. Based on assessment of gully morphologic change and substrate characteristics, 22 archaeological sites along Wupatki Wash were identified as having a high vulnerability to erosion.