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Geomorphology

Geomorphology

  • Indicators of Hydrologic Alteration (IHA) is a software program, developed by The Nature Conservancy, that provides useful information for those trying to understand the hydrologic impacts of human activities or trying to develop environmental flow recommendations for water managers. Nearly 2,000 water resource managers, hydrologists, ecologists, researchers and policy makers from around the world have used this program to assess how rivers, lakes and groundwater basins have been affected by human activities over time – or to evaluate future water management scenarios.
     
  • Karen Schlatter gives an Update on The Vegetation Response to Environmental Flows and Restoration Treatments in the Colorado River Delta at TC's 2016 Annual Conference. 

  • The purpose of the Stream Stewardship and Recovery Handbook is to create an educational resource for private landowners to better understand their streamside properties in the context of the larger watershed, what they can do to practice good stream stewardship and when/how they should engage outside help for stewardship or recovery projects.

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    A 184-Year Record of River Meander Migration from Tree Rings, Aerial Imagery, and Cross-Sections on The Powder River, Montana
     
    Derek M. Schook*1, Sara L. Rathburn2, Jonathan M. Friedman3
     
    1 Department of Forest and Rangeland Stewardship, Colorado State University, Fort Collins, CO, USA; derek.schook@colostate.edu
    2 Department of Geosciences, Colorado State University, Fort Collins, CO, USA; sara.rathburn@colostate.edu
    3 US Geological Survey, Fort Collins, CO, USA; friedmanj@usgs.gov
     
     
    Channel migration is the primary mechanism of floodplain turnover in meandering rivers and is essential to the maintenance of floodplain ecosystems. Channel migration is dictated by river flows, and even modest perturbations to the flow regime may decrease migration rates. Ongoing research on Montana’s Powder River began in 1975 and has contributed to a diverse array of fluvial geomorphology literature. Although the past research thoroughly describes processes occurring along the Powder River, it is unknown how representative documented conditions are compared to those that occurred before agricultural expansion, incremental water development, and climate change. We calculated channel migration rates from topographic cross-sections collected between 1975-2014. We then extended the spatiotemporal perspective of channel migration up to two centuries by delineating the river channel in air photos (1939-2013) and by aging transects of cottonwoods (1829-2014). Channel migration calculated from the recent cross-sections occurred at 0.63 m/yr, compared to 1.68 m/yr for the medium-length air photo record and 2.78 m/yr for the long cottonwood record. Examining early- and late-periods from within the air photo record supported these findings; the post-1978 photos showed a similar migration rate to that calculated from cross-sections surveyed in the same period (0.81 vs. 0.63 m/yr), which was half the rate found over the entire 74-year air photo period. All lines of evidence suggest that channel migration and floodplain turnover have decreased in recent decades, and the recent intensively studied period is not representative of past fluvial geomorphic processes. Corresponding to the decreased channel migration rates is a decrease in channel width (111 vs. 52 m for 1939 vs. 2013), an increase in sinuosity (1.55 vs. 2.01 for 1939 vs. 2013), decrease in flood peaks, and an exotic shrub invasion. We conclude that even the modest degree of landscape change and flow management in the watershed has caused channel migration and floodplain turnover to decrease, threatening the native floodplain ecosystem that depends on dynamic fluvial processes.
     
     
     
  • Bioengineering practices provide resiliency for streambanks, enhance wildlife habitat, enhance organic matter inputs to streams, improve water quality, increase floodplain roughness, and heighten landscape aesthetics so important to countless residents, visitors, and businesses. Accordingly, the authors have created the following manuscript to:
    • Provide guidelines for a comprehensive bioengineering strategy;
    • Incorporate design elements that impart site stability and resilience;
    • Include project recommendations that minimize risk during periods of vulnerability;
    • Increase understanding of how to properly apply bioengineering and revegetation techniques;
    • Provide background resources on the combined forces of water and gravity as they pertain to bioengineered structures; and
    • Create a searchable Revegetation Matrix for the primary native restoration species useful for flood recovery and other riparian areas throughout Colorado.
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    Designing for Ecological Disturbance in River Restoration to Promote Native Species Regeneration: A Look at the River Bluffs Project on the Poudre River
     
    Johannes Beeby1*, Travis Stroth1, and Sharon Bywater-Reyes2   
     
    1Stillwater Sciences, Boulder, CO, USA; jbeeby@stillwatersci.com, Tstroth@stillwatersci.com
    2University of Northern Colorado, Greeley, CO, USA; sharon.bywaterreyes@unco.edu
     
     
    Disturbance is a natural process in rivers and many riparian species actually require disturbance to regenerate. Nevertheless, disturbance is often seen as a negative by communities and practitioners alike. The 2013 Floods along the Front Range of Northern Colorado provided this necessary disturbance in many instances but also resulted in many restoration projects in river corridors to “fix” the disturbance. Many restoration designs continue to build static river corridors where constructed channels are expected to stay put, and project success is evaluated based on whether the channel is “stable”, i.e., sediment in equals sediment out, there is no bank erosion, no future channel migration, and no wood that may cause disturbance. At the River Bluffs Project on the Poudre River, we used a process-based approach to design a dynamic channel with disturbance built-in. As a metric of project success, our team is monitoring project outcomes with an eye on disturbance as a key metric of restoring system processes in hopes of returning a once frozen river corridor to a more dynamic system that can continue to move and adjust as needed through time. Our monitoring efforts include 1) grain size analysis to monitor sediment transport including riffle flushing and floodplain deposition, 2) topographic analysis via cross-section surveying and structure from motion point clouds, and 3) vegetation surveys to monitor planted and naturally recruited vegetation. Preliminary results indicate 1) mixed grain-size trends, 2) building of mid-channel bars and off-channel erosion, and 3) natural recruitment of Plains Cottonwood after one peak flow event equal to the Q2. By removing berms, reconnecting floodplain, narrowing the channel, and utilizing large wood structures, the river is now set up to create the disturbance needed to help promote new Plains Cottonwood gallery forests now and into the future.
     
     
     
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    The Importance of Plant Traits on River Processes and How to Incorporate them into Revegetation Strategies
     
    Sharon Bywater-Reyes1*, Rebecca Diehl2, Li Kui3, John Stella4, and Andrew Wilcox5
     
    1Earth and Atmospheric Sciences, University of Northern Colorado, Box 100, 501 20th St., Greeley, CO 80639, USA, sharon.bywaterreyes@unco.edu
    2Diehl, Rebecca, Gund Institute for Environment, The University of Vermont, Burlington, VT 05405, USA, Rebecca.Diehl@uvm.edu
    3Kui, Li, Marine Science Institute, The University of California Santa Barbara, Santa Barbara, CA 93106, USA, li.kui@ucsb.edu
    4Stella, John, Forest and Natural Resource Management, SUNY-ESF, Syracuse, NY 13210, USA, stella@esf.edu
    5Wilcox, Andrew, Department of Geosciences, University of Montana, Missoula, MT 5981, USA, Andrew.Wilcox@mso.umt.edu
     
     
    The strength of interactions between plants and river processes is dependent on plant traits such as stem density, plant frontal area, and stem bending properties. A combination of flow regulation, river management, and exotic species invasion have altered the distribution of vegetation in many waterways, with subsequent shifts in the distribution of plant traits. For example, in most U.S. Southwest waterways, Tamarix has invaded, displacing native pioneer vegetation such as Populus. Our team investigated whether plant-trait differences between Tamarix and Populus differentially affect hydraulics, sediment transport, and river morphology with a combination of flume, field, and remote sensing approaches spanning the individual seedling to river-corridor scales. We found that Tamarix requires more force to bend compared to Populus, has greater stem densities and a different crown morphology, resulting in a greater influence on near-bed flow velocities, and subsequently sediment transport (greater aggradation rates). In the Bill Williams (Arizona) watershed, at the patch and corridor scales, remote sensing observations confirmed greater aggradation for denser vegetation patches. Furthermore, long-term channel adjustments were faster for Tamarix versus Populus dominated reaches. More broadly, because the plant traits that influence hydraulics and sediment transport are correlated to plant functional traits (e.g., specific leaf area and stem-tissue density), they should be explicitly considered in riparian management and restoration efforts. Restoration designs should use a collaborative approach that includes the views of fluvial geomorphologists and riparian ecologists such that plants are distributed in a manner with desirable outcomes.
     
     
     
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    Channel Morphologic Changes Associated with Invasive Vegetation Removal
     
    Celeste Wieting1*, Sara Rathburn2, Lindsay Reynolds3, Jonathan Friedman4, Derek Schook5
     
    1,2Colorado State University, Fort Collins, CO, USA; celeste.wieting@colostate.edu,  sara.rathburn@colostate.edu
    3Bureau of Land Management, Denver, CO, USA; lreynolds@blm.gov
    4US Geological Survey, Fort Collins, CO, USA; friedmanj@usgs.gov
    5Department of Forest and Rangeland Stewardship, Colorado State University, Fort Collins, CO, USA; derek.schook@colostate.edu
     
     
    Invasive vegetation poses a great threat to riparian ecosystem diversity in Western North America. Invasive species such as tamarisk (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) dominate much of the riparian corridor of American Southwest rivers. In addition to altering riparian ecosystems, the spread of invasive vegetation causes channel morphologic changes including channel narrowing and incision. Over the past decades, various methods of river restoration have been applied at different scales to remove invasive species along river corridors to benefit native vegetation, local wildlife, and restore channel morphology. As widespread removals become more prevalent in river restoration projects, it is critical to understand how channels will respond. Comprehensive post-removal channel morphologic response studies are lacking, largely because post-removal monitoring generally focuses on vegetation or wildlife response. Our work will help fill this knowledge gap through i) a literature review of channel morphologic changes resulting from invasive species removal throughout the Southwestern US; ii) ongoing reach- to segment-scale field monitoring of channel geometry changes resulting from different types of vegetation removal; and iii) explicitly linking vegetation characteristics to channel and floodplain surfaces through flow and sediment transport dynamics. Some of the broader questions related to channel-vegetation feedbacks include: What are the fundamental controls that govern the suite of probable channel responses following invasive vegetation removal? How do different removal methods compare in terms of resulting stream morphologic changes? Does mechanically removing the whole plant lead to the greatest stream restoration benefits? Which invasive species impart the greatest fluid drag and promote deposition and channel narrowing? This research will involve a coarse-scale analysis of channel geometry changes following removal of invasive species on rivers with ongoing research. Examples of variables to be noted include vegetation type, method of vegetation removal, time since removal, number of high flows since removal, and major channel changes such as avulsions. Additional analyses will utilize satellite imagery and existing data from remote sensing applications. Stream channel response to invasive vegetation presence and removal will be analyzed on a smaller scale at Canyon de Chelly National Monument (CACH) and Big Bend National Park (BIBE). Work within CACH will include repeat Unmanned Aircraft System (UAS) flights for high-resolution terrain data and digital elevation models (DEM), and repeat channel cross-section surveys building on previous work. Identifying controls on incision versus widening will be important at CACH because the channel has been actively incising, putting cultural resources within the canyon at risk. At BIBE, there are complex channel-vegetation interactions with tamarisk, giant cane (Arundo donax), and willows and past and future vegetation management practices along the Rio Grande River. Existing terrestrial laser scans (TLS) will be used to map invasive vegetation extent and changes that alter hydraulics and sediment transport characteristics. Our research results will be useful to federal land managers addressing ongoing invasive vegetation issues and will assist in predicting future post-removal channel change to protect the time and financial investments of large-scale invasive species removal projects.
     
     
     
  • The Upper Colorado River Endangered Fish Recovery Program has requested experimental flow releases from Flaming Gorge Dam for (1) elevated summer base flows to promote larval endangered Colorado pikeminnow, and (2) midsummer spike flows to disadvantage spawning invasive smallmouth bass. This white paper explores the effects of these proposed flow modifications on riparian vegetation and sediment deposition downstream along the Green River. Although modest in magnitude, the elevated base flows and possible associated reductions in magnitude or duration of peak flows would exacerbate a long-term trend of flow stabilization on the Green River that is already leading to proliferation of vegetation including invasive tamarisk along the channel and associated sediment deposition, channel narrowing and channel simplification. Midsummer spike flows could promote establishment of late-flowering plants like tamarisk. Because channel narrowing and simplification threaten persistence and quality of backwater and side channel features needed by endangered fish, the proposed flow modifications could lead to degradation of fish habitat. Channel narrowing and vegetation encroachment could be countered by increases in peak flows or reductions in base flows in some years and by prescription of rapid flow declines following midsummer spike flows. These strategies for reducing vegetation encroachment would need to be balanced with flow needs of other riverine resources. Use of high flows to remove unwanted vegetation is constrained by current operational guidance for Flaming Gorge Dam, which attempts to limit spills (i.e., flows greater than 8600 ft3/s) that might contribute to cavitation and lead to dam safety concerns. Therefore, reversing vegetation encroachment is more likely to succeed if implemented while plants are still small. Annual monitoring of near-channel vegetation and topography would enable managers to prescribe a timely hydrologic response in case the proposed flow experiments lead to vegetation encroachment and habitat degradation.

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    Assessment of Geomorphic Impacts of Vegetation Removal on the Colorado River in the Grand Valley, Colorado
     
    Gigi Richard1*
    1Fort Lewis College, Durango, CO, USA; garichard@fortlewis.edu
     
     
    Recent and expanding efforts to remove tamarisk and Russian olive (TRO) from riparian zones may contribute to increased channel mobility and bank erosion, as evidenced by significant bank erosion associated with the 2011 peak flow in areas where tamarisk had been removed along the Colorado River in the Grand Valley, Colorado. The purpose of this study was to assess changes in channel mobility following tamarisk removal along a 51-km reach of the Colorado River in western Colorado via GIS analysis of repeat aerial photos and field surveying of channel cross-sections at vegetation removal sites. The study included field surveys of channel cross-sections at three TRO removal sites (2013-2018), GIS analysis of channel change using repeat aerial photos from 2002 to 2016, and aerial drone surveys of the three study sites in 2017.  Results revealed that channel change and bank erosion occur along this reach of river regardless of vegetation removal efforts. During the 2007-2012 time, period erosion sites where TRO removal occurred were significantly wider (nearly 50%, p<0.05) than erosion sites where vegetation removal did not occur. The results of this study do not clearly indicate if time after vegetation removal plays a role in changes in rate of erosion. For example, two of the sites experience 10 meters of erosion three to eight years following TRO removal, while three other sites experience decreasing rates of erosion following TRO removal. Method of TRO removal was not included in the analysis but could provide more insights into the variability in erosion rates following TRO removal.
     
     
     
     
     
  • A great deal of effort has been devoted to developing guidance for stream restoration and rehabilitation. The available resources are diverse, reflecting the wide ranging approaches used and expertise required to develop stream restoration projects. To help practitioners sort through all of this information, a technical note has been developed to provide a guide to the wealth of information available. The document structure is primarily a series of short literature reviews followed by a hyperlinked reference list for the reader to find more information on each topic. The primary topics incorporated into this guidance include general methods, an overview of stream processes and restoration, case studies, and methods for data compilation, preliminary assessments, and field data collection. Analysis methods and tools, and planning and design guidance for specific restoration features, are also provided. This technical note is a bibliographic repository of information available to assist professionals with the process of planning, analyzing, and designing stream restoration and rehabilitation projects. 
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    Floodplain Plant Community and Stream Channel Response More than Ten Years Following Tamarisk and Russian Olive Removal in Canyon De Chelly National Monument, Arizona
     
    Lindsay Reynolds1*, Kristin Jaeger2, Keith Lyons3, Celeste Weiting7, Julianne Scamardo8, David Cooper4, Ellen Wohl5, and Sara Rathburn6
     
    1Colorado State University, Fort Collins, CO, USA; lreynolds@blm.gov;
    2United States Geological Survey, Tacoma, WA, USA; kjaeger@usgs.gov
    3National Park Service, Canyon de Chelly National Monument, Chinle, Arizona, USA; keith_lyons@nps.gov
    4Colorado State University, Fort Collins, CO, USA; david.cooper@colostate.edu
    5Colorado State University, Fort Collins, CO, USA; ellen.wohl@colostate.edu
    6Colorado State University, Fort Collins, CO, USA; sara.rathburn@colostate.edu
    7Colorado State University, Fort Collins, CO, USA; celeste.weiting@colostate.edu
    8Colorado State University, Fort Collins, CO, USA; julianne.scamardo@colostate.edu
     
     
    Riparian areas throughout Canyon de Chelly National Monument (CACH) in northeastern Arizona were invaded by the exotic tree species tamarisk (Tamarix ramosissima, T. chinensis, and hybrids) and Russian olive (Elaeagnus angustifolia) starting in the early 20th century. By 2005 tamarisk and Russian olive dominated the floodplains. Prior to exotic tree invasion, streams in Canyon de Chelly were wide, shallow and braided, with native cottonwood and willows along the margins. Some reaches in the lower, mainstem Chinle Wash, still remain wide and shallow, however, the two primary tributary canyon streams have narrowed and incised 1-5 m over the last 50 years. Both the invasion of exotic trees and the incision of stream channels throughout CACH have created challenges for land managers and Navajo residents. Lower water tables and the lack of regular overbank flooding associated with stream down-cutting have dramatically altered the landscape, hampered farming and traditional grazing, and threatened road-stream crossings throughout the canyon. In an effort to restore native vegetation and an active floodplain in the canyons, the National Park Service, in collaboration with the Navajo Nation, began clearing tamarisk and Russian olive throughout the canyon system in 2005. Between 2005 and 2008, we monitored four intensive study sites in CACH for stream channel characteristics and floodplain vegetation prior to and following exotic plant removal. We compared cut-stump (above-ground stem removal only) and whole-plant (above-ground stem and below-ground root) removal of tamarisk and Russian olive trees to control sites that were not treated. We found that channel change, largely through channel widening, was greatest in the whole-plant removal treatments, but that the cut-stump treatments were more beneficial for promoting native plant communities. However, both the floodplain vegetation and channel morphologic changes were limited by the existing degree of channel incision, the bed and bank material, and subsequent entrenchment. Since 2008, most large floods have been insufficient to inundate historic floodplain surfaces in the intensive study sites, potentially limiting morphologic change through bank erosion, channel widening and reestablishment of native riparian plants. In the summer of 2019, we focused new sampling efforts on quantifying floodplain vegetation composition and channel entrenchment in exotic removal sites. We resampled four, 1 km reaches of floodplain plant communities and associated stream channel in Canyon de Chelly after more than 10 years since exotic tree removal. Preliminary results show that floodplain and terrace vegetation continues to be dominated by exotic and weedy species, but native grasses and herbs persist in large numbers and are doing especially well inside grazing exclosures and native seeding areas. Channel widening is occurring in plant removal sites that have coarse bank materials and less initial incision, leading to the formation of new inset point bars and small inset floodplains. However, channel incision has continued in sites with deeper initial entrenchment and clay layers armoring the banks. Initial findings show that bank material, specifically the presence of clay, may exert a local control on on-going channel incision and widening processes.
     
     
     
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    Biogeomorphic Feedbacks in the Southwestern USA: Exploring the Mechanisms of Geomorphic Change and the Effectiveness of Mitigation Measures
     
    David Dean1* and David Topping1
     
    1U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff Arizona, USA; djdean@usgs.gov
     
     
    Human water use, water resource development, and the proliferation of riparian plants have resulted in extensive geomorphic change to rivers worldwide. In many dryland rivers of the Southwestern U.S., hydrological change, combined with the expansion of riparian plants, have contributed to processes of channel narrowing, floodplain aggradation, and loss of fluvial habitat. Using a series of field studies, we demonstrate how sediment accumulation and vegetation proliferation have resulted in biogeomorphic feedbacks that have promoted channel narrowing and floodplain formation.
     
    In the Little Colorado River in Arizona, water management practices, variations in climate/hydrology, and the resultant expansion of riparian vegetation have resulted in channel narrowing of up to 88%. Narrowing has occurred concurrently with increases in sinuosity and channel roughness and decreases in channel slope. These changes have created a biogeomorphic feedback by increasing flood-wave travel time and contributing to the attenuation of flood peak magnitude, thereby resulting in additional sediment accumulation. In the Rio Grande in Big Bend National Park, channel narrowing and floodplain aggradation has led to the loss of channel capacity, an increase in overbank flooding and continued floodplain accretion even though discharge has declined. In the Rio Grande and Green River, vegetation expansion onto active channel bars has resulted in bar stabilization, caused vertical aggradation of these surfaces, and has converted them to floodplains. Analyses in the Rio Grande, Little Colorado River, and Green River show that vegetation expansion into once active channel environments occurs during consecutive years of low peak flow magnitude.
     
    An understanding of the mechanisms that have driven geomorphic changes in river channels may help to formulate effective mitigation measures. Vegetation removal can have local and reach-scale effects on channel morphology; however, the effectiveness of these actions is dependent upon many variables including the flow regime and upstream sediment supply. At larger scales, the comprehensive measurement of sediment transport, e.g., our measurement programs in the Rio Grande and Colorado, Yampa, and Green rivers, can help managers tailor upstream water releases required to maintain sufficient channel complexity or to maximize sediment export and erosion.
     
     
     
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    Author(s): R. Roy Johnson; Steven W. Carothers; Deborah M. Finch; Kenneth J. Kingsley; John T. Stanley
     
    Fifty years ago, riparian habitats were not recognized for their extensive and critical contributions to wildlife and the ecosystem function of watersheds. This changed as riparian values were identified and documented, and the science of riparian ecology developed steadily. Papers in this volume range from the more mesic northwestern United States to the arid Southwest and Mexico. More than two dozen authors - most with decades of experience - review the origins of riparian science in the western United States, document what is currently known about riparian ecosystems, and project future needs. Topics are widespread and include: interactions with fire, climate change, and declining water; impacts from exotic species; unintended consequences of biological control; the role of small mammals; watershed response to beavers; watershed and riparian changes; changes below large dams; water birds of the Colorado River Delta; and terrestrial vertebrates of mesquite bosques. Appendices and references chronicle the field’s literature, authors, "riparian pioneers," and conferences. >> Volume 2 is also available on Treesearch: https://www.fs.usda.gov/treesearch/pubs/60500
  • Geomorphic Consequences of Russian Olive Invasion and Prospects for Restoration along the Escalante River, Utah
     
    Michael L. Scott1*, Lindsay V. Reynolds2, Patrick B. Shafroth3 and John R. Spence4
     
    1Faculte Affiliate, Colorado State University, Fort Collins, CO, USA; scottmikeski@gmail.com
    2 Riparian & Wetland Lead, National Operations Center, Bureau of Land Management, Lakewood, CO, USA; lreynolds@blm.gov
    3Fort Collins Science Center, U.S. Geological Survey, 2150 Centre Ave., Bldg C, Fort Collins, CO, USA; shafrothp@usgs.gov
    4 National Park Service, Glen Canyon National Recreation Area, Science & Resource Management Division, P.O. Box 1507, Page, AZ, USA; 86040, USA; John_Spence@nps.gov
     
     
    Along rivers, feedbacks between vegetation and fluvial processes contribute to the complexity and dynamics of riparian ecosystems. For example, native and invasive species may establish and persist on active channel bedforms as part of channel narrowing. Using historical aerial photography and dendrochronology, we quantified spatial and temporal patterns of narrowing and vegetation expansion, including native Fremont cottonwood (Populus fremontii) and non-native Russian olive (Elaeagnus angustifolia), along the largely unregulated Escalante River in the southwestern USA. Narrowing was initiated during a mid-20th-century drought. Cottonwood rapidly colonized higher, bar surfaces between the 1950s and 1981. Small numbers of Russian olive established in moist sites during this initial period as the channel narrowed by nearly 80%. After 1981, there was no obvious cottonwood establishment but low channel bars and banks were rapidly colonized by Russian olive, narrowing the channel further. Exponential growth of this large-seeded, shade-tolerant species lagged its introduction by 30 years, apparently because of delayed reproductive maturity, limited seed availability and widespread availability of favorable establishment sites following initial channel narrowing. Sediment trapping, levee formation and modification of channel form by dense, channel-edge bands of Russian olive progressively limited new establishment sites and by 2000, recruitment declined sharply. Catchment-scale removal of Russian olive began in 2010. Anecdotal evidence suggests that post-removal erosion of levees and channel realignment are occurring in some locations but forward-looking monitoring is needed to assess the effectiveness of removal on overall recovery of channel form.
     
     
     
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    Riparian Vegetation Response to High-Magnitude Dam Releases on the Dolores River, SW Colorado
     
    Cynthia Dott1*, Julie Knudson2*
     
    1 Department of Biology, Fort Lewis College, Durango CO USA; dott_c@fortlewis.edu  
    2 Purgatoire Watershed Partnership, Trinidad CO US; jknudson@purgatoirepartners.org
      
     
    The Dolores River has a unique history of flow management, including truly unprecedented variation over the last three years due to large snowpack and historic drought in back-to-back years.    
    The Dolores provides habitat for rare native fish species and supports unique assemblages of native riparian vegetation.  McPhee Dam, completed in 1985, reduced spring snowmelt-dominated high flows >50%, from 3000-8000 cfs pre-dam to 800-2000 cfs post-dam, but also raised late summer baseflows.  In drought years, no peak occurs and flows vary from 15-80 cfs.  Since dam completion, only three years have had sufficient snowmelt to allow releases from McPhee that reached magnitudes of 4000 cfs: 1993, 2005 and 2017 (the 2019 peak was nearly 3500 cfs).  Because of reduced peak flows and multiple years of drought and low flow, the banks of the lower Dolores have become armored by vegetation – primarily willows (Salix exigua) and giant reed grass (Phragmites australis) and also tamarisk (Tamarix spp)—with simultaneous losses of bare ground and potential cottonwood seedling germination sites. In 2017 a high magnitude, long duration spill of 4000 cfs was planned to support native fish and riparian ecosystems below the dam.  A consortium of scientists worked with managers to plan for pre- and post-spill monitoring.  We hypothesized that bank scouring and vegetation removal would occur, along with sediment deposition on the floodplain.  We re-occupied several sites below the dam to allow us to compare vegetation structure and composition pre- and post-peak flow release.  Data collected in 2010 showed average willow stem densities of 34.3/m2 at one site below the dam compared to 1/m2 above the dam. In many reaches, mature cottonwoods have been in decline and new cottonwood establishment is limited.  If the 2017 high flow was sufficient to emulate pre-dam floods, we predicted willow stem densities would decline, cover by bare ground would increase, and cottonwood seedlings would establish.  Because of cooler than anticipated spring temperatures, the high flow release was shortened and 4000 cfs flows were maintained for only 3 days, though the total duration of the spill was longer than expected. While we observed no significant changes in willow stem densities post-spill, we did see an increase in cover of bare ground.  Timing of post-spill drawdown was apparently wrong for cottonwood seedling establishment on new open sites in 2017, but increased water table recharge due to high flows benefits established cottonwoods over the long run.  The fact that 2017 was followed by the exceptional drought of 2018, and then another year of prolonged high discharge in 2019 is also likely to have long-term consequences for riparian habitat on the Dolores River.
     
     

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