The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American Southwest R E S E A R C H A N D D E V E L O P M E N T EPA/600/R-08/134 ARS/233046 November 2008 www.epa.gov The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American Southwest by Lainie R. Levick, David C. Goodrich, Mariano Hernandez USDA-ARS Southwest Watershed Research Center Tucson, Arizona Julia Fonseca Pima County Office of Conservation Science and Environmental Policy Tucson, Arizona Darius J. Semmens USGS – Rocky Mountain Geographic Science Center Denver, Colorado Juliet Stromberg, Melanie Tluczek Arizona State University Tempe, Arizona Robert A. Leidy, Melissa Scianni USEPA, Office of Water, Region IX San Francisco, California D. Phillip Guertin University of Arizona Tucson, Arizona William G. Kepner USEPA, ORD, NERL Las Vegas, Nevada The information in this report has been funded wholly by the United States Environmental Protection Agency under an interagency assistance agreement (DW12922094) to the USDA, Agricultural Research Service, Southwest Watershed Research Center. It has been subjected to both agencies peer and administrative review processes and has been approved for publication. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy. Mention of trade names and commercial products does not constitute endorsement or recommendation for use. U.S. Environmental Protection Agency Office of Research and Development i Washington, DC 20460 5765leb08 Acknowledgements We gratefully acknowledge the following people for their valuable comments and review that significantly improved the quality, value, and accuracy of this document: Dave Bertelsen (University of Arizona Herbarium), Trevor Hare (Sky Island Alliance), Jim Leenhouts (USGS), Kathleen Lohse (University of Arizona), Waite Osterkamp (USGS), Sam Rector (Arizona Department of Environmental Quality), Phil Rosen (University of Arizona), Marty Tuegel (U.S. Fish & Wildlife Service), Dale Turner (The Nature Conservancy), and Wilson Yee (USEPA). Unless noted otherwise, all photographs are © Lainie Levick. Suggested citation: Levick, L., J. Fonseca, D. Goodrich, M. Hernandez, D. Semmens, J. Stromberg, R. Leidy, M. Scianni, D. P. Guertin, M. Tluczek, and W. Kepner. 2008. The Ecological and Hydrological Significance of Ephemeral and Intermittent Streams in the Arid and Semi-arid American Southwest. U.S. Environmental Protection Agency and USDA/ARS Southwest Watershed Research Center, EPA/600/R-08/134, ARS/233046, 116 pp. ii Abstract This report represents a state-of-the-art synthesis of current knowledge of the ecology and hydrology of ephemeral (dry washes) and intermittent streams in the American Southwest, and may have important bearing on establishing nexus to traditional navigable waters (TNW) and defining connectivity relative to the Clean Water Act. Ephemeral and intermittent streams make up approximately 59% of all streams in the United States (excluding Alaska), and over 81% in the arid and semi-arid Southwest (Arizona, New Mexico, Nevada, Utah, Colorado and California) according to the U.S. Geological Survey National Hydrography Dataset. They are often the headwaters or major tributaries of perennial streams in the Southwest. This comprehensive review of the present scientific understanding of the ecology and hydrology of ephemeral and intermittent streams will help place them in a watershed context, thereby highlighting their importance in maintaining water quality, overall watershed function or health, and provisioning of the essential human and biological requirements of clean water. Ephemeral and intermittent streams provide the same ecological and hydrological functions as perennial streams by moving water, nutrients, and sediment throughout the watershed. When functioning properly, these streams provide landscape hydrologic connections; stream energy dissipation during high-water flows to reduce erosion and improve water quality; surface and subsurface water storage and exchange; ground-water recharge and discharge; sediment transport, storage, and deposition to aid in floodplain maintenance and development; nutrient storage and cycling; wildlife habitat and migration corridors; support for vegetation communities to help stabilize stream banks and provide wildlife services; and water supply and water-quality filtering. They provide a wide array of ecological functions including forage, cover, nesting, and movement corridors for wildlife. Because of the relatively higher moisture content in arid and semi-arid region streams, vegetation and wildlife abundance and diversity in and near them is proportionally higher than in the surrounding uplands. In the rapidly developing southwest, land management decisions must employ a watershed-scale approach that addresses overall watershed function and water quality. Ephemeral and intermittent stream systems comprise a large portion of southwestern watersheds, and contribute to the hydrological, biogeochemical, and ecological health of a watershed. Given their importance and vast extent, it is concluded that an individual ephemeral or intermittent stream segment should not be examined in isolation. Consideration of the cumulative impacts from anthropogenic uses on these streams is critical in watershed-based assessments and land management decisions to maintain overall watershed health and water quality. iii iv Table of Contents Acknowledgements ....................................................................................................................ii Abstract......................................................................................................................................iii Table of Contents .......................................................................................................................v List of Tables............................................................................................................................vii List of Figures..........................................................................................................................viii 1. Introduction ............................................................................................................................1 2. Location of Ephemeral and Intermittent Streams...................................................................2 3. Definitions ..............................................................................................................................6 4. The Watershed Context ..........................................................................................................7 5. Characteristics, Functions, and Ecosystem Significance .......................................................9 a. Hydrologic Features .........................................................................................................13 i. Variability of arid and semi-arid region flows and floods ............................................15 ii. Types of arid and semi-arid region floods....................................................................17 iii. Transmission losses.....................................................................................................18 iv. Ground-water recharge................................................................................................22 v. Landscape and hydrologic connections........................................................................24 vi. Energy dissipation .......................................................................................................26 vii. Sediment mobilization, storage, transport, and deposition ........................................26 b. Geomorphic Characteristics .............................................................................................28 i. Channel-forming processes ...........................................................................................31 ii. Geomorphic response to land management..................................................................34 iii. Map scale in determining channel network and stream order.....................................35 c. Biogeochemical Functions ...............................................................................................36 i. Cycling of elements and compounds.............................................................................36 ii. Detention of imported elements and compounds.........................................................37 iii. Particulate detention....................................................................................................38 iv. Organic carbon export .................................................................................................39 d. Plant Community Support ................................................................................................40 i. Physiognomy, density, and species composition. .........................................................41 ii. Primary productivity and plant water sources..............................................................43 iii. Temporal and spatial patterns of species diversity......................................................44 iv. Influences of vegetation on ecosystem processes. ......................................................45 v. Vegetation and channel morphology............................................................................46