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Ecosystem Stressors

Ecosystem Stressors

  • This bibliography is a compendium of state-of-knowledge publications about the threats affecting western U.S. riparian ecosystems. The bibliography is ordered alphabetically and the type of threats discussed in each publication is highlighted. These threats include agriculture, climate change, dam construction, disease, drought, invasive species, fire, floods, flow regulation, forest harvesting, grazing, groundwater depletion, insects, mining, recreation, roads, water diversions, urbanization, and water quality.
  • This document provides an introduction to environmental flows, models for E-flow recovery, describes rivers in peril and watershed health, and provides recommendations for restoring E-flow. 

  • This report spotlights successful, sustainable and economically sensible steps ten communities are taking to make sure they will have water in the decades to come. 

  • Written by 44 of the field's most prominent scholars and scientists, this volume compiles 25 essays on tamarisk--its biology, ecology, politics, management, and the ethical issues involved with designating a particular species as "good" or "bad". The book analyzes the controversy surrounding tamarisk's role in our ecosystems and what should be done about it.


  • The purpose of the Study, funded by the Bureau of Reclamation, was to define current and future imbalances in water supply and demand in the Basin and the adjacent areas of the Basin States that receive Colorado River water over the next 50 years (through 2060), and to develop and analyze adaptation and mitigation strategies to resolve those imbalances.

  • The Colorado River Basin Water Supply and Demand Study (Study), initiated in January
    2010, was conducted by the Bureau of Reclamation’s (Reclamation) Upper Colorado and
    Lower Colorado regions, and agencies representing the seven Colorado River Basin States.
    As defined in the Plan of Study, the purpose of the Study is to define current and
    future imbalances in water supply and demand in the Basin and the adjacent areas of the
    Basin States that receive Colorado River water over the next 50 years (through 2060), and to
    develop and analyze adaptation and mitigation strategies to resolve those imbalances. The
    Study does not result in a decision as to how future imbalances will or should be addressed.
    Rather, the Study provides a common technical foundation that frames the range of potential
    imbalances that may be faced in the future and the range of solutions that may be considered
    to resolve those imbalances.
  • The Seven Colorado River Basin States are implementing a proactive program to meet the needs of the Colorado River Basin and to provide continued stewardship of the Colorado River. As part of this program, 12 potential options were evaluated in terms of water quality, technical feasibility, reliability, environmental factors, and permitting considerations.

  • This biodiversity scorecard provides a snapshot of the current conservation status of Colorado's rare and imperiled species, and its most widespread ecological systems. The Colorado Natural Heritage Program
    took a systematic and repeatable approach to these assessments, focusing on: quality, quantity, threats, and level of current protection. Resulting scores for these factors were then combined to produce an overall conservation status score. Successful implementation of a comprehensive conservation strategy should result in maintaining or improving these scores over time.
  • Abstract:
    Successful rangeland management maintains or restores the ability of riparian plant communities to capture sediment and stabilize streambanks. Management actions are most effective when they are focused on the vegetated streambank closest to the active channel, the greenline, where vegetation most influences erosion, deposition, landform, and water quality. Effective grazing management plans balance grazing periods, especially those with more time for re-grazing, with opportunities for plant growth by adjusting grazing timing, duration, intensity, and/or variation of use and recovery.
    Emphasizing either: a) schedules of grazing and recovery, or b) limited utilization level within the same growing season, is a fundamental choice which drives management actions, grazing criteria, and methods for short-term monitoring. To meet resource objectives and allow riparian recovery, managers use many tools and practices that allow rather than impede recovery. Economic decisions are based on both evaluation of investments and ongoing or variable costs, themselves justified by reduced expenses, increased production, or improved resource values. Ongoing management adjusts actions using short-term monitoring focused on chosen strategies. Long-term monitoring refocuses management to target priority areas first for needed functions, and then for desired resource values. Once riparian functions are established, management enables further recovery and resilience and provides opportunities for a greater variety of grazing strategies.
  • 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.
  • The Roadmap for Considering Water for Arizona’s Natural Areas contains information on the current scientific understanding of water for natural areas and existing legal considerations for providing water to natural areas, examples of where natural areas are already included in water management decisions, and an overview of available paths forward for including natural areas alongside human uses.

  • Based on research conducted on the lower White River in Colorado, it was found that tamarisk establishment enhanced not only sediment deposition that leads to channel narrowing, but also to new vegetation establishment. Plants increased the friction in the channel,thus decreasing water velocity close to plants. Low velocity areas became susceptible to furthervegetation encroachment, particularly if they did not have high velocities for a series of ~4 or more years. As vegetation encroached and changed the shape of the channel, the importance of common and large floods, for vegetation establishment and sediment transport, changed.
    Application of this process-based understanding to future flow regimes will help managers anticipate locations along the channel that are susceptible to vegetation encroachment andchanges to channel width.
  • This document presents a statewide assessment on the potential future influences of a changing climate on species and ecosystems of particular importance to the Bureau of Land Management within Colorado, with the goal of facilitating development of the best possible climate change adaptations to meet future conditions. 
  • This Information Brief by the National Park Services discusses changes in the Santa Cruz River in southern Arizona. 

  • Authors:
    Kent R. Mosher, Heather L. Bateman
    Amphibians and reptiles (herpetofauna) have been linked to specific microhabitat characteristics, microclimates, and water resources in riparian forests. Our objective was to relate variation in herpetofauna abundance to changes in habitat caused by a beetle used for Tamarix biocontrol (Diorhabda carinulata; Coleoptera: Chrysomelidae) and riparian restoration. During 2013 and 2014, we measured vegetation and monitored herpetofauna via trapping and visual encounter surveys (VES) at locations affected by biocontrol along the Virgin River in the Mojave Desert of the southwestern United States. Twenty-one sites were divided into four riparian stand types based on density and percent cover of dominant trees (Tamarix, Prosopis, Populus, and Salix) and presence or absence of restoration. Restoration activities consisted of mechanically removing non-native trees, transplanting native trees, and restoring hydrologic flows. Restored sites had three times more total lizard and eight times more yellow-backed spiny lizard (Sceloporus uniformis) captures than other stand types. Woodhouse’s toad (Anaxyrus woodhousii) captures were greatest in unrestored and restored Tam-Pop/Sal sites. Results from VES indicated that herpetofauna abundance was greatest in the restored Tam-Pop/Sal site compared with the adjacent unrestored Tam-Pop/Sal site. Tam sites were characterized by having high Tamarix cover, percent canopy cover, and shade. Restored Tam-Pop/Sal sites were most similar in habitat to Tam-Pop/Sal sites. Two species of herpetofauna (spiny lizard and toad) were found to prefer habitat components characteristic of restored Tam-Pop/Sal sites. Restored sites likely supported higher abundances of these species because restoration activities reduced canopy cover, increased native tree density, and restored surface water.
    New Findings on the Climate Sensitivity of the Water Balance of the Upper Colorado River Basin
    Chris Milly1
    1 Integrated Modeling and Prediction Division, U.S. Geological Survey
    The structure and function of rivers and riparian environments depend on many factors, and these include both climate and human disturbances. Under a changing climate, river restoration efforts may benefit from information about the future trajectory of climate and its impact on water balance. Because of the central role played by the Colorado River for water supply in the Southwest, many scientific investigations have been undertaken over the years in an effort to define the climate sensitivity of runoff in the Upper Colorado River Basin (UCRB). These investigations have yielded frustratingly inconsistent results. We found that the widely varying estimates of sensitivity of UCRB runoff to climate can be reconciled by recognizing certain shortcomings in the methods that have been used until now. By avoiding these shortcomings, we have considerably narrowed the uncertainty in climate sensitivity of the UCRB. Our analysis shows that (and, importantly, explains why) the amount of reduction in flow that results from atmospheric warming is strongly dependent on the seasonal cycle of snow cover; this finding may be useful in predicting which sub-basins within the UCRB are least/most sensitive to warming.
  • Author(s): Steven W. Carothers; R. Roy Johnson; Deborah M. Finch; Kenneth J. Kingsley; Robert H. Hamre
    In the Preface to volume 1, we discuss the development of riparian ecology as one of the newest of ecological fields that gained significant momentum in the 1950s and 1960s as part of the general “riparian movement” in the United States. The field expanded rapidly throughout the latter half of the 1900s. Volume 2 involves more than two dozen authors - most with decades of experience - who expand upon riparian and other topics introduced in volume 1. Two important recent developments are global climate change and impacts of introduced tamarisk leaf beetles (Diorhabda spp.) in the American West. Other chapters in volume 2 that provide current information evaluate the losses of riparian habitat, including “extirpation” of a large number of mesquite bosques (woodlands) in the Southwest; the restoration of riparian ecosystems damaged by anthropogenic activities; the importance of a watershed; and the importance of riparian ecosystems to recreation. The combination of volumes 1 and 2 examines the evolving understanding of scientific implications and anthropogenic threats to those ecosystems from Euro-American settlement of the region to present. >> Volume 1 is also available in Treesearch:
    Riparian System Responses to Fire and Flood Disturbance in Capulin Canyon, Bandelier National Monument, NM
    Jamie Gottlieb1*, Patrick Shafroth2, Michael Scott3, Craig Allen4
    1Northern Arizona University, Flagstaff, AZ, USA;
    2U.S. Geological Survey, Fort Collins, CO, USA;
    3Faculty Affiliate, Colorado State University, Fort Collins, CO, USA;
    4U.S. Geological Survey, Los Alamos, NM, USA;
    Disturbance is a driver of riparian ecosystem dynamics, and riparian areas are typically resilient to disturbance events.  However, responses to extreme disturbances are not well-documented.  We examined riparian responses to extreme disturbance in a montane canyon in northern New Mexico.  Multiple severe fires burned extensive areas of live and dead organic material in the eastern Jemez Mountains between 1996 and 2011.  Runoff, stormflow discharges, and sediment transport increased greatly after the fires. Capulin Creek flows through a canyon which drains part of the east Jemez Mountains and was severely burned in 2011 and severely flooded in 2013.  Here we report results of repeat sampling of canyon bottom geomorphology and riparian vegetation along six transects in Capulin canyon, sampled before (in 2006) and after (2019) the 2011-2013 fire and flood disturbances.  Sampling included repeat topographic and landform surveys along six monumented cross-sections to determine geomorphic change and woody vegetation cover, basal area, and stem density by species.  We found a dramatic decrease in riparian vegetation between 2006 and 2019.  For example, the mean total basal area (per transect), in 2006 was 17.35 ± 12.92 m2/ha (mean ± standard deviation); whereas in 2019 it was only 0.32 ± 0.49 m2/ha.  Pinus ponderosa, Alnus oblongifolia, Acer negundo, and Juniperus scopulorum accounted for the majority of the basal area pre disturbance, with mean basal areas of 10.15 m2/ha, 4.57 m2/ha, 1.66 m2/ha, and 1.55 m2/ha, respectively.  Post-fire and -flood, mean basal areas for P. ponderosa, A. oblongifolia, and J. scopulorum were all 0 and for A. negundo it was 0.23 ± 0.51 m2/ha.  Other relatively abundant species in 2019, were Quercus gambelii and Salix lutea with mean basal areas of 0.075 ± 0.17 m2/ha and 0.01 ± 0.03 m2/ha, respectively.  Post-disturbance cross-sections revealed significant erosion and a shift from a trapezoidal channel/valley geometry to a wider, braided geometry.  Our results will help to inform future management decisions regarding potential restoration actions in highly disturbed canyon ecosystems at Bandelier National Monument and elsewhere around the West. 
  • 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. 
    • Invasive species disturb ecosystems and threaten biodiversity. Invasive species management, such as biological control, can cause additional disturbances, so quantifying how native species respond to invasive control is important
    to inform best management practices
    • We quantifed southwestern bird communities in sites that varied in the amount of the non-native plant tamarisk (Tamarix spp.), before and after biological control efforts
    • Following biocontrol, we found significant differences in community composition and diversity, and several bird species declined by ≥30%
    • Bird declines were ameliorated in the presence of native vegetation, consistent with the hypothesis that tamarisk biocontrol decreases prey availability and alters microclimate
    • We recommend land managers monitor areas dominated by tamarisk after biocontrol, and if re-establishment of native vegetation is slow or lacking, consider the feasibility of active restoration
    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:
    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;  
    2 Purgatoire Watershed Partnership, Trinidad CO US;
    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.
  • The U.S. Geological Survey’s Grand Canyon Monitoring and Research Center convened a workshop June 23-25, 2015, in Flagstaff, Ariz. for practitioners in restoration science to share general principles, successful restoration practices, and discuss the challenges that face those practicing riparian restoration in the southwestern United States. Presenters from the Colorado River and the Rio Grande basins, offered their perspectives and experiences in restoration at the local, reach and watershed scale. Outcomes of the workshop include this Proceedings volume, which is composed of extended abstracts of most of the presentations given at the workshop, and recommendations or information needs identified by participants. The organization of the Proceedings follows a general progression from local scale restoration to river and watershed scale approaches, and finishes with restoration assessments and monitoring.

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