Bitterroot River Geomorphic FINAL Summary: Mainstem Channel and Bridge Crossings Ravalli County, Montana Karin Boyd Applied Geomorphology, Inc. 211 N Grand Ave, Suite C Bozeman, MT 59715 406-587-6352 Prepared for: Montana Department of Transportation Tony Thatcher DTM Consulting, Inc. 2701 Prospect Avenue 211 N Grand Ave, Suite J PO Box 201001 Bozeman, MT 59715 Helena, MT 59620-1001 406-585-5322 May 1, 2008 Table of Contents 1.0 Executive Summary................................................................................................ 1 2.0 Introduction............................................................................................................. 3 2.1 Project Objectives............................................................................................... 3 2.2 Defining Channel Stability.................................................................................. 4 2.3 Primary Findings................................................................................................. 6 2.4 Document Organization...................................................................................... 7 2.5 Acknowledgements ............................................................................................. 8 2.6 Disclaimer........................................................................................................... 8 3.0 Physical and Biological Setting .............................................................................. 9 3.1 Geology............................................................................................................. 10 3.2 Hydrology ......................................................................................................... 10 3.3 Geomorphology ................................................................................................ 11 3.4 Riparian Vegetation.......................................................................................... 15 3.5 Fisheries............................................................................................................ 15 3.6 Water Quality.................................................................................................... 16 4.0 Geomorphic Assessment Methodology ................................................................ 19 4.1 Base Maps......................................................................................................... 19 4.2 Pre-Field Assessment and Project Reach Characterization .............................. 19 4.3 Project Reach Inventory.................................................................................... 19 4.3.1 Bank Erosion Severity .............................................................................. 20 4.3.2 Bank Protection –Inferred Purpose........................................................... 20 4.4 Bridge Assessment............................................................................................ 20 4.4.1 Historic Photo Analysis ............................................................................ 21 4.4.2 Channel Stability Assessment................................................................... 21 4.4.3 Historic Cross-sections ............................................................................. 25 4.4.4 Channel Geometry.................................................................................... 25 4.5 Channel Profile ................................................................................................. 25 4.6 Geographic Information System (GIS) Applications........................................ 25 5.0 Geomorphology of the Mainstem Bitterroot River............................................... 27 5.1 Slope and Sinuosity........................................................................................... 29 5.2 Floodplain Width and Belt Width..................................................................... 30 5.3 Level I Channel Type........................................................................................ 32 5.4 Diversions ......................................................................................................... 33 5.5 Large Woody Debris......................................................................................... 34 5.6 Bank Erosion..................................................................................................... 35 5.7 Bank Armor....................................................................................................... 37 6.0 Geomorphic Conditions at Assessed Bridges....................................................... 41 6.1 Channel Profile and Pattern.............................................................................. 41 6.2 Floodplain Encroachment ................................................................................. 45 6.3 Active Channel Encroachment.......................................................................... 48 6.4 Channel Stability............................................................................................... 50 6.5 Cross Section .................................................................................................... 53 6.5.1 Silver Bridge ............................................................................................. 53 6.5.2 Woodside Bridge....................................................................................... 54 6.5.3 Victor Bridge............................................................................................. 54 AGI and DTM Bitterroot River Geomorphic Summary i 6.5.4 Bell Crossing............................................................................................. 55 6.5.5 Stevensville Bridge ................................................................................... 56 6.5.6 Florence Bridge......................................................................................... 57 6.5.7 Cross Section Summary............................................................................ 57 6.6 Bank Erosion..................................................................................................... 58 6.7 Bank Armor....................................................................................................... 60 6.8 Site-Specific Bridge Summaries....................................................................... 60 6.8.1 Silver Bridge (RM 55.9) ........................................................................... 62 6.8.2 Woodside (RM 52.9)................................................................................. 66 6.8.3 Victor (RM 44.3)....................................................................................... 68 6.8.4 Bell Crossing (RM 41.4)........................................................................... 72 6.8.5 Stevensville (RM 34.4) ............................................................................. 75 6.8.6 Florence (RM 23.8)................................................................................... 78 7.0 Conceptual Strategies to Minimize Geomorphic Impacts at Bridges ................... 81 7.1 Minimizing Encroachment................................................................................ 81 7.1.1 Active Channel Encroachment.................................................................. 81 7.1.2 Floodplain Isolation .................................................................................. 82 7.1.3 Secondary Channel Continuity ................................................................. 82 7.2 Maintaining alignment at bridges ..................................................................... 83 7.2.1 Tapering the Braid Belt to Improve Long-Term Angle of Approach....... 84 7.3 Minimizing Habitat Loss on Armored Banks ................................................... 87 7.4 Large Woody Debris Passage ........................................................................... 88 7.5 Maintaining Fish Passage.................................................................................. 89 7.6 Removal of Derelict Features ........................................................................... 89 8.0 References............................................................................................................. 91 Appendix A. Glossary of Terms .......................................................................... 95 Appendix B. Project Reach Maps (Plates)........................................................... 99 Appendix C. Historic Aerial Photo Sheets at Bridges ....................................... 101 List of Figures Figure 3-1. Project location map........................................................................................ 9 Figure 3-2. Mean annual hydrograph, Bitterroot River at Darby USGS Gage 12344000. ................................................................................................................................... 10 Figure 3-3. Annual peak discharges, Bitterroot River at Darby 1940-2006; USGS Gage 12344000................................................................................................................... 11 Figure 5-1. Map of project reach showing Bitterroot River morphological zones (Gaueman, 1977)....................................................................................................... 28 Figure 5-2. Longitudinal channel profile of the main stem Bitterroot River, Ravalli County MT...............................................................................................................