Bitterroot River

FINAL Geomorphic 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...... 29 Figure 5-3. Average slope and sinuosity for morphologic zones...... 30 Figure 5-4. Widths of inundated floodplain cross section and active braid belt measured at 1-mile increments, Bitterroot River; mile marker is upstream end of increment. 31 Figure 5-5. Number of active channel threads measured at 1-mile increments, Bitterroot River...... 31

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Figure 5-6. Average 100-year floodplain width belt width, and average number of active channels within Bitterroot River geomorphic zones...... 32 Figure 5-7. Level I Channel type (Rosgen, 1994) and associated belt width, Bitterroot River...... 33 Figure 5-8. Distribution of mapped irrigation structures on main stem Bitterroot River.34 Figure 5-9. Distribution of Large Woody Debris aggregates on the main stem Bitterroot River...... 35 Figure 5-10. Bank condition (erosion and armor) summed by 1-mile increments...... 36 Figure 5-11. Bank erosion extent within Bitterroot River morphological zones...... 37 Figure 5-12. Relative lengths of features protected by bank armor, Bitterroot River. .... 38 Figure 5-13. Bank protection extents by 1 mile increments, Bitterroot River...... 39 Figure 6-1. Longitudinal bed profile of Bitterroot River near Silver Bridge...... 42 Figure 6-2. Longitudinal profile of Bitterroot River near Woodside Bridge...... 42 Figure 6-3. Longitudinal profile of Bitterroot River near Victor Crossing...... 43 Figure 6-4 Longitudinal profile near Bell Crossing...... 43 Figure 6-5. Longitudinal profile near Stevensville Bridge...... 44 Figure 6-6. Longitudinal bed profile near Florence Bridge...... 44 Figure 6-7. Average bed slopes at bridges and across corresponding morphological zones...... 45 Figure 6-8. Extents of bridge encroachment into floodplain and floodway, Bitterroot River...... 46 Figure 6-9. Percent floodplain and floodway encroachment by assessed bridges, Bitterroot River...... 47 Figure 6-10. Encroachment of bridge approaches into active channel corridor, Bitterroot River...... 49 Figure 6-11. Channel stability ratings at selected bridges, Bitterroot River...... 50 Figure 6-12. 1939 and 1994 surveyed cross sections of the Bitterroot River at Silver Bridge...... 53 Figure 6-13. 1952 and 1993 surveyed channel cross sections of Bitterroot River at Woodside Bridge...... 54 Figure 6-14. 1963 and 1993 surveyed channel cross sections of Bitterroot River at Bell Crossing Bridge...... 55 Figure 6-15. 1950, 1993, and 2000 surveyed cross sections of Bitterroot River at Stevensville Bridge...... 56 Figure 6-16. 1954 and 1993 cross sections of Bitterroot River at Florence Bridge...... 57 Figure 6-17. Width to depth ratios and percent floodplain and floodway encroachment near six bridges on the Bitterroot River...... 58 Figure 6-18. Percent of severely eroding bank line, Bitterroot River...... 59 Figure 6-19. Severely and moderately eroding bank extents at each bridge showing average erosion extents for corresponding geomorphic zone (red line), Bitterroot River...... 59 Figure 6-20. Extent of bank armoring within ½ mile of bridges relative to geomorphic zone average...... 60 Figure 6-21. 100-year floodplain and floodway width plotted from USDA-NRCS (1995) cross section data...... 61 Figure 6-22. Schematic drawing of angle of approach of river to bridge pier...... 62

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Figure 6-23. Valley bottom cross section from Tucker Crossing showing higher elevation of bed of west branch channel relative to east branch channel...... 72 Figure 7-1. Conceptual strategy for maintaining capillary and secondary channel connectivity on floodplain by installing culverts at road embankments in locations of existing channels...... 83 Figure 7-2. Plan view of permeable training dike and fish habitat structures...... 85 Figure 7-3. Schematic section showing permeable dike concept (Cross section A-A’ from Figure 7-2)...... 85 Figure 7-4. Permeable training dikes to control river approach angle upstream of bridge...... 86 Figure 7-5. Strategies to minimize habitat loss on armored banks include: A) Construction of an inset floodplain bench surface at bankfull discharge, or B) Inter- planting riprap with riparian vegetation...... 88

List of Tables Table 1. TMDL 303(d) listings (2006) for the Bitterroot River...... 17 Table 2. Locations of six bridges evaluated in this study...... 21 Table 3. Stability indicators by Johnson et al. (1999)...... 23 Table 4. Morphological Zones proposed by Gaueman (1977)...... 27 Table 5. Sinuosity and channel gradient of the Bitterroot River by geomorphic section. 29 Table 6. Mapped extent of eroding bank line, Darby to Florence...... 36 Table 7. Extents of observed bank protection types, Bitterroot River...... 37 Table 8. Total length of bank protection in terms of feature protected...... 38 Table 9 Floodplain and floodway encroachment measured just upstream of each assessed bridge...... 46 Table 10. Bridge span length relative to typical channel width in vicinity of bridge...... 49 Table 11. Results of a rapid channel stability assessment at six bridges on the main stem Bitterroot River...... 52 Table 12. Summary of geomorphic conditions at Silver Bridge...... 64 Table 13. Summary of conditions at Woodside Bridge...... 67 Table 14. Summary of conditions at Victor Bridge...... 69 Table 15. Summary of conditions at Bell Crossing...... 73 Table 16. Summary of conditions at Stevensville Bridge...... 76 Table 17. Summary of conditions at Florence Bridge...... 78

List of Photos Photo 1. Silver Bridge looking downstream at the middle pier, October 2002...... 63 Photo 2. View downstream of new bridge to replace Silver Bridge, November 2007.... 63 Photo 3. View northwest towards Bitterroot River showing bridge span over point bar; river channel is at far end of bridge, November 2007...... 64 Photo 4. Looking downstream from Silver Bridge deck with rock riprap on left bank; the new bridge has been constructed at this location, October 2002...... 65 Photo 5. View downstream of Woodside Bridge, October 2002...... 67

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Photo 6. Looking downstream at Victor Bridge (west branch Bitterroot River) , October 2002...... 69 Photo 7. View downstream of Victor Bridge showing left bank bar formation, October 2002...... 70 Photo 8. Culvert on left side of Victor Bridge, October 2002...... 70 Photo 9. View downstream of Bell Crossing Bridge, October 2002...... 74 Photo 10. View upstream to southwest of Stevensville Bridge, October 2002...... 76 Photo 11. View upstream from Stevensville Bridge showing permeable training dikes and vigorous willow community, October 2002...... 77 Photo 12. View downstream from Stevensville Bridge deck, October 2002...... 77 Photo 13. View downstream of Florence Bridge, October 2002...... 79 Photo 14. View west across Bitterroot River on downstream side of Florence Bridge, October 2002...... 79 Photo 15. A new culvert at Victor Crossing provides secondary channel and floodplain connectivity, but may impede fish passage, October 2002...... 89

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1.0 Executive Summary The Bitterroot River is a geomorphically complex system characterized by a wide floodplain, one or more primary channels, and multiple smaller channels that dissect the broad floodplain area. The100-year floodplain of the river is locally over 10,000 feet wide, and the width of the primary channel typically exceeds several hundred feet. The primary river channels contain coarse, shifting bar deposits, and as such, they are prone to lateral migration, bank erosion, and bendway cutoff. The natural rates and patterns of channel migration on the Bitterroot River are altered where the active channel is narrowed at bridge crossings. Some of the geomorphic impacts identified at bridge crossings include impeded down-valley migration of meander bends and altered sediment transport capacities. Sediment deposition upstream of bridges is especially prone to occur when channel changes upstream of a bridge result in delivery of a sediment pulse to the constricted area. These processes can result in deflection of flow paths and increased erosion potential on adjacent riverbanks.

Although natural geomorphic processes are altered to some extent at bridge crossings, these impacts are local in nature, and confined to an area within a few thousand feet of the structure. When summarized by 1-mile increments, the extent of bank erosion and bank armor is typically similar between channel segments that have bridges and segments that do not. Approximately 12% of the banks inventoried in the project reach are armored, however the majority of that armor is protecting non-agricultural private properties and agricultural lands. Less than 2% of the total bank line between Darby and Florence has been armored to protect bridges.

The bridges assessed for this project include Silver Bridge, Woodside Bridge, Victor Bridge, Bell Crossing, Stevensville, and Florence Bridge. Collectively, these bridges depict an array of management strategies that effectively demonstrate means of limiting the impacts of the structures on river process. The consideration of these and other strategies in future bridge design and/or rehabilitation efforts will potentially reduce impacts to the stream corridor, and thereby reduce bridge maintenance requirements associated with bank erosion, accelerated deposition, and shifting channel alignments.

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2.0 Introduction Applied Geomorphology, Inc. (AGI) and DTM Consulting, Inc. (DTM) were contracted by the Montana Department of Transportation (MDT) to finalize a draft report describing a stream study on the Bitterroot River through Ravalli County, Montana. Inter-Fluve, Inc. submitted the original report to MDT in April 2004. In response to that draft submittal, MDT personnel developed a series of review comments. This report is intended to finalize the draft report by addressing those original comments, as well as by updating air photo interpretations with more recent imagery. Additional revisions include a more extensive data analysis, expanded graphical representation of results, and extensive rewriting and reorganization of the text. The field data presented in this report are largely derived from the original field inventory of October 2002 performed by AGI, DTM and Inter-Fluve personnel, although several bridges were briefly revisited in November 2007.

The intent of this report is to provide MDT with a planning-level document and associated spatial data that introduces conceptual options and strategies for minimizing the impacts of transportation infrastructure (bridges, culverts, and road embankments) within the Bitterroot River corridor. The strategies may be considered when new projects are designed, and when existing projects are maintained or reconstructed.

2.1 Project Objectives The objective of this study is to evaluate the impacts of a series of bridges on geomorphic processes of the Bitterroot River, and to assess the relative severity of those impacts in the context of overall river corridor land uses. The assessment includes a 2002 bank inventory of the main channel from near Darby, to the Ravalli-Missoula County line, a distance of approximately 61 river miles. Based on that inventory data and additional field observations, the effects of the transportation system on the pattern and profile of the Bitterroot River are evaluated. Assessments of the channel and bridge configurations as well as changes in geomorphology through time were performed at six bridge structures: Silver Bridge, Woodside Bridge, Victor Bridge, Bell Crossing, Stevensville Bridge, and Florence Bridge. This information is integrated to develop strategies to reduce the impacts of bridges on natural geomorphic processes of the Bitterroot River. The feasibility of strategies presented herein will depend on cost, right-of-way extent, opportunities for collaboration with adjacent landowners, and site-specific engineering analysis.

The bridges evaluated in this study extend from Florence upstream to Hamilton. The Silver Bridge is the southernmost bridge assessed, as specified in the project scope of work. It should be noted that there are three other major bridges south of Silver Bridge, including the Main Street Bridge (RM 58.2) and Angler’s Roost Bridge (RM 62.6) near Hamilton, and the Darby Bridge, which is approximately 4.5 miles upstream of the town of Darby. These bridges are labeled on numerous figures, however they were not included in the overall impacts assessment.

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The first part of this study provides an extensive literature search, identifying previous reports and manuscripts relevant to the project objective. The field inventory component of the project was conducted by floating the entire project reach, utilizing a Global Positioning System (GPS) and data logger to map and store relevant linear and point features. The Bitterroot River Geomorphic Summary, supported by a Geographic Information System (GIS) database, gives the user the ability to visually and spatially analyze multiple scales of data and information that may be updated, expanded and iterated in the future. Likewise, the GIS database provides a foundation for further studies. It is important to recognize that the accompanying Bitterroot River GIS database on CD-ROM can be used in congruence with this report. All data is stored in an ESRI ArcGIS 9.2 Personal Geodatabase. An ArcGIS 9.2 project file (MXD) is included.

2.2 Defining Channel Stability A portion of this assessment is dedicated to describing the “stability” of the Bitterroot River in the vicinity of several bridges. The concept of channel stability is often somewhat contradictory between the fields of engineering and geomorphology, and a discussion of these contradictions is warranted. From a geomorphic perspective, channel stability defines a condition of sediment transport equilibrium, or geomorphic equilibrium. This condition is one in which a stream channel transports inputs of water and sediment at the same rate they are delivered. As such, the channel can transport its water and sediment load over a typical range of conditions without requiring significant changes in channel cross section, slope, or roughness. From a geomorphic perspective, an unstable channel is one that is out of balance with its inputs, such that the channel requires changes to its morphology (e.g. shape and structure) to achieve an equilibrium state. Unstable channels are therefore prone to adjustments in channel cross section or slope due to tendencies to aggrade (experience sediment deposition) or degrade (experience erosion). It is important to note, however, that geomorphic stability does not preclude lateral channel migration and associated bank erosion. These processes are natural for many channels that are geomorphically stable. However, these processes can be accelerated when inputs change and a channel becomes unstable. For example, excessive sediment loading often causes bar formation and increased rates of channel migration, which, if accompanied by loss of channel cross section area or increase in channel slope (shortening), is an unstable state. On alluvial river systems like the Bitterroot River, the channel is considered geomorphically stable if it maintains its overall sediment transport capacity through time, even while experiencing lateral shift and bank erosion.

Geomorphic stability is a concept that reflects the balance between inputs of sediment, flow energy, and channel morphology. It can be assessed on the scale of a channel cross section, or more extensively by reach. On a local scale, bank stability, which reflects the condition of a site-specific stream bank, does not necessarily correlate to channel stability. Rapidly eroding banks are commonly referred to as “unstable”, although they may occur within a channel that is, as a whole, within a state of geomorphic equilibrium.

From an engineering perspective, stability typically describes a level of protection or non-deformability for an engineered feature in a river corridor. For example, bank armor

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designed at a “100-year level of stability” is designed to remain immobile for all discharges up to the 100-year event. In general, engineering stability reflects a level of resilience and maintenance of a certain condition through time.

Throughout this document, concepts of stability with regard to reach-scale geomorphology, site-specific bank conditions, and engineering works will not be used interchangeably. Researchers have differed on their conclusions regarding the geomorphic stability of the Bitterroot River, although in general, the system appears to be in a state that is close to sediment transport equilibrium, as no evidence of long-term systemic aggradation or degradation has been identified. However, this long-term condition is punctuated by localized deposition, erosion, channel shift, and avulsion, especially during flood events. The overall picture is one of a very dynamic channel environment.

Because of the differing perspectives regarding “stability” in river environments, it can be difficult to determine if a stable condition is an optimal condition for river managers, transportation engineers, and corridor stakeholders. Some of the benefits of geomorphic stability, including unimpeded channel migration, to biological systems include the following: · regeneration of riparian vegetation on open bars, creating diverse, multi-age communities of woody riparian species; · creation and maintenance of complex fish habitat through local scour caused by active migration; · creation and maintenance of complex fish habitat through recruitment of woody debris from eroding banks as trees are undercut and fall into the channel; · recruitment of spawning gravels from the channel banks; and, · development of undercut banks and associated fisheries habitat on actively migrating bank lines.

Although channel migration, even if it is associated with geomorphic stability, tends to be a beneficial process for biological systems, it can be difficult to manage at bridges, where a high level of engineered stability is required for protecting the structure and for general public safety. Channel migration results in bank erosion, recruitment of large woody debris, creation of sediment pulses, and changing flow alignments to bridge piers. Bridges are typically designed to optimize water and sediment conveyance based on the channel alignment present at the time of design. Maintaining or recovering that alignment commonly requires extensive river engineering. As such, even in the context of geomorphic stability, designing, constructing, and managing bridge structures within a dynamic river corridor such as the Bitterroot is an inherently challenging prospect.

The presence of a bridge structure in a geomorphically unstable setting may pose significant threats to the infrastructure. Engineered infrastructure can be threatened by three primary means if a river is undergoing adjustments in cross section, slope, or roughness to accommodate changing inputs: · downcutting and threatening of infrastructure elements such as pilings; · aggradation and loss of conveyance through the bridge opening; and, · accelerated bank erosion.

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Even in geomorphically stable settings, bridge structures that encroach into river corridors commonly result in a narrowing of the channel and the creation of backwater during floods. These impacts can alter channel morphology and affect channel alignment to the bridge piers.

A primary objective of this assessment is to assess the geomorphology of the Bitterroot River both regionally and in the immediate vicinity of bridges. Based on this assessment, the impacts of transportation infrastructure on channel processes are summarized. Lastly, several strategies are described that have the potential to reduce those impacts. Most of the strategies presented are at least partially in place at existing bridges.

In summary, the following points should be considered when the concept of “stability” is discussed with respect to bridges and river environments: · Geomorphic stability is a different concept than engineered stability; · The pros and cons of geomorphic stability, including bank erosion and channel migration, vary depending on perspective; · Channel migration is typically considered beneficial to riparian systems and fisheries; · Channel migration can be challenging in bridge environments due to its association with bank erosion, large woody debris production, and changing alignments; · Bridge encroachment can cause local changes to channel morphology; and, · Systemic geomorphic instability can pose significant threats to the bridge structures.

2.3 Primary Findings The key findings of the study are summarized below and discussed in detail in the following sections.

1. The Bitterroot River is a dynamic alluvial river. For example, lateral bend migration distances of up to 1,500 feet since 1995 resulted from high bedload transport conditions. Such events are beneficial for the recruitment of large woody debris to the active channel; creation of complex fish habitat and cover; and regeneration of woody riparian plant communities, such as cottonwood and willow.

2. Geomorphically significant floods that drive the majority of changes in channel pattern and river course occur once or twice per decade. Such flood magnitudes occurred in 1947-48, 1956, 1972-74, 1996-97, and 2003.

3. A hierarchy of channel types exists on the Bitterroot River floodplain; these include the primary active channel, secondary overflow channels, and intricate capillary floodplain channels that are supported by a shallow groundwater table.

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4. Bridges are an established and important component of the Bitterroot River valley transportation infrastructure. It is therefore impractical to propose the exclusion of bridges and associated road embankments from the stream corridor. As a result, some impact by the infrastructure is inevitable, including the displacement of natural features by the bridge footprint. On a dynamic river such as the Bitterroot, it is cost prohibitive to design bridges that do not affect channel processes in some way.

5. The highway system’s most significant impacts to the river morphology occur at bridge crossings. These impacts include encroachment into the floodplain and channel, disruption of down-valley migration of meander bends, and accelerated sediment deposition upstream of the structures. The impacts from sedimentation and bar aggradation are localized, and recovery from channel encroachment, occurs rapidly in the downstream direction.

6. While roadway encroachments and bridge spans have local impacts to channel alignment and pattern, they do not appear to control or trigger reach-scale change in channel pattern. Systemically, channel pattern appears to recover after a short distance (1,000 to 3,000 feet) below bridge spans.

7. This analysis did not find that the selected bridge structures significantly altered channel geometry as defined by changes in width to depth (W/D) ratios measured from USDA-NRCS (1995) cross-section survey data.

8. Over the 61-mile project reach, approximately 12% of the stream banks are armored. Of that total extent, approximately 70% is protecting private property (residences, agriculture, and private roads), 17% is protecting railroad infrastructure, and 12% is protecting highway bridges and grades. Less than 2% of the total bank line has been armored to protect bridges.

9. Inventory and mapping of bank erosion on the left and right banks indicate that there are no significant increases or decreases in bank stability near bridges.

2.4 Document Organization This document is organized in eight primary sections, followed by a Glossary of Terms (Appendix A), detailed Project Reach Maps (Appendix B), and Historic Aerial Photo Sheets at the bridge sites (Appendix C).

Sections 1 and 2 describe project goals and objectives and provide a short summary of the key project findings. Section 3 presents the highlights of the literature review and describes the general character of the Bitterroot River system. Section 3 also provides important information on the physical processes acting on the river, as well as information on the various elements influencing the river.

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Section 4 contains a detailed discussion of the methodologies used in this study, including the project reach inventory, the bridge site assessment, channel profile analysis, and the associated Geographic Information System (GIS).

Sections 5 and 6 present the results of the geomorphic inventory and detailed bridge assessments.

Section 7 discusses a variety of conceptual strategies to minimize the geomorphic effects of transportation system development in a dynamic river system such as the Bitterroot River.

Section 8 contains a list of cited references.

2.5 Acknowledgements This report represents the completion of an original draft document in 2004 by Inter- Fluve, Inc. (Inter-Fluve, 2004). We acknowledge Inter-Fluve’s efforts in the original data collection and completion of a draft report. We also extend our gratitude to Inter-Fluve for their assistance in transferring the original data for use in this report finalization. Kraig McLeod of Montana Department of Transportation provided effective oversight with both contracting and project execution. Our understanding of the Bitterroot River fishery and its limiting factors was greatly enhanced by conversations with Chris Clancy of Montana Fish Wildlife and Parks. We extend our thanks to MDT personnel including Mark Goodman, Nigel Mends, Pat Basting, Bonnie Steg, Linda Dworak, and KC Yahvah for providing insightful review and discussion regarding the draft submittal.

2.6 Disclaimer This planning level document is intended to provide conceptual strategies for the reduction of geomorphic impacts associated with several bridge structures located within the Bitterroot River corridor. Any of the strategies presented would require engineering analysis, risk assessment, and cost feasibility assessments prior to implementation.

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3.0 Physical and Biological Setting The Bitterroot River flows northward approximately 75 miles from near Darby, Montana, to its confluence with the Clark Fork River near Missoula. The river flows through the entire length of the Bitterroot Valley, and its entire watershed encompasses an area on the order of 2,800 square miles. The portion of the Bitterroot River evaluated in this study extends from a point just south of the U.S. 93 Bridge near the Rye Creek confluence northward to the Missoula-Ravalli County line, a distance of approximately 61 river miles (Figure 3-1). The following sections provide a summary of the physical and biological characteristics of the project reach, including a literature review of the geomorphology of the river corridor.

Figure 3-1. Project location map.

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3.1 Geology The Bitterroot Valley is an elongate, north-trending basin defined by the Sapphire Mountains to the east and the Bitterroot Mountains to the west. North of Hamilton, the average width of the valley bottom is approximately 7 miles. Between Hamilton and Darby, the valley bottom is considerably narrower. The river valley margins are comprised of Tertiary and Quaternary age sediments that form high river terraces and fans, mountain foothills, glacial, lacustrine, and glacio-lacustrine landforms (Lonn and Sears, 2001). Within this valley, the active floodplain of the Bitterroot River consists of coarse gravelly alluvial deposits that are up to 3 miles wide. Natural lateral boundaries of the modern Bitterroot River floodplain typically consist of alluvial fans built by tributary streams, and terrace remnants that are present on both the east and west sides of the valley (Gaueman, 1997).

3.2 Hydrology The hydrology of the Bitterroot River is typical of a snowmelt driven system, in which flows increase rapidly during late April and May, and peak runoff tends to occur in late May to early June (Figure 3-2). Flows typically drop through June and July, when the majority of the snowmelt hydrograph recession occurs. Base flows of less than 500 cfs at Darby tend to persist until runoff begins during onset of the next snowmelt cycle in April.

Bitterroot River at Darby Mean Annual Hydrograph Jan Feb 4500 Mar Apr 4000 May Jun 3500 3000 Jul Aug 2500 Sep Oct

(cfs) 2000 Nov Dec 1500 1000 500 Mean Daily Discharge 0 0 50 100 150 200 250 300 350 400 Day of Year

Figure 3-2. Mean annual hydrograph, Bitterroot River at Darby USGS Gage 12344000.

Over the last several decades, flooding on the Bitterroot River has threatened private property, infrastructure, and roads. Floods of over 10,000 cfs were recorded at the Darby gaging station in 1947-48, 1956, 1972, 1974, 1997, and 2003 (Figure 3-3). Since the original field work was performed in fall of 2002, it should be noted that any impacts

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associated with the flood event of 2003 is not reflected in the field inventory data summaries.

Bitterroot River at Darby Peak Annual Streamflow 12000 10000 8000 6000 (cfs) 4000 2000

Peak Annual Discharge 0 1940 1943 1946 1949 1952 1955 1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 Year

Figure 3-3. Annual peak discharges, Bitterroot River at Darby 1940-2006; USGS Gage 12344000.

Five major irrigation diversions are present on the Bitterroot River. Irrigation diversions are also common on tributary streams, resulting in low flow depletions within the system during the irrigation season. Currently, agreements are in place to maintain minimum flows of 400 cfs at Bell Crossing with managed releases from Painted Rocks Reservoir.

3.3 Geomorphology The Bitterroot River consists of a main channel thread that is coarse grained and commonly braided. In places, the channel is relatively narrow and forms a single thread, even at low flow. Elsewhere, braided channel segments, which have multiple threads at low flow conditions, can be identified as relatively wide swaths of unvegetated coarse alluvium (poorly consolidated, recent stream deposits). The wide floodplain corridor of the Bitterroot River also contains an extensive network of relatively small, narrow, sinuous channels that dissect that active floodplain. Groundwater commonly discharges into these channels. Reaches that contain this complex network of channels are referred to as “anastomosing”. According to Gaueman (1997), “the complex channel network found in anastomosing reaches of the Bitterroot River may be best understood in terms of a composite system in which a semi-independent floodplain and groundwater discharge network is superimposed on a braided channel system”.

The Bitterroot River is prone to accelerated rates of erosion, transport, and deposition during large flood events. Undercutting of non-cohesive gravelly alluvial banks and subsequent slumping of the uppermost, relatively cohesive, fine-grained soils appears to be the dominant mode of failure within the project reach. The river typically shifts from a single-thread, straight configuration where the channel parallels a natural lateral boundary, such as a high terrace escarpment or bedrock outcrop, to a braided pattern as the channel transverses the wide floodplain. Valley crossover zones, located where the river traverses from one side of the floodplain to the other, are typically associated with

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high rates of channel shift, and associated recruitment of large woody debris (LWD refers to dead woody material typically greater than 20 inches in diameter) and sediment from the eroding banks. Field observations indicate that these areas also typically support riparian successional trends, as well as complex aquatic habitat.

The physical attributes of the Bitterroot River has been evaluated by several researches over the last several decades (McMurtrey, 1972; Simons et. al. 1981; Cartier, 1984; Gaeuman, 1997).

Senger (1975) provided a synthesis of published and unpublished water resource information, including estimates of average annual water budgets, water quality, flow records, and groundwater data. This report also included an assessment of the effects of logging on water quantity and quality. Simons et al (1981) showed a climatic change occurred in the Bitterroot Basin during the middle of the 20th century. Nearly two decades of drought, and subsequent low snowpack and streamflow (1930-1946) were followed by an increase in precipitation from 1947 to 1978, nearly a 50 percent increase in mean annual peak flow. Consecutive major peak floods in 1947 and 1948 mark the beginning of a wetter period in the watershed. Increase in peak flows and such short intervals between two high flood years have been observed to cause channel instability, especially between Hamilton and Stevensville (Simons et al., 1981).

Cartier (1984) studied the geomorphology, sediment transport and streamflow characteristics of the Bitterroot River. With respect to sediment transport, Cartier concluded that the channel bed of the Bitterroot River is not armored, such that it has an active mobile surface layer, and is therefore likely unlimited by sediment supply. He also suggested that the Bitterroot River is near a “geomorphic threshold” which attributes to low channel-pattern stability, shown by a combined braided and meandering condition. The threshold condition implies that the river is susceptible to perturbations and changes in hydrology and sediment loading.

Ault and Cartier (1984) suggested that active faults trending southwest-northeast cross the Bitterroot River valley at both Darby and Stevensville, and that movement on the faults has resulted in a downdropping of the valley floor within the core of the Bitterroot Valley. The authors attribute the dynamic nature of the Bitterroot River between Darby and Stevensville to this actively downdropping fault block, due to continual sediment deposition within the structural depression.

Gaeuman (1997) suggests that evidence of recent tectonic activity in the Bitterroot Valley is highly speculative due to a lack of supporting seismic or geologic offset data. The Master’s Thesis completed by Gaeuman (1997) provides the most recent, comprehensive analysis and historic geomorphic characterization of the Bitterroot River. The primary objective of Gaueman’s thesis was to document and quantitatively describe historic channel changes between Hamilton and Stevensville. With this data, he identified potential causal mechanisms for these changes. Within his complete study area, which extended from near Darby to the Missoula/Ravalli County line, Gaueman digitized channels on 1937, 1955, and 1987 aerial photography, and compared conditions through

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time. Additional information was derived from hydrologic modeling and physiographic data.

The channel types and features mapped by Gaeuman (1997) on the 1937, 1955, and 1987 aerial photography include the following: 1. Braid belt: The region occupied by bed material and essentially lacking vegetation; 2. Overflow channels: Gravelly swales that exit the braid belt and arc a short distance over the floodplain before terminating or re-entering the braid belt; 3. Secondary channels: Relatively narrow, stable, often sinuous channels outside the braid belt and lacking a visible gravel bed; 4. Capillary channels; Similar to secondary channels but distinguished from them on aerial photographs by frequent and irregular bifurcation, smaller size, and lack of continuity.

Using summary results derived from the digitized features as well as groundwater data, field investigations, and hydrologic analysis, Gaueman (1997) made the following conclusions: · The Bitterroot River consists of a predominantly braided channel that is embedded in a network of narrow, sinuous, minor channels. · The area of channel that consists of unvegetated gravels (braid belt) has widened and straightened since 1937. From 1937 to 1987, the braid belt widened by a total of 28 percent. Most of that widening occurred prior to 1955. This widening may be due to the presence of an anomalously narrow braid belt in the 1930’s due to low flow conditions characteristic of the early 1930’s. · Lateral shifts in the primary channel location are predominantly caused by meander cutoff rather than rapid channel migration. Both braid belt widening and cutoffs are attributed to a high frequency of overbank flow. Local overbank flooding occurs relatively frequently due to the presence of low banks throughout the corridor. · The complex network of minor channels (secondary and capillary channels) in the valley bottom appears to be related to groundwater discharge on the valley floor. Static well level data indicate that shallow water tables and large fluctuations in groundwater depth are associated with a high density of secondary or capillary channels. In the field, capillary channels were observed as commonly terminating at their upstream end at a headcut formed in fine grained floodplain deposits, and commonly, groundwater was emerging at the base of the headcut. · Capillary channels tend to grow and sometimes capture the main braid belt when the channels extend up the river valley due to headcutting, and/or overbank flows drive enlargement and extension of the minor channels. · Groundwater sapping is also a contributing factor in channel widening and bank failure. Fluctuations in the water table appear to promote bank undercutting and caving failure due to rapid drawdown during periods of dropping stage (water depth).

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· Where the valley is steep, floodplain channels tend to split, and the braid belt is relatively wide, whereas where the gradient is low, the channel pattern tends to be relatively simple and narrow. · Within the study reach (Darby to the Missoula/Ravalli county line), there are nine locations where the braid belt splits around large permanent islands. The points at which the braid belt splits tend to be consistently located through time, in areas of anomalously steep valley gradients. However, the relative proportions of flow into each channel around islands can change significantly through time. · Within the central portion of the valley, there are commonly low areas on the eastern margin of the floodplain. These low swales often contain tributary channels that emerge from the foothills and then turn northward to parallel the main river. Relatively high ground separates the main channel from the eastern swales. However, it is unclear as to whether there are breaks in this relatively high ground that could convey floodwaters to the eastern side of the valley. If such gaps exist, “the potential is great for a large-scale avulsion of the main Bitterroot River to the eastern part of the central valley. Based on limited data, it is speculated that such an avulsion is most likely to occur in the vicinities of Willow Creek (Corvallis), Tucker Crossing, or north of Victor Crossing”. · Wide scale aggradation of the Bitterroot River is not evident. Local instances of aggradation, indicated by partially buried debris on the stream bank, were observed, and these instances were typically located in the vicinity of bridges and other structures.

The geomorphic character of the Bitterroot River and its floodplain changes significantly between Darby and Florence. To that end, Gaeuman (1997) identified four geomorphic zones distinguished by changes in dominant valley, channel, and floodplain characteristics, and describes them as follows:

1. The “Darby Section (RM 83-56)” extends from Darby to Hamilton. Within this section of river, the valley bottom and braid belt are both relatively narrow. The river tends to be confined largely to a single active channel. Within this single thread, the channel cross sections shows substantial variability from narrow, single thread conditions to localized braiding. Capillary channels are generally absent in this reach. 2. The “Victor Section (RM 56-36)”, which extends from Hamilton to just south of Stevensville, is much more complex than the Darby Section, as reflected by a wider valley bottom, wider braid belt, and numerous flow splits. Large islands are present due to splitting of the braid belt, and “the wide bottom is intricately etched with numerous secondary and capillary channels”. Much of the braid belt widening that occurred between 1937 and 1955 occurred in this reach. 3. The “Stevensville Section (RM 36-28)”, extending from near Stevensville to approximately 3.5 miles south of Florence, is predominantly straight and single thread, however old channel remnants are identifiable as floodplain swales. Where the valley bottom is relatively wide, capillary channels are common. The valley is locally confined by the Burnt Fork alluvial fan as well as artificial structures, which are common in this reach.

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4. The “Lower Section (RM 28-22)” reaches from south of Florence to the Missoula/Ravalli County line. Within this reach, the channel morphology is highly variable, ranging from a fairly straight, narrow configuration to wide areas supporting multiple channel threads.

The bridges evaluated in this report are located in the Victor Section (Silver, Woodside, Victor, Bell Crossing), the Stevensville Section (Stevensville), and the Lower Section (Florence). Three bridges are present in the Darby Section; however these bridges were not included in the assessment. The areas identified by Gaeuman (1997) as potentially prone to avulsion to the eastern portion of the floodplain are at the Woodside Bridge, and both upstream and downstream of Victor Bridge.

3.4 Riparian Vegetation Sandy fine-grained floodplain soils and coarse gravelly bank and bed materials support a mosaic of woody plant species dominated by an overstory of Ponderosa pine and cottonwood. Lodgepole pine, Black cottonwood, dogwood, birch, and several willow species are also common on the banks and floodplain of the Bitteroot River. Regeneration and recruitment of cottonwood, willow, and various conifer species on newly deposited, exposed bars and floodplain surfaces indicate riparian habitat is relatively healthy with multiple age classes represented throughout the project reach. Despite vigorous and diverse plant community types, even dense vegetative cover appears to contribute little erosion resistance to the channel banks.

3.5 Fisheries The Bitterroot River supports a robust cold water fishery. Game fish present in the main river include brook trout, brown trout, mountain whitefish, rainbow trout, and westslope cutthroat trout. Additional species present include largescale sucker, longnose dace, longnose sucker, northern pike minnow, peamouth, redside shiner, and slimy sculpin (MTFWP: http://fwp.mt.gov). Known primary stressors on the fishery include whirling disease which has been documented in the upper river above Darby and the East Fork Bitterroot River, and dewatering on the mainstem and its tributaries (Chris Clancy, pers. comm., 2007). Dewatering can result in siltation, habitat fragmentation and increased water temperatures.

Chronic dewatering occurred on the Bitterroot River in the 1970s and 1980s. In an effort to reduce the impacts of dewatering, an agreement was made in 2004 by Montana Trout Unlimited, Montana Fish, Wildlife, and Parks, and the state Department of Natural Resources and Conservation to set aside 10,000 acre feet of water from Painted Rocks Reservoir to guarantee a minimum flow of 400 cfs at Bell Crossing (http://www.missoulian.com). This minimum flow agreement was considered a major improvement to mid-1980’s conditions when the river nearly ran dry at Bell Crossing. The flows are typically released from Painted Rocks Reservoir between mid-July and mid-September.

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With regard to fisheries habitat preferences in the Bitterroot River system, tributaries and mainstem side channels clearly provide important spawning and rearing habitat areas. A recent study on Skalkaho Creek, a tributary that enters the Bitterroot River from the east near Hamilton, included electrofishing at irrigation diversions. Researchers captured westslope cuttthroat trout which they considered to be from the Bitterroot River, moving upstream to spawn. Their preliminary results also indicated that small fish moving downstream towards the Bitterroot mainstem were entrained by irrigation ditches (Montana Water Center, 2003).

Senger (1975) concluded that on the Bitterroot River, requirements for a productive fishery include three general components, including water quality, water quantity, and channel and floodplain characteristics that offer adequate spawning sites, food production, resting, security from predators, and wintering habitat. Lentz (1998) examined the distribution and habitat of recently emerged young-of-the-year (YOY) trout in two tributaries, Sleeping Child Creek and Daly Creek. He found that YOY relied heavily on secondary channels where water was shallow and nutrient rich, and that the fish were concentrated at tributary confluences.

Montana Fish Wildlife, and Parks (MTFWP) has developed a Bitterroot River Fisheries Management Plan that outlines fisheries habitat constraints, identifies multiple river users, and suggests that a ‘corridor’ management strategy is necessary with inter-agency, public interest, and private landowner collaboration. The Montana FWP 2007 fishing regulations designate two reaches of the Bitterroot River as artificial lures only, and catch-and-release for all trout. The first reach is one mile downstream of Darby Bridge (RM 78.8; Plate 18) to Como Bridge (RM 72.25; Plate 16), which is managed as a Large Trout Management Area. The other reach extends from the old Tucker Bridge crossing (RM 47.9; Plate 9) to Florence Bridge (RM 23.8; Plate 1), and is designated a Habitat Limited Area.

According to MTFWP (1991), the most significant limiting factors with respect to the Bitterroot River fishery include dewatering of the middle portion of the main stem Bitterroot River, impacts to critical tributaries, and high fish mortality due to irrigation facilities and practices. Marginal fish populations in the Tucker Crossing and Bell Crossing area are considered to be directly related to dewatering (MTFWP, 1991).

3.6 Water Quality Information regarding potential water quality problems on the Bitterroot River has been obtained from the Montana Department of Environmental Quality (MDEQ), which has compiled a 2006 303(d) listing of water quality impairments in the system (http://www.deq.state.mt.us). For these water quality planning purposes, the mainstem Bitterroot River has been divided into segments, each of which has probable causes and sources of water quality impairments identified (Table 1). Both of the segments included in the project area are considered partially supporting of the cold water fishery and aquatic life, and fully supporting with regard to industrial, agricultural, and drinking water beneficial uses.

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The upper-most listed stream segment in the project reach, which extends from the confluence of the east and west forks downstream to Skalkaho Creek near Hamilton, is listed for degradation of riparian vegetation, as well as for copper. Within this reach, MDEQ reported that increased summertime flow releases from Painted Rock Reservoir have helped the trout fishery, and that whereas rainbow trout number have declined in this reach, cutthroat numbers have increased steadily since 1990. However, MDEQ did indicate that summertime temperatures are near the high end of optimal for a mixed salmonid population, and that brown trout populations decline rapidly below Darby, apparently due to increased dewatering. The degradation of the riparian corridor within this reach was reported by MDEQ as evident within grazing areas and along the railroad grade. The copper listing in this segment is due to the fact that three of four copper samples collected upstream of Lost Horse Creek exceeded the chronic standard value. Probable sources of these impairments identified by MDEQ include grazing and streambank modifications, although no probable source is identified for the high copper concentrations.

Table 1. TMDL 303(d) listings (2006) for the Bitterroot River. Bitterroot River Probable Causes Probable Sources Segment East and West Forks Alteration in stream-side or Grazing in Riparian or Shoreline Zones; to Skalkaho Creek littoral vegetative covers Rangeland Grazing; Streambank modifications (Hamilton) and destabilization Copper Source Unknown Low flow alterations Agriculture Irrigated Crop Production Nitrate/Nitrate Agriculture Irrigated Crop Production Wet Weather Discharges Skalkaho Creek Total Phosphorous Agriculture (Hamilton) to Irrigated Crop Production Eightmile Creek Wet Weather Discharges (Florence) Sedimentation/Siltation Agriculture Habitat Modification – Other than hydromodification Wet Weather Discharges Temperature, Water Agriculture Wet Weather Discharges

Between Skalkaho Creek (Hamilton) and Eightmile Creek (Florence), MDEQ identified moderate water quality impairments, primarily due to chronic dewatering and elevated temperatures up to 70 degrees F in the summer. The high temperatures are considered partly related to reduced flows and irrigation returns. MDEQ noted that below Threemile Creek, macroinvertebrates have been impacted from sediment loading, nutrients and organic enrichment. The probable sources for these impairments include agriculture, irrigated crop production, habitat modification, and wet weather discharges (urban- related runoff and stormwater).

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4.0 Geomorphic Assessment Methodology The geomorphic assessment includes an analysis of both existing documents and field data collected in fall of 2002.

4.1 Base Maps To support this study, a new series of black and white aerial photography was collected on May 15, 2002 at a scale of 1:12,000. These photographs were scanned and georeferenced in the project GIS. These photos provided detailed coverage of the primary active river corridor. They were supplemented with USGS orthophotos from 1995 in order to provide complete photographic coverage of the river corridor. A total of nine laminated mapping sheets (24” X 36”) at a scale of 1-inch equals 1,000 feet were used as the base maps for the field mapping effort. These field maps were used for navigating, recording notes, and mapping bank and floodplain features, supplementing GPS data collection efforts.

4.2 Pre-Field Assessment and Project Reach Characterization In order to attain a sense of the regional geomorphology, physical characteristics of the river and floodplain corridor were coarsely evaluated using stereo pairs of black and white aerial photographs. Large scale geomorphic features such as active floodplain, alluvial terraces, alluvial fans, bedrock, and upland surfaces were identified to provide a reconnaissance-level understanding of the physical setting of the project reach.

In order to assess downstream trends in channel and valley conditions, several geomorphic parameters were quantified at 1-mile increments for the entire project reach. The regularly-spaced increments are referenced with respect to their locations in the four morphological zones identified by Gaeuman (1997). The 100-yearfloodplain width was measured from Flood Hazard Area maps (USDA- NRCS, 1995) along a transect at each river mile perpendicular to the main corridor axis. The cumulative width of isolated, non-flooded areas in the corridor was subtracted from the total 100-year flood hazard width. Total meander belt width was measured as the cross-valley width of active channel occupation. Active channels were identified as having observable unvegetated substrate on the air photos.

4.3 Project Reach Inventory The field work for this study, performed from October 20-25, 2002, consisted of floating the main Bitterroot River corridor, 61 river miles, from the U.S. 93 Bridge south of Darby (“Darby Bridge”) to the Missoula-Ravalli County Line. A sub-meter real time differential Global Positioning System (GPS) system was used to collect continuous linear features at 5-meter intervals, as well as specific point features, while floating. All data are referenced to a continuous main channel centerline with linear distance from the Bitterroot River confluence with the Clark Fork River. This comprehensive mapping effort recorded erosion extent and severity, armor extent, and qualitative descriptions of

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primary bed materials. Bank armor mapping units are attributed in terms of type of armor (e.g. full-bank riprap, toe riprap, car bodies, cabled logs, rootwads). Point data include large woody debris (LWD) jams, and man-made structures such as irrigation diversions, bridges, and grade control structures.

4.3.1 Bank Erosion Severity The relative condition of un-armored bank line was described using a methodology adapted from the NRCS (1983) Channel Evaluation Workshop (Zaroban and Sharp, 2001). The NRCS stream bank erosion inventory is a field method that estimates stream bank and channel stability, length of active eroding banks, and bank geometry. Field evaluations of stream bank characteristics are assigned a categorical rating ranging from zero to three. The rating categories are: 1) general stability and evidence of erosion; 2) bank condition and evidence of erosion severity; 3) protection provided by vegetation or rocks; 4) configuration of bank and channel; 5) erodibility of channel bed; and 6) evidence of recent deposition at site. For the first several miles of the inventory, a cumulative rating score of: Mild (0-4), Moderate (5-8), and Severe (9+) was recorded and tallied for each eroding bank segment. Once the overall bank conditions associated with the rating scores were established, an erosion severity was assigned to bank segments based on those conditions (i.e. detailed ratings were not performed for the remainder of the reach due to time limitations). The extent and severity of bank erosion for both banks throughout the entire project reach were recorded using the GPS data logger. Data was subsequently imported into the project GIS for display and analysis.

4.3.2 Bank Protection –Inferred Purpose In an attempt to quantify impacts directly attributable to the highway system, individual segments of bank protection on the channel margins were attributed with regard to the type of feature protected. The feature classes assigned to the bank protection line segments include: highway grade or bridge, railroad grade or bridge, irrigation canal or diversion, and private (agricultural lands, roads, undeveloped property, and buildings). It should be recognized that these attributes reflect an interpretation of current land uses in the vicinity of, and apparently protected by, the armor. As such, the attributes do not define current project ownership, and may not capture original intent where old armor is now protecting a new land use. In general, however, the attributes allow a comparison of the extent of armor protecting various land uses in the river corridor.

4.4 Bridge Assessment Assessments were performed at six bridges to assess the impacts of the structures on geomorphic processes within the river corridor. Four of these bridges are located between Hamilton and Stevensville in the morphological zone referred to as the “Victor Section”, one is at Stevensville, and one is at Florence.

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Table 2. Locations of six bridges evaluated in this study. Bridge River Mile Morphological Zone* Silver 55.9 Victor Woodside 52.9 Victor Victor 44.3 Victor Bell Crossing 41.4 Victor Stevensville 34.4 Stevensville Florence 23.8 Lower Section *Section 3.3 Geomorphology

Two other major bridges on the system near and upstream of Hamilton, including the Main Street Bridge (RM 58.2) and the Angler’s Roost Bridge (RM 62.6), were not evaluated as part of this study. These bridges are labeled on numerous figures, however they were not included in the overall impacts assessment.

4.4.1 Historic Photo Analysis For each bridge, a series of historic air photos have been compiled to determine relative changes in channel morphology and floodplain characteristics since the late 1930’s (Appendix C). Aerial photos from 1937, 1955, 1972, 1995, 2002, and 2005 were assessed. A composite panel containing overlays of the channel centerlines is included to show how the river course has shifted throughout the time period of record.

4.4.2 Channel Stability Assessment When assessing the impacts of bridges on channel morphology, it is important to consider the geomorphic stability of the channel immediately at the bridge crossing. Indicators of geomorphic instability can highlight conditions of systemic instability that is independent of the bridge structure, local instabilities caused by the bridge, or potentially a combination of both. The field effort included a rapid channel stability assessment at each of the six bridge crossings based on a methodology developed by Johnson et al. (1999). This technique combines elements of several published channel stability assessment methods (e.g., Pfankuch, 1978; Lagasse et al., 1995; Simon and Downs, 1995; Thorne et al., 1996). The assessment consists of an evaluation of twelve indicators of regional and local stability, with the assignment of ratings ranging from excellent to poor for each indicator (Table 3). The advantages of this rapid assessment method include: 1) use of a combination of non-ambiguous qualitative and quantitative criteria; 2) lack of a single variable that dominates the rating of channel stability; 3) placement of greater weight on more defined criterion; and 4) use of stability indicators covering both regional and local variables.

The twelve qualitative stability indicators used in the rapid channel stability assessment are presented in Table 3 and include the following:

1. Bank soil texture and coherence; 2. Average bank slope angle;

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3. Extent and condition of vegetative bank protection; 4. Degree of bank cutting; 5. Degree of mass wasting or bank failure; 6. Extend of bar development; 7. Debris jam potential; 8. Obstructions, flow deflectors, and sediment traps; 9. Channel bed material consolidation and armoring; 10. Angle of channel approach to the bridge or culvert; 11. Bridge or culvert distance from meander impact point; and 12. Percentage of channel constriction.

Regional instability indicators include numbers 4, 5, and 9, and local instability indicators include 7, 8, and 10-12. The shear stress stability indicator (Johnson et al. 1999), was not calculated at the bridges due to a lack of hydraulic and sediment gradation data. This assessment assumes general transport equilibrium at each bridge at bankfull conditions, and relies on other descriptive parameters to distinguish depositional potential.

The 12 stability indicators were rated, weighted, and summed using the original weighting factors of Johnson et al. (1999). Overall rating ranges and total possible scores were modified to accommodate the absence of the excess shear stress (te) stability indicator. The total possible range of weighted ratings was divided equally into four groups representing modified stability rating ranges that delineate between excellent, good, fair, and poor stability.

Because the shear stress stability indicator was not included in the ratings, a sensitivity analysis was performed on the results to determine if that parameter would have altered the stability category at each bridge. This involved applying the extreme end members of the shear stress rating values, and recalculation of scores. The sensitivity analysis indicated that at two bridges, applying extremely low shear stress values results in a shift in results. These results, which are for Silver and Florence Bridges, are discussed specifically to each bridge in Chapter 6.0.

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Table 3. Stability indicators by Johnson et al. (1999). Ratings Stability Indicator Excellent (1-3) Good (4-6) Fair (7-9) Poor (10-12)

1. Bank soil texture and coherence Clay and silty clay; cohesive Clay loam to sandy clay loam Sandy clay to sandy loam Loamy sand to sand; noncohesive material material 2. Average bank slope angle Bank slopes < 3H:1V (18° or Bank slopes up to 2H:1V Bank slopes to 1.7H:1V (31° or Bank slopes over 60% common on (Pfankuch, 1978) 33%) on both banks. (27° or 50%) on one or 60%) common on one or both one or both banks. occasionally both banks. banks. 3. Vegetative bank protection Wide band of woody vegetation Medium band of woody Small band of woody vegetation Woody vegetation band may vary (Pfankuch, 1978; Thorne et al., with at least 90% density and vegetation with 70-90% plant with 50-70% plant density and depending on age and health with 1996) cover. Primarily hard wood, leafy, density and cover. A majority cover. A majority of soft wood, less than 50% plant density and deciduous trees with mature, of hard wood, leafy, piney, coniferous trees with cover. Primarily soft wood, piney, healthy, and diverse vegetation deciduous trees with young or old vegetation, lacking coniferous trees with very young, located on the bank. Woody maturing, diverse vegetation. in diversity located on or near the old and dying, and/or monostand vegetation oriented vertically. located on the bank. Woody top of the bank. Woody vegetation located off of the bank. vegetation oriented 80-90° vegetation oriented at 70-80° Woody vegetation oriented at less from horizontal with minimal from horizontal often with than 70° from horizontal with root exposure. evident root exposure. extensive root exposure. 4. Bank cutting (Pfankuch, 1978) Little or none evident. Infrequent Some intermittently along Signific ant and frequent. Cuts Almost continuous cuts, some over raw banks less than 15 cm high channel bends and at 30-60 cm high. Root mat 60 cm high. Undercutting, sod-root generally. prominent constrictions. Raw overhangs. overhangs, and side failures banks may be up to 30 cm. frequent. 5. Mass wasting or bank failure No or little evidence of potential Evidence of infrequent and/or Evidence of frequent and/or Frequent and extensive mass (Pfankuch, 1978) or very small amounts of mass minor mass wasting. Mostly significant occurrences of mass wasting. The potential for bank wasting. Uniform channel width healed over with vegetation. wasting that can be aggravated failure, as evidenced by tension over the entire reach. Relatively constant channel by higher flows, which may cracks, massive undercuttings, and width and minimal scalloping cause undercutting and mass bank slumping is considerable. of banks. wasting of unstable banks. Channel width is highly irregular Channel width quite irregular and banks are scalloped. and scalloping of banks is evident. 6. Bar development (Lagasse et Bars are mature, narrow relative Bars may have vegetation Bar widths tend to be wide and Bar widths are generally greater al., 1995) to stream width at low flow, well and/or be composed of coarse composed of newly deposited than ½ the stream width at low vegetated, and composed of gravel to cobbles, but coarse sand to small cobbles flow. Bars are composed of coarse gravel to cobbles. minimal recent growth of bar and/or may be sparsely extensive deposits of fine particles evident by lack of vegetation vegetated. up to coarse gravel with little to no on portions of the bar. vegetation.

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Table 3. Stability indicators by Johnson et al. (1999). Ratings Stability Indicator Excellent (1-3) Good (4-6) Fair (7-9) Poor (10-12)

7. Debris jam potential (Pfankuch, Debris or potential for debris in Small amounts of debris Noticeable accumulation of all Moderate to heavy accumulations 1978) channel is negligible. present. Small jams could be sizes. Moderate downstream of various size debris present. formed. debris jam potential possible. Debris jam potential significant. 8. Obstructions, flow deflectors, Rare or not present. Present, causing cross Moderately frequent and Frequent and often unstable and sediment traps currents and minor bank and occasionally unstable causing a continual shift of (Pfankuch,1978) bottom erosion. obstructions, cause noticeable sediment and flow. Traps are erosion of the channel. easily filled causing channel to Considerable sediment migrate and/or widen. accumulation behind obstructions. 9. Channel bed material Assorted sizes tightly packed, Moderately packed with Loose assortment with no Very loose assortment with no consolidation and armoring overlapping, and possibly some overlapping. Very apparent overlap. Small to packing. Large amounts of (Pfankuch, 1978) imbricated. Most material > 4 small amounts of material < 4 medium amounts of material < 4 material < 4 mm. mm. mm. mm. 10. High flow angle of approach 0° < a < 5° 5° < a < 10° 10° < a < 30° a > 30° to bridge or culvert (Simon and Downs, 1995) a

11. Bridge or culvert distance Dm > 35 m 20 < Dm < 35 m 10 < Dm < 20 m 0 < Dm < 10 m from meander impact point (Simon and Downs, 1995)b 12. Percentage of channel 0-5% 6-25% 26-50% > 50% constriction (Simon and Downs, 1995) Note: Ranges of values in ratings columns provide possible rating values for each factor. a a = approach flow angle to bridge or culvert. b Dm= distance from bridge or culvert upstream to meander impact point.

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4.4.3 Historic Cross-sections Historic cross-sections surveyed beneath the bridge deck centerline were compared at the six bridge sites by plotting MDT hydraulic surveys and/or design drawings with more recent NRCS sections surveyed in 1992 and 1993. Natural Resources Conservation Service (NRCS) sections are tied to the North American Vertical Datum of 1988 (NAVD-88). It was assumed that all historic bridge surveys prior to 1988 were based on 1934 U.S. Coast and Geodetic Survey (USCGS) points. Subsequent NAVD-88 based cross section points were estimated to be 3.5 feet higher than the USCGS-based data. X- Y coordinates were manually extracted from hardcopies of historic pre-bridge channel surveys and plotted in Microsoft Excel spreadsheets with the NRCS sections from 1992- 93. The horizontal axis was determined by identification of common points where possible. The accuracy of this exercise was limited, however, the overall goal was to determine gross changes in channel geometry and relative bed elevations that might indicate net aggradation or degradation under the bridge structures.

4.4.4 Channel Geometry Widths to depth ratios (W/D) of the primary (main stem) channel were estimated near the six bridge sites using NRCS survey data from 1992 and 1993. Estimates of bankfull width were determined from bankfull indicators observed in the field such as the lower limit of woody vegetation, changes in bank substrate, aerial photographs, and corresponding grade breaks measured from plotted NRCS sections near and at the six bridges. A total of five cross sections were analyzed in vicinity of the bridge structures. Generally, sections were spaced 1500-3000 feet above and below the bridge, immediately upstream and downstream from the bridge, and right under the bridge/roadway centerline. Maximum bankfull depth was estimated by subtracting the corresponding bankfull elevation from the channel (thalweg) bed elevation. A bankfull depth immediately under the bridge (centerline) was calculated by averaging the bankfull depth of the two sections immediately above and below the bridge. However, channel constriction and scour immediately under the bridge suggest that this is only an approximation at best.

4.5 Channel Profile The data used in the assessment of channel profile are derived from the NRCS (1995) Flood Plain Management Study. This report contains channel bed survey data for all of the selected bridges except Victor Bridge. Longitudinal profiles of the channel bed (thalweg) near each bridge were plotted for a distance of approximately 4,000 to 7,000 feet, depending on the extent of survey data. The effects of the bridge structures on channel grade were noted such as slope breaks and the shape of the bed profile upstream and downstream of the bridge crossing.

4.6 Geographic Information System (GIS) Applications To support this project, a project GIS was developed using ESRI ArcGIS 9.2. All field mapping data, GPS features, imagery and associated base data were integrated to allow

AGI and DTM Bitterroot River Geomorphic Summary 25 for spatial analysis and presentation of project information. Project specific data collected during the field investigation are compiled in an ESRI Personal Geodatabase. Associated base data such as roads, boundaries and images are generally stored as ESRI Shape files or georeferenced raster images.

Key data layers included with this GIS are: · 1995 and 2002 vertical aerial photograph images; · digital photos from field investigation and associated photo points; · channel bed and bank attributes, segment lengths, and their relative condition; · bank protection types and estimated “responsible party”; · point data; large woody debris, irrigation diversion structures, weirs, and miscellaneous (other); and · roads, bridges, infrastructure, and in-channel structures.

Hardcopy map plates generated from the GIS are presented in Appendix B.

The GIS and the supporting database may be easily updated and expanded with new information, data, and tools for effectively monitoring the geomorphology of the Bitterroot River.

AGI and DTM Bitterroot River Geomorphic Summary 26

5.0 Geomorphology of the Mainstem Bitterroot River A summary of the general geomorphology of the mainstem Bitterroot River provides some context for the assessment of more site-specific conditions at bridges. Geomorphic parameters on the mainstem are summarized by 1-river mile increments. The mile markers are based on the 1995 river centerline, starting at the confluence with the Clark Fork River. References to river miles reflect the upstream end of the increment; that is, data depicted for RM 45 reflect conditions from RM 45 to RM 44. A second spatial scale presented is based on the morphological zones described by Gaueman (1997). These zones reflect the general channel pattern (planform) of the Bitterroot River between Darby and the Missoula/Ravalli County line (Table 4; Figure 5-1).

Table 4. Morphological Zones proposed by Gaueman (1977). Morphological RM Location General Planform Character Zone/Section Darby 83-56 Darby to Hamilton Relatively narrow valley bottom and braid belt; typically single thread. Victor 56-36 Hamilton to just Wide valley bottom, wide braid belt, south of and numerous flow splits, numerous Stevensville secondary and capillary channels. Stevensville 36-28 Near Stevensville Straight and single thread, with channel to Florence remnants as floodplain swales. Where the valley bottom is relatively wide, capillary channels are common. Lower 28-22 South of Florence Highly variable morphology, ranging to county line from a fairly straight, narrow configuration to wide areas supporting multiple channel threads.

AGI and DTM Bitterroot River Geomorphic Summary 27

Figure 5-1. Map of project reach showing Bitterroot River morphological zones (Gaueman, 1977).

AGI and DTM Bitterroot River Geomorphic Summary 28

5.1 Slope and Sinuosity Channel profile data for this study was limited to USDA-NRCS (1995) floodway survey data collected in 1992 and 1993. A plot of the invert bed elevation (thalweg) of the main stem of the Bitterroot River channel project reach is concave in shape, which is typical of a mountainous alluvial valley (Figure 5-2). The channel bed slope is steeper near Darby and it gradually flattens towards the northern end of the valley at Florence.

Bitterroot River Bed Profile

4000

3900

3800

3700 ---Florence ---Stevensville ---Bell Crossing ---Victor ---Woodside ---Silver ---Main Street ---Angler's Roost ---Darby 3600

3500

3400

3300 Elevation (feet) NAVD-88 3200 Note: Data from USDA-NRCS (1995) Floodplain Management Study. 3100

3000 20 30 40 50 60 70 80 River Miles

Figure 5-2. Longitudinal channel profile of the main stem Bitterroot River, Ravalli County MT.

The average slope of the total project reach is 0.21% (Table 5; Figure 5-3). When broken down by segments, the Darby Zone and the Victor Zone are the steepest, with bed gradients in excess of 0.2%. Downstream of Stevensville, average bed slopes are approximately 0.1 %, or half as steep. Sinuosity, defined as the channel length divided by the valley length, provides a measure of how tortuous a river course is. Based on 2002 aerial photographs, the Bitterroot River is characterized by relatively consistent and moderate sinuosity of approximately 1.2.

Table 5. Sinuosity and channel gradient of the Bitterroot River by geomorphic section.

Average Bridge/Zone Bed Slope Sinuosity Total Project Reach 0.21% 1.16 Lower Section 0.11% 1.18 Stevensville Section 0.10% 1.09 Victor Secton 0.26% 1.2 Darby Section 0.22% 1.15

AGI and DTM Bitterroot River Geomorphic Summary 29

Bitterroot River Slope/Sinuosity

0.30% 1.5 Slope 0.25% Sinuosity 1.4 0.20% 1.3 0.15% 1.2 0.10% Sinuosity Channel Slope 0.05% 1.1 0.00% 1 Total Reach Project Zone Victor Zone Darby Zone Lower Zone Stevensville

Figure 5-3. Average slope and sinuosity for morphologic zones.

5.2 Floodplain Width and Belt Width Belt width refers to the width of the main channel corridor, which generally reflects the amplitude of major bendways. Between Darby and Hamilton (Silver Bridge), in the upper portion of the Bitterroot Valley, the belt width is typically less than 1000 feet wide (Figure 5-4). Downstream of Silver Bridge, belt width increases, maintaining a relatively constant width of approximately 2000 feet between Silver Bridge and Florence Bridge. The width of the cross section inundated during a 100-year flood event shows more variability than that of the meander belt; however general downstream patterns are similar. In the upper portion of the river valley, the floodplain is typically less than 2000 feet wide; at Silver Bridge, the floodplain progressively widens downstream to over 9000 feet between Victor Bridge and Bell Crossing (Figure 5-4). Downstream of Bell Crossing, the floodplain narrows towards Stevensville. Below Stevensville Bridge, it widens again to approximately 1 mile as it approaches Florence Bridge. One notable area with respect to both the floodplain and meander belt widths is the area just downstream of Stevensville Bridge, where the river corridor is markedly narrow.

AGI and DTM Bitterroot River Geomorphic Summary 30

Bitterroot River 100-yr Floodplain Width and Active Belt Width 12000 Belt Width

10000 100-yr Floodplain ---Bell Xing ---Victor Br

8000 ---Florence Br ---Woodside Br

6000 ---Stevensville Br ---Angler's Roost Width (ft) ---Darby Br 4000 ---Main St ---Silver Br

2000

0 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 Miles above Clark Fork Confluence

Figure 5-4. Widths of inundated floodplain cross section and active braid belt measured at 1-mile increments, Bitterroot River; mile marker is upstream end of increment.

A count of the number of active channels at each 1-mile increment indicates a pattern similar to that shown by floodplain width; that is, the section of Bitterroot River between Silver Bridge and Bell Crossing is characterized by an increasing number of active channel threads (Figure 5-5). These results indicate that upstream of Silver Bridge (Hamilton), the Bitterroot River floodplain is relatively narrow and typically characterized by a single channel. Downstream of Silver Bridge, the floodplain widens and the number of active channel threads increases, commonly reaching three to five active channels. Downstream of Stevensville Bridge, the floodplain width remains relatively high at approximately 1-mile; however, the river is typically confined within a single channel.

Bitterroot River Number of Active Channels 6

5 ---Bell Xing 4 ---Victor Br ---Florence Br ---Stevensville Br ---Woodside Br 3 ---Angler's Roost ---Main St ---Darby Br 2 Number of Channels ---Silver Br 1

0 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 Miles above Clark Fork Confluence

Figure 5-5. Number of active channel threads measured at 1-mile increments, Bitterroot River.

AGI and DTM Bitterroot River Geomorphic Summary 31

When summarized by morphological zone, the channel segment between Darby and Hamilton (“Darby Zone”) has the narrowest average 100-year floodplain width at 1700 feet (Figure 5-6). The average belt width through this section is 772 feet, which is significantly lower than that of the other zones. The widest floodplain extent occurs from Hamilton to near Stevensville (“Victor Zone”), averaging approximately 6500 feet with an average active channel belt width of 1800 feet. This section of river also has, on average, the most active channels (Figure 5-6). The average number of active channel segments for all of the cross sections between Hamilton and Stevensville is 2.6. The cross sections in each of the other zones have an average of approximately 1.5 active channels.

Bitterroot River Corridor Width and Channel Threads by Zone 10000 3.00 Average 100-yr Floodplain Width 9000 Average Belt Width 2.50 8000 Number of Active Channels 7000 2.00 6000 5000 1.50 4000 1.00 Mean Width (ft) 3000 Active Channels Average Number of 2000 0.50 1000 0 0.00 Lower Stevensville Victor Darby Geomorphic Zone

Figure 5-6. Average 100-year floodplain width belt width, and average number of active channels within Bitterroot River geomorphic zones.

5.3 Level I Channel Type Channel pattern by river mile was examined on 2002 aerial photo coverage and classified using a Rosgen (Level I) analysis. As indicated, the Bitterroot River channel pattern shifts back and forth as several channel types: straight, entrenched-B4; meandering-C4; braided- D4; and anastomosing-Da4 (Rosgen, 1994). In general, more confined B4 channel types are located upstream of Silver Bridge and are associated with total belt widths of less than 500 feet (Figure 5-7). Relatively wide, braided (D) and meandering/braided (C/D) channel types are prevalent between Woodside Bridge and Stevensville Bridge. While roadway encroachments and bridge spans have local impacts to channel alignment and pattern, they do not appear to control or trigger reach-scale change in channel pattern.

AGI and DTM Bitterroot River Geomorphic Summary 32

Channel Type and Belt Width

B B/C C 3500 C/D D Da 3000

2500 ---Florence ---Stevensville ---Bell Crossing ---Victor ---Woodside ---Silver ---Main St ---Angler's Roost ---Darby 2000

1500 Belt Width (ft) 1000

500

0 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 River Mile

Figure 5-7. Level I Channel type (Rosgen, 1994) and associated belt width, Bitterroot River.

5.4 Diversions Field data collection included a comprehensive inventory of point data including in- channel irrigation diversion structures. A total of 17 structures were observed in the field (Figure 5-8). The structures mapped include boulder and concrete sills, gravel berms, and rock weirs. One of the largest irrigation diversion structures mapped serves the Republican Ditch at RM 65.5 (Appendix B, Plate 14), immediately below the confluence with Sleeping Child Creek. The diversion is about a 12- foot drop constructed from large concrete and boulders. The backwater above the diversion has resulted in substantial aggradation of fines, which appear to be derived and deposited by Sleeping Child Creek sub-watershed. Other major diversion structures include Hedge Ditch (RM 69.5; Appendix B, Plate 15) and Woodside Ditch (RM 55.8; Appendix B, Plate 11) immediately downstream of Silver Bridge. Less than 2% of the mapped bank armor on the Bitterroot River was identified as directly protecting diversion structures or associated canals.

AGI and DTM Bitterroot River Geomorphic Summary 33

Diversion Structures

Boulder Sills Concrete Gravel Berm 4 Other Rock Weir 3.5

3

2.5 ---Florence ---Stevensville ---Bell Crossing ---Victor ---Woodside ---Silver ---Main St ---Darby 2 ---Angler's Roost 1.5

Structures per Mile 1

Number of Irrigation Diversion 0.5

0 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 River Mile

Figure 5-8. Distribution of mapped irrigation structures on main stem Bitterroot River.

5.5 Large Woody Debris Large Woody Debris (LWD) is typically defined as dead woody material greater than 20 inches in diameter. On the Bitterroot River, LWD tends to form aggregates, which are features consisting of multiple pieces of wood that affect the channel form by inducing local scour or deflecting flow paths. LWD is considered to be an important habitat component within cold water fisheries of Montana, and high densities of LWD aggregates typically reflect dynamic channel conditions where trees are recruited into the channel due to active bank erosion. The 2002 inventory revealed that between Darby and Florence, large woody debris aggregates are most prevalent within the Victor Section from RM 56 to RM36 (Figure 5-9), which corresponds with a wide floodplain area, a high meander belt width, and a network of multiple braiding and anastomosing channels. This section includes the highest concentration of bridges in the reach, including Silver Bridge, Woodside Bridge, Victor Bridge, and Bell Crossing Bridge. Within this reach, LWD aggregate concentrations are not suppressed below typical levels observed on the remainder of the mainstem channel.

AGI and DTM Bitterroot River Geomorphic Summary 34

Large Woody Debris

7

6

5 ---Florence ---Stevensville ---Bell Crossing ---Victor ---Woodside ---Silver ---Main St ---Darby 4 ---Angler's Roost 3

2 Aggregates per Mile

Number of Woody Debris 1

0 20 25 30 35 40 45 50 55 60 65 70 75 80 River Mile

Figure 5-9. Distribution of Large Woody Debris aggregates on the main stem Bitterroot River.

5.6 Bank Erosion The entire 61 miles of the main stem of the Bitterroot River corridor was canoed with a set of base maps compiled from aerial photography and a GPS unit with data logger. The bank lines were inventoried with respect to their erosion severity or presence and type of armor. Appendix B contains the results of the inventory, with inventory results presented graphically on base maps. The maps show a continuous centerline labeled by river mile tics every one tenth of a mile. The centerline marks the path floated along the approximate channel thalweg. The inventoried left and right bank conditions are shown as offsets from that centerline. It is important to note that the offset lines do not represent a mapped bank line location. Also, the inventory only represents those bank protection features visible from the boat at the time of inventory.

The mapped project reach between Darby and Florence is 61 river miles long. The total mapped extent of bank line for the inventory is therefore 122 miles, accounting for both banks. Of this 122 miles of bank line, 36%, or 44.2 miles of bank was mapped as eroding in October 2002 (Table 6). An average of 17% of the total bank length was mapped as experiencing mild erosion, 14% was experiencing moderately severe erosion, and 5% was severely eroding. If the extent of armored bank is excluded from the total mapped extent of bank line, 42% of the unarmored bank line was mapped as eroding.

A plot of total percent eroding bank line shows that between reaches, total eroding bank extents is typically between 20% and 40% (Figure 5-10). When the extent of bank armor is also taken into consideration, the total combined eroding or armored bank lengths exceed 80% in only two reaches, which are both at the town of Darby. On average, approximately half (48%) of the bank line of the entire reach is either eroding or armored.

AGI and DTM Bitterroot River Geomorphic Summary 35

There appears to be no correlation between high extents of bank erosion and the presence of a bridge, although some bridges such as Woodside and Stevensville, have relatively high armor extents, which reduces the amount of exposed bank line susceptible to erosion. When the data are grouped in terms of geomorphic zone, it is apparent that the most extensive erosion was mapped in the upper portion of the Darby Zone, upstream of RM 76. Within this zone, approximately 15% of the total bank line was mapped as severely eroding; within the other zones, severely eroding bank line extents did not exceed 5% of the total mapped bank.

Table 6. Mapped extent of eroding bank line, Darby to Florence. Length Length Mapped Erosion Percent of Total Eroding Eroding Severity Bank Length Bank (ft) Bank (mi) Mild 11,0425 20.9 17% Moderate 90,909 17.2 14% Severe 32,052 6.1 5% Total 233,386 44.2 36%

Bitterroot River Bank Armor Severe Erosion Bank Condition Moderate Erosion 100% Mild Erosion

80% ---Darby ---Main St ---Victor Bridge ---Angler's Roost ---Silver Bridge ---Woodside Bridge ---Bell Crossing

60% ---Florence Bridge ---Stevensville Bridge

40%

Percent of Total Bankline 20%

0% 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 75 77 79 81 83 Miles Above Clark Fork Confluence

Figure 5-10. Bank condition (erosion and armor) summed by 1-mile increments.

AGI and DTM Bitterroot River Geomorphic Summary 36

Bitterroot River Bank Erosion by Zone 60% Severe 50% Moderate Mild 40%

30%

20%

10% Percent Eroding Bank

0% RM 24-21 RM 35-25 RM 55-36 RM 75-56 RM 76-83

Lower Stevensville Victor Lower Darby Upper Darby Geomorphic Zone

Figure 5-11. Bank erosion extent within Bitterroot River morphological zones.

5.7 Bank Armor Approximately 12% of the observed channel bank line within the project reach is armored with some form of bank protection (Table 7). Full bank riprap is the most common form of bank protection within the project reach, as almost 10% of the mapped bank line consists of combined full bank and toe riprap.

Table 7. Extents of observed bank protection types, Bitterroot River. Total Length (ft) Percent of Percent of Type of Bank Left Right Protection Bank Line Protection Bank Bank Total Type Protected Cabled logs 0 998 998 1% 0.2% Car Bodies 1,803 1359 3162 4% 0.5% Concrete (gunnite) 168 0 168 0% 0.0% Concrete Rubble 3,165 251 3415 4% 0.5% Logs 626 1845 2470 3% 0.4% Other 1,003 104 1107 1% 0.2% Toe Riprap 1,216 718 1934 2% 0.3% Full Bank Riprap 34,736 26,283 61,019 78% 9.5% Rootwads 268 3722 3990 5% 0.6% Total Protection 42,983 35,279 78,262 100% 12.0 %

In order to assess the type of bank protection in terms of its purpose, each mapped segment of bank protection is attributed in terms of the type of feature it is protecting. This attribute reflects a designation of land use behind the armor as visible on air photos, and does not imply ownership or condition of the feature. The results indicate that the protection of non-agricultural private property accounts for 53% of the total observed bank protection present in the system. These areas reflect primarily rural residential land

AGI and DTM Bitterroot River Geomorphic Summary 37

use. Approximately 5% of the bank protection is against agricultural fields. The bank protection adjacent to the bridges and highway grades themselves constitute approximately 11% of the total bank protection in the system.

Table 8. Total length of bank protection in terms of feature protected. Total Percent of Feature Protected Length (ft) Total Highway Bridge 8,726 11% Highway Grade 785 1% Agriculture 3,565 5% Canal 882 1% Diversion 208 0% Other Road 9260 12% Private Property 41,090 53% Railroad Bridge 1359 2% Railroad Grade 12,059 15% Total 77,934 100%

Bitterroot River Bank Protection Protected Features 45000 40000 35000 30000 25000 20000 15000

Total Length (ft) 10000 5000 0 Canal Bridge Grade Grade Bridge Private Railroad Railroad Highway Property Diversion Agriculture Other Road Protected Feature

Figure 5-12. Relative lengths of features protected by bank armor, Bitterroot River.

Although on average, 12% of the bank line in the project reach is armored, there is a large degree of variability in bank armor densities throughout the reach. When evaluated on 1 mile increments, bank protection extents range from 0% to over 45% near Woodside Bridge (Figure 5-13).

AGI and DTM Bitterroot River Geomorphic Summary 38

Bitterroot River Bridge Highway Grade Agriculture Canal Protected Features Diversion Other Road 50% Private Property Railroad Bridge 45% Railroad Grade 40% 35% ---Woodside Br

30% ---Angler's Roost ---Main St ---Stevensville Br 25% ---Darby Br ---Victor Br ---Florence Br 20% ---Silver Br ---Bell Crossing 15% 10% Percent of Bank Armored 5% 0% 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 River Mile

Figure 5-13. Bank protection extents by 1 mile increments, Bitterroot River.

AGI and DTM Bitterroot River Geomorphic Summary 39

AGI and DTM Bitterroot River Geomorphic Summary 40

6.0 Geomorphic Conditions at Assessed Bridges The following section contains a description of geomorphic parameters measured in the vicinity of the bridges evaluated in this study (Table 2). Where possible, these conditions are compared to trends measured at a larger scale of geomorphic zone. A local channel profile is included for each bridge site.

The results of this assessment may be summarized as follows: · Channel gradient through the bridges is typically very close to the average reach gradient. Some profiles show localized inflections at the bridges which appear to reflect local scour beneath the bridge (e.g. Woodside Bridge) local deposition upstream of the structure (e.g. Victor Bridge), or an increased gradient due to bendway cutoff (e.g. Stevensville Bridge). · Because the Bitterroot River floodplain is commonly over a mile wide, the road embankments disrupt floodplain continuity at the 100-year flood event. There is no evidence that this encroachment into the floodplain by the road embankment has negatively affected channel stability at the bridges. · At Bell Crossing, Victor, and Woodside, the channel width as measured across proximal bar deposits areas is high due to sediment storage within the reach. As a result, encroachment into this width ranges from 33% (Bell Crossing) to 52% (Woodside). These bridges are prone to impacting downstream migration of depositional features, however the geomorphic impacts of such disruptions appear confined to the area immediately upstream of the bridge.

6.1 Channel Profile and Pattern Where channel slope depicts local anomalies at a bridge structure, these anomalies may be due to vertical grade controls created by the structure, deposition upstream due to backwatering generated by the structure, or local scour effects.

The channel gradient near Silver Bridge (Figure 6-1) is 0.0025 and shows a relatively linear trend. Immediately under the bridge span, about 1.5 feet of local scour is evident. The 0.25% average gradient for the Bitterroot River is consistent with the average value for the Victor Morphological Zone.

AGI and DTM Bitterroot River Geomorphic Summary 41

Silver Bridge

3535

3530 Bed Profile 3525 Linear (Bed Profile) 3520

3515

3510 ---Silver Bridge

3505

3500 Slope = 0.0025 3495

3490

Elevation (feet) NAVD-88 3485

3480

3475 54 55 56 57 58 River Mile

Figure 6-1. Longitudinal bed profile of Bitterroot River near Silver Bridge.

The channel gradient near Woodside Bridge is 0.0027 (Figure 6-2). At the bridge structure itself, the bed elevation lies below the average profile, indicating a few feet of local scour at the bridge.

Woodside Bridge

3490 3485 Bed Profile 3480 Linear (Bed Profile) 3475

3470 Bridge

3465 ---Woodside

3460

3455

3450 Slope = 0.0027

3445 Elevation (feet) NAVD-88 3440

3435

3430 51 52 53 54 55 River Miles

Figure 6-2. Longitudinal profile of Bitterroot River near Woodside Bridge.

The channel gradient at Victor Crossing on the west branch is 0.0024 (Figure 6-3). It is important to note that this bridge was replaced in 2000, and therefore, present bed profile conditions may be quite different than what is shown from 1993 survey data (NRCS, 1995). The Victor profile shows nearly four feet of local scour under the span and a convex profile upstream. The increased bed elevation upstream of Victor Bridge relative to the average profile trend suggests that accelerated sediment deposition has occurred for a distance of approximately ½ mile upstream of the structure.

AGI and DTM Bitterroot River Geomorphic Summary 42

Victor Bridge

3390

3380 Bed Profile Linear (Bed Profile) 3370 Slope = 0.0024

3360 ---Victor Bridge

3350

3340

Elevation (feet) NAVD-88 3330

3320 42 43 44 45 46 47 River Miles

Figure 6-3. Longitudinal profile of Bitterroot River near Victor Crossing.

The Bell Crossing profile (Figure 6-4) also shows a slightly elevated, convex bed profile upstream of the span, which supports field observations of sediment deposition above the structure. The average bed slope through the reach is .0020, which is somewhat lower than the reach average (.0026).

Bell Crossing

3360 Bed Profile 3350 Linear (Bed Profile)

3340

3330 ---Bell Crossing Slope = 0.0020

3320 Elevation (feet) NAVD-88 3310

3300 39 40 41 42 43 44 River Miles

Figure 6-4 Longitudinal profile near Bell Crossing.

The channel gradient near Stevensville Bridge is 0.0007, which is the lowest of the six bridge segments (Figure 6-5), is indicative of a straight, well entrenched, sediment transport reach. The profile shows a distinct concavity upstream of the bridge, which may represent local shortening of the channel upstream of the structure.

AGI and DTM Bitterroot River Geomorphic Summary 43

Stevensville

3280 Bed Profile Linear (Bed Profile) 3275

3270

3265

---Stevensville Bridge Slope = 0.0007

3260

Elevation (feet) NAVD-88 3255

3250 33.0 34.0 35.0 36.0 River Miles

Figure 6-5. Longitudinal profile near Stevensville Bridge.

The channel gradient at Florence Bridge is 0.0015 (Figure 6-6). An increase in bed slope immediately upstream of the span is likely associated with the major chute cut-off or channelization activity that occurred between 1972 and 1995. Although the profile might suggest Florence Bridge acts as a local grade control, the channel bed appears well armored above and below the span.

Florence

3215

3210 Bed Profile Linear (Bed Profile) 3205

3200

Slope = 0.0015 3195 ---Florence Bridge

3190

Elevation (feet) NAVD-88 3185

3180 21 22 23 24 25 26 27 River Miles

Figure 6-6. Longitudinal bed profile near Florence Bridge.

When compared to the reach averages, the average bed profiles are similar at the bridges to general river trends (Figure 6-7). The bed slopes at the Bell Crossing and Stevensville bridges are somewhat lower than the average condition, whereas the local slope at the Florence Bridge is steeper than the reach as a whole. In general, however, the bridges do not show any consistent trend with regard to the relationship between local bed slope and reach-scale conditions.

AGI and DTM Bitterroot River Geomorphic Summary 44

Bitterroot Bridges Slope 0.30% Local Slope 0.25% Zone Average 0.20%

0.15%

0.10%

0.05% Average Bed Slope (%) 0.00% Victor Silver Bell Florence Crossing Woodside Stevensville

Figure 6-7. Average bed slopes at bridges and across corresponding morphological zones.

6.2 Floodplain Encroachment Roadway embankments and bridges cause floodplain and channel encroachment where the road transverses the valley bottom, and bridge structures potentially constrict the active channel and floodplain. For this study the extent of floodplain and channel encroachment at each bridge was measured from the NRCS Floodplain Management Study (USDA- NRCS, 1995). Floodplain base maps contained within this study show the lateral extent of both the 100-year floodplain and the 100-year floodway.

The mapped 100-year floodplain boundaries for the Bitterroot River reflect the lateral extent of floodwater inundation expected during a 100-year discharge event. The floodway is defined in Montana as the corridor that could contain the entire 100-year discharge with a maximum increase in depth of 0.5 feet relative to unconfined conditions. Within that corridor, the floodway is designated as appropriate for land uses that are compatible with periodic flooding. Residential and commercial structures are not allowed in the floodway. Between the floodway margin and the extent of the 100-year floodplain, buildings are allowed under state law if the structure is elevated two feet above the 100-year flood elevation. This area is called the flood fringe. The variability in 100-year floodplain and floodway widths derived from USDA-NRCS (1995) data provides a valuable tool to compare natural valley constriction and expansion with human induced encroachment.

One concern regarding the presence of bridges in river corridors is encroachment into the active floodplain. Where road embankments traverse a floodplain, they can be assessed in terms of their encroachment by the extent of their course that extends above the 100-

AGI and DTM Bitterroot River Geomorphic Summary 45 year water surface elevation. Such floodplain and floodway encroachment is calculated as a percentage of total floodplain width. Floodway and floodplain encroachment are calculated as the non-inundated length of road embankment divided by the 100-year floodway width or the 100-year floodplain width respectively. For example, if 500 feet of road embankment is non-inundated crossing a 1,000- foot wide 100-year floodplain, then percent floodplain encroachment is 50 percent.

For this study, the relative degree of floodplain encroachment at the six bridge structures was evaluated from flood hazard area maps included in the Flood Plain Management Study Bitterroot River, Ravalli County (USDA-NRCS, 1995). The widths of the floodplain and floodway were measured at the cross section located immediately upstream of the bridge (Table 9, Figure 6-8).

Table 9 Floodplain and floodway encroachment measured just upstream of each assessed bridge. Non-inundated Non-inundated Embankment Embankment 100- YR 100- YR Length on Length in Floodplain Floodway Floodplain Floodway Floodplain Floodway Encroachment Encroachment Bridge Width (ft) Width (ft) (ft) (ft) (%) (%) Silver 887 887 500 500 56 56 Woodside 6043 2271 5350 1150 89 51 Victor 7838 6835 5350 5350 68 78 Bell 9052 2060 7700 1600 85 78 Stevensville 1935 1119 1400 700 72 63 Florence 3743 1199 3450 700 92 58

Bitterroot River 100-yr floodplain Bridge Encroachment Non-inundated embankment in floodplain 10000 100-yr floodway 9000 Non-inundated embankment in floodway 8000 7000 6000 5000 4000 Width (ft) 3000 2000 1000 0 Silver Woodside Victor Bell Stevensville Florence

Figure 6-8. Extents of bridge encroachment into floodplain and floodway, Bitterroot River.

AGI and DTM Bitterroot River Geomorphic Summary 46

The results of this analysis indicate that in terms of overall area, the most extensive floodplain encroachment occurs on those bridges located between Hamilton and Stevensville (Woodside Bridge, Victor Bridge, and Bell Crossing). Within these reaches, the floodplain is over 1 mile wide, and hence the encroachment of roads that cross the river corridor is substantial. At all bridges, the floodplain and floodway widths are narrowed by at least 50% (Figure 6-9).

Bitterroot River Bridge Encroachment Floodplain 100 Floodway 90 80 70 60 50 40 30 20 Percent Encroachment 10 0 Silver Woodside Victor Bell Stevensville Florence

Figure 6-9. Percent floodplain and floodway encroachment by assessed bridges, Bitterroot River.

The encroachment of roadways into the floodplain and floodway can alter channel hydraulics by blocking the path of water as it flows down the valley on the floodplain surface. This impediment to overbank flows can result in a concentration and deepening of flows in the main channel beneath the bridge. With respect to channel process, this focusing of flow can result in two primary impacts, including deposition upstream of the structure if a backwater condition is generated, and scour at and downstream of the structure due to flow concentration through the bridge opening. The extent of this impact depends on the nature of floodplain hydraulics, the susceptibility of the channel to altered hydraulics, as well as the bridge configuration.

This assessment did not include a quantitative assessment of floodplain and channel hydraulics on the Bitterroot River. However, based on field observations and bed profile plots, there is no evidence to suggest that the hydraulic impact of the bridges has caused systemic degradation downstream of the structures. For example, at the Florence Bridge, over 90% of the floodplain width is blocked by the highway embankment, yet this bridge site has a high geomorphic stability rating (Section 6.4).

There is some evidence that aggradation has occurred upstream of Bell Crossing and Victor Bridge, suggesting that backwater conditions may limit sediment transport through the reach. However, without a detailed hydraulic and sediment transport study, it is impossible to ascertain whether floodplain encroachment (versus pier alignments with respect to the flow of the river or active channel encroachment) has led to any disruptions

AGI and DTM Bitterroot River Geomorphic Summary 47

in sediment transport continuity. The infrequency of the 100-year flood limits the long- term record of its impact; as such, the empirical assessment of floodplain encroachment should better performed immediately after such an event through detailed channel surveys both upstream and downstream of each bridge.

The width of the 100-year floodplain along the Bitterroot River is typically over 1000 feet wide, reaching 9000 feet in width at Bell Crossing. Preventing any encroachment by transportation infrastructure in such a wide floodplain environment is cost prohibitive. Some degree of impact of the transportation infrastructure on the floodplain environment is therefore inevitable in a system such as the Bitterroot. These impacts include displacement of floodplain habitat by the footprint of the embankment, and potentially by disrupting the continuity of small channels that dissect the floodplain. Other impacts may include reduced floodplain access downstream of the embankment, and a reduction in groundwater recharge volumes during floods. The preservation of secondary channel continuity through embankments is an important aspect of maintaining the geomorphic integrity of the system, and is discussed as a strategy to minimize impacts in Chapter 7.0.

Encroachments into the Bitterroot River floodplain also occur where features such as railroads, roadways, levees, or bank protection parallel the stream valley, and locally encroach into the floodplain area. In many instances, longitudinal roadways and bank protection cut off a portion of the floodplain, alter channel migration patterns, and restrict floodwater access to the floodplain (Thompson, 2000). Longitudinal encroachment by U.S. 93 does not appear common, although a good example of the highway-cutting north across the 100-year floodplain is 2 miles north of Darby at RM 75.7 (Appendix B, Plate 17).

6.3 Active Channel Encroachment Another important factor in the consideration of transportation infrastructure encroachment is the extent to which bridge approaches extend into the active channel corridor. Because of the natural variability in the width of the active channel corridor, and the difficulty in defining its boundaries, it is difficult to evaluate this encroachment condition on the Bitterroot River. Bridges are typically constructed where the active channel width is relatively low, hence encroachment can be overestimated by measurements made off-site. It is still important to consider the channel width at the bridge structure relative to that width measured away from the bridge to help identify areas of encroachment into the corridor. Such encroachment can affect the hydraulic conditions during relatively frequent (e.g. 2-year) flows, and can also affect long-term accommodation of downstream migrating features such as mid-channel bars or point bars. It is critical, however, that results are carefully considered with regard to the local setting, as well as any physical evidence of geomorphic response to channel narrowing.

The bridge span lengths measured off of air photos are typically on the order of 300 feet long (Figure 6-10). The extent to which this results in channel encroachment varies by bridge. Active channel widths, as measured at depositional areas within ½ mile of the bridge, range from less than 350 feet at Stevensville and Florence to 600 feet at Bell

AGI and DTM Bitterroot River Geomorphic Summary 48

Crossing. The relative encroachment of the bridge approaches into the typical corridor width ranges from 7% at Florence Bridge to 52% at Woodside Bridge (Table 10).

Table 10. Bridge span length relative to typical channel width in vicinity of bridge. Typical Channel Approximate Bridge Width in Vicinity Encroachment Comment Bridge Span (ft) of Bridge (ft) Florence Measured just downstream; bridge Bridge 325 350 7% located at narrowing section Stevensville Measured just downstream; bridge Bridge 320 350 9% located at narrowing section Measured immediately upstream on Bell Crossing 400 600 33% mid-channel bar Victor Crossing 340 500 32% Large bar 0.4 miles upstream Woodside Bridge 240 500 52% Series of bars downstream Silver Bridge 400 450 11% Bar immediately upstream

Active Channel Encroachment

Bridge Span Width 700 Active Channel Width 600

500

400

300 Bridge (ft) 200

100

0 Florence Stevensville Bell Victor Woodside Silver Bridge Typical Active Channel Width Away From Bridge Bridge Crossing Crossing Bridge

Figure 6-10. Encroachment of bridge approaches into active channel corridor, Bitterroot River.

The encroachment of a bridge into the active channel reflects an artificial constriction of the corridor that contains depositional features such as point bars or mid-channel bars. At Bell Crossing, Victor Crossing, and Woodside, the channel is relatively wide such that encroachment is more significant than at Stevensville, Florence, or Silver Bridge. As a result, these bridges are more prone to affect the downstream migration patterns of depositional features. At Bell Crossing, this impact is apparent due to the presence of a large depositional bar immediately upstream of the bridge (Section 6.8.4). The other

AGI and DTM Bitterroot River Geomorphic Summary 49

bridges to not display any current disruption in bar form migration, and the effect at Bell Crossing appears limited to the area immediately upstream of the structure.

6.4 Channel Stability Results of the rapid channel stability assessment at the six bridges are presented in Figure 6-11 and Table 11). The six individual bridge structures provide a relatively wide range of channel stability conditions. Stevensville and Florence Bridge received a rating of good, Silver, Woodside, and Victor Bridge rated fair; and Bell Crossing received a rating of poor. A sensitivity analysis of the stability ratings was performed to address the exclusion of the “shear stress stability indicator” in the assessment process, as this parameter was not quantifiable due to data limitations. Results of the sensitivity analysis showed that at Silver Bridge, a low shear stress indicator (reflecting relatively low sediment transport energy) would improve its condition from Fair to Good, and at Florence Bridge, similar results would modify its rating to Good to Excellent. No other bridge rating categories are affected by the excluded parameter.

Channel Stability Ratings

80 Poor 70 60 Fair 50 40 Good Rating 30 20 Excellent 10 0 Bell Silver Victor Florence Woodside Stevensville Bridge

Figure 6-11. Channel stability ratings at selected bridges, Bitterroot River.

Bell Crossing has the highest rating with a score of 72.2, which reflects “poor” conditions of geomorphic stability. Florence Bridge, rated “good” at a score of 28.4, however a low shear stress ratio (minimal erosion potential) would increase its rating to “excellent”. These two ratings depict a range of channel stability conditions at bridge crossings. The most impaired stability indicator at Bell Crossing was the presence of a large mid- channel bar feature immediately upstream that has caused the channel to split and erode adjacent banks on the west and east side of the floodplain. Additionally, the overall distance from the meander impact point is low, vegetation bank protection is degraded, and the average bank slope angle is high. The 100-year floodplain width immediately upstream of Bell Crossing is over 9,000 feet wide, and the active channel is locally on the

AGI and DTM Bitterroot River Geomorphic Summary 50 order of 600 feet wide. As a result, encroachment extent is relatively high at Bell Crossing, which may relate to the poor stability rating

In contrast, Florence Bridge is located at the upstream end of a major valley constriction, a good location for a bridge span based on local geologic control. The right and left floodplain boundary are composed of late Pleistocene alluvial outwash terrace and fan complex (Lonn and Sears, 2001) associated with Eightmile Creek on the east and One Horse Creek on the west valley margin. The right abutment at Florence Bridge is constructed on this 20- foot high terrace bank, a natural boundary that limits lateral channel migration to the east. Florence Bridge encroachment into the active channel is relatively minor, estimated at 7% of the local channel width.

AGI and DTM Bitterroot River Geomorphic Summary 51

Table 11. Results of a rapid channel stability assessment at six bridges on the main stem Bitterroot River.

Silver Woodside Victor Bell Stevensville Florence

Weighted Weighted Weighted Weighted Weighted Weighted Stability Indicator Weight Rating Rating Rating Rating Rating Rating Rating Rating Rating Rating Rating Rating Bank soil texture/coherence 0.6 10 6 10 6 5 3 9 5.4 9 5.4 7 4.2 Avg. bank slope angle 0.6 6 3.6 5 3 3 1.8 10 6 5 3 5 3 Vegetative bank protection 0.8 5 4 5 4 5 4 10 8 3 2.4 3 2.4 Bank cutting 0.4 3 1.2 4 1.6 6 2.4 11 4.4 5 2 4 1.6 Mass wasting or bank failure 0.8 1 0.8 4 3.2 4 3.2 8 6.4 7 5.6 3 2.4 Bar development 0.6 10 6 8 4.8 11 6.6 12 7.2 7 4.2 2 1.2 Debris jam Potential 0.2 5 1 4 0.8 12 2.4 5 1 5 1 4 0.8 Obstructions, deflectors, sediment traps 0.2 9 1.8 4 0.8 5 1 9 1.8 6 1.2 4 0.8 Bed material consolidation/armoring 0.8 11 8.8 9 7.2 10 8 10 8 6 4.8 4 3.2 High flow angle of approach to bridge 0.8 11 8.8 10 8 11 8.8 9 7.2 5 4 4 3.2 Distance from meander impact point 0.8 4 3.2 12 9.6 11 8.8 11 8.8 4 3.2 2 1.6 Percentage of channel constriction 0.8 9 7.2 7 5.6 10 8 10 8 5 4 5 4 Total Rating Score Fair 52.4 Fair 54.6 Fair 58 Poor 72.2 Good 40.8 Good 28.4 Rating with Low Shear Stress Stability Indicator Good Excellent * Modified from Johnson et.al (1999). Overall Rating Ranges Score Excellent R < 22 Good 2267

AGI and DTM Bitterroot River Geomorphic Summary 52

6.5 Cross Section A comparison of channel cross sections through time can help identify temporal adjustments in channel geometry. For this study, cross-sections surveyed immediately under the bridge deck centerline are compared at the six bridge sites using historic MDT hydraulic surveys and/or design drawings and more recent 1992/1993 NRCS sections. All cross sections are plotted as if looking downstream with the horizontal distance in feet starting on the left floodplain margin.

6.5.1 Silver Bridge Historic cross-section comparisons at Silver Bridge are shown in Figure 6-12. The 1939 pre-bridge section indicates the thalweg (deepest point on the cross section) was nearly 3 feet deeper than in 1993, but has remained in the same location along the left channel bed margin. In 1939, undulating topography on the right bank reflects bar deposits; those undulations are gone in 1993, and there is a net degradation in mean bed elevation of about 5 feet. At Silver Bridge, the historic sections have maintained the same cross sectional width, but bar height has decreased and the thalweg bed elevation has aggraded, the result is an increase the width to depth (W/D) ratio from 1939 to 1993.

Silver Bridge MDT 1939 3525 NRCS 1994

3520

3515

3510

3505 Elevation (ft) NAVD-88 3500

3495 100 150 200 250 300 350 400 450 500 550 600 Distance (ft)

Figure 6-12. 1939 and 1994 surveyed cross sections of the Bitterroot River at Silver Bridge.

AGI and DTM Bitterroot River Geomorphic Summary 53

6.5.2 Woodside Bridge At Woodside Bridge, the pre-bridge 1952 section shows a distinct low flow channel on the right side of the cross section (Figure 6-13). The thalweg is almost 5 feet higher in 1993 and has shifted about 50 feet to the left, indicating local aggradation at the thalweg. However, the bar on the left bank has degraded, creating a more trapezoidal shaped active channel. Overall, the width to depth ratio has increased through time at Woodside Bridge.

Woodside Bridge

3480 1952 MDT 3475 1993 NRCS

3470

3465

3460 Elevation (ft) NAVD-88 3455

3450 0 50 100 150 200 250 300 350 400 Distance (ft)

Figure 6-13. 1952 and 1993 surveyed channel cross sections of Bitterroot River at Woodside Bridge.

6.5.3 Victor Bridge No historic cross-section surveys were available for useful comparison on the west branch of the Bitterroot River at Victor Bridge.

AGI and DTM Bitterroot River Geomorphic Summary 54

6.5.4 Bell Crossing The channel stability assessment at Bell Crossing indicates that the geomorphic stability of the channel at the bridge is low. A comparison of 1963- MDT and 1993- NRCS sections indicates major change in bed form and elevation over that 30-year time frame (Figure 6-14). In 1963, the left side of the cross section contains a distinct secondary channel. A 5-foot high bar about 150 feet wide separates the overflow channel from the main channel on the right. In 1993, the channel capacity has greatly increased; removal of the bar and channel bed scour of over 8 feet has occurred. Likewise, formation of a large secondary channel on the right bank margin and a steep-faced narrow bar indicates temporal shifts in channel form and capacity. Due to the dynamic channel behavior upstream of Bell Crossing, shifts in channel geometry at the bridge structure will likely continue.

Bell Crossing

3345 1963 MDT 1993 NRCS 3340

3335

3330

Elevation (ft) NAVD-88 3325

3320 0 100 200 300 400 500 600 Distance (ft)

Figure 6-14. 1963 and 1993 surveyed channel cross sections of Bitterroot River at Bell Crossing Bridge.

AGI and DTM Bitterroot River Geomorphic Summary 55

6.5.5 Stevensville Bridge Historic cross-section comparisons at Stevensville are shown in Figure 6-15. Three years of historic cross-sections were available for this study. MDT hydraulic cross-section data was surveyed in 1950 and again in 2000 prior to bridge deck widening and structural improvement. In 1950, the channel thalweg was located towards the left (west) bank with a 6-foot high, 90-foot wide bar and overflow channel on the far right bank. In 1993, this bar feature is absent, the channel is trapezoidal, and total channel area has increased. By 2000, the channel downcut over 3.5 feet and the thalweg shifted to the (east) middle of the channel and in filled the right bank area.

Stevensville Bridge

3285 1950 MDT 1993 NRCS 3280 2000 MDT

3275

3270

3265 Elevation (ft) NAVD-88 3260

3255 0 50 100 150 200 250 300 350 Distance (ft)

Figure 6-15. 1950, 1993, and 2000 surveyed cross sections of Bitterroot River at Stevensville Bridge.

AGI and DTM Bitterroot River Geomorphic Summary 56

6.5.6 Florence Bridge Historic cross-section comparisons at Florence Bridge indicate that the cross section remained stable between 1954 and 1993 (Figure 6-16). During this time frame, the bed elevation of the active channel has decreased by less than one half foot. The channel has maintained a uniform trapezoidal geometry, typical of its well-armored bed and relatively low width to depth ratio. Design and construction of the west highway embankment and left bridge abutment placed up to 16 feet of fill across the existing (pre-1956) floodplain. The result is high floodplain encroachment at Florence, but negligible impacts to channel geometry, channel pattern, sediment transport, and flow conveyance.

Florence Bridge

3220

3215

3210

3205

3200 Elevation (ft) NAVD-88 3195 1954 MDT 1993 NRCS 3190 0 100 200 300 400 500 600 Distance (ft)

Figure 6-16. 1954 and 1993 cross sections of Bitterroot River at Florence Bridge.

6.5.7 Cross Section Summary Width to depth ratios (W/D) of the primary (main stem) channel were estimated near the six bridge sites using NRCS survey data from 1992 and 1993 (Figure 6-17). The W/D ratio values upstream and downstream of the bridges are scattered with no discernible pattern. This analysis did not find that the selected bridge structures significantly altered channel geometry as defined by changes in W/D ratios measured from USDA-NRCS (1995) cross-section survey data.

AGI and DTM Bitterroot River Geomorphic Summary 57

100 100

Woodside Bell Crossing 90 90 Victor Stevensville 80 80

70 70 Florence Silver 60 60

50 50

40 40

Width to Depth Ratio 30 30 Percent (%) Encroachment 20 20 Width to Depth Ratio 10 Floodway Encroachment 10 Floodplain Encroachment 0 0 35 30 25 20 15 10 5 0 River Miles from County Line

Figure 6-17. Width to depth ratios and percent floodplain and floodway encroachment near six bridges on the Bitterroot River.

6.6 Bank Erosion A plot of total severe bank erosion extent is shown in Figure 6-18. Severely eroding banks increase in the uppermost project reach near Darby (RM 83 to 76; Plates 17-19); and between Woodside and Bell Crossing (RM 52 to 41; Plates 7-10). The high rate of bank erosion in this upper reach is likely a function of erosive, steep valley walls and vertical alluvial terrace banks that confine the active channel. Likewise, the upper project reach above Darby is heavily armored with ‘private’ bank protection between RM 83 and 74 (Plates 17-19), some of which shows evidence of failing. Upstream of RM 80, between 13% and 19% of the bank line is eroding either moderately or severely, but has some vestiges of bank armor.

AGI and DTM Bitterroot River Geomorphic Summary 58

Bitterroot River Severely Eroding Bankline 40%

35% ---Darby Bridge

30%

25%

20% ---Woodside Bridge 15% ---Main St ---Angler's Roost Percent of Bankline 10% ---Stevensville Bridge ---Silver Bridge ---Bell Crossing 5% ---Victor Bridge ---Florence Bridge

0% 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 75 77 79 81 83 Miles Above Clark Fork Confluence

Figure 6-18. Percent of severely eroding bank line, Bitterroot River.

In order to assess the relative extents of bank erosion in the vicinity of bridges, the inventory data were summarized for a channel length ½ mile upstream and ½ mile downstream of each assessed structure. These data indicate that the extents of moderately and severely eroding banks as mapped within the vicinity of the assessed bridges are less than the average erosion extents mapped for the corresponding geomorphic zone (Figure 6-19). The exception to this trend is the Stevensville Bridge, where local erosion is more extensive than on average.

Bank Erosion at Bridges

30% Severe Erosion 25% Moderate Erosion

20%

15%

10%

Percent Eroding Bank 5%

0% Bell Lower Victor Darby Total Bridge Section Section Section Average Average Average Bridge Florence Crossing Average Bridge Section Average Increment Woodside Stevensville Stevensville Silver Bridge Victor Bridge Bridge

Figure 6-19. Severely and moderately eroding bank extents at each bridge showing average erosion extents for corresponding geomorphic zone (red line), Bitterroot River.

AGI and DTM Bitterroot River Geomorphic Summary 59

6.7 Bank Armor The extent of bank protection within ½ mile of the bridges is typically higher than the average value for the corresponding geomorphic zone (Figure 6-20). When these data are plotted in terms of the type of feature protected, it is clear that in the vicinity of the bridges, there is substantial armor protecting private property as well as bridge infrastructure.

Bank Protection Within 1/2 mile of Bridges 60% Private Property Protection 50% Bridge Protection/Alignment Zone Average 40%

30%

20% Percent of Bank Armored 10%

0% Bell Silver Victor Florence Bridge Woodside Stevensville Figure 6-20. Extent of bank armoring within ½ mile of bridges relative to geomorphic zone average.

6.8 Site-Specific Bridge Summaries This section contains a summary of geomorphic conditions at each of the assessed bridges. This summary is based on geomorphic data presented in previous chapters, additional observations made during the field investigation, and a review of aerial photography of each bridge. A sequence of historic aerial photographs at each bridge structure is presented as a time series in Appendix C. Historic aerial photo coverage from 1937, 1955, 1972, 1995,2002, and 2005 were analyzed.

As described in Section 6.2, the widths of the Bitterroot River floodplain and floodway varies dramatically between bridges (Figure 6-21). At Silver Bridge, the floodplain is relatively narrow (less than 1000 feet); downstream of this point, the floodplain width increases eight-fold (to over 8000 feet) by Bell Crossing. The corridor narrows towards Stevensville, and then moderately widens at Florence. The encroachment of the highway and bridge system into the floodplain varies accordingly. The bridges that crosses the river corridor at Victor and Bell Crossing, for example, are associated with a high degree of floodplain encroachment. However, encroachment of a highway and bridge complex onto a broad 100-year floodplain does not necessarily induce a significant channel

AGI and DTM Bitterroot River Geomorphic Summary 60

response if flow energy on the floodplain is low. To that end, the intent of this chapter is to summarize overall conditions at each bridge and to identify as possible the nature and scale of geomorphic impacts associated with the infrastructure on the channel itself. These impacts may include broad encroachment of the adjacent floodplain, as well as local impacts on channel planform and sediment transport continuity.

Bitterroot River Floodplain/Floodway Widths

12000 Floodplain Floodway 10000 ---Bell Crossing ---Victor Bridge 8000 ---Florence Br ---Woodside Br ---Main St ---Angler's Roost ---Darby Bridge 6000 Width (ft) 4000 ---Stevensville Br ---Silver Br

2000

0 20 25 30 35 40 45 50 55 60 65 70 75 80 85 River Mile

Figure 6-21. 100-year floodplain and floodway width plotted from USDA-NRCS (1995) cross section data.

One parameter described in this section is the Bitterroot River’s “angle of approach” to bridge support structures. The angle of approach reflects the river course at the bridge with respect to the trend of the bridge piers. A zero degree angle of approach means the river approaches the bridge piers parallel to their trend (Figure 6-22). A higher angle of approach indicates that the flow path is at an angle to the piers. On many bridges the piers are oriented perpendicularly to the bridge centerline, in these cases, the river is flowing parallel to the piers when it approaches the bridge at a right angle. In other instances, the bridge piers are not perpendicular to the structure, and these cases are noted. Typically, disruptions to sediment transport through bridge structures increases with an increasing angle of approach between the river and the bridge piers due to increased obstruction of the channel cross section.

AGI and DTM Bitterroot River Geomorphic Summary 61

Figure 6-22. Schematic drawing of angle of approach of river to bridge pier.

6.8.1 Silver Bridge (RM 55.9) Silver Bridge was constructed in 1940, replacing a historic bridge at the same general location and alignment as the previous roadway. Since the original field inventory was completed in 2002, Silver Bridge has again been replaced by a new bridge located about 450 feet downstream. During the field visit of November 2007, both bridges were in place, although the new bridge was not yet open to vehicle traffic. The upstream bridge structure, which is slated for removal, is a steel truss structure with one, centered, oval or round-nose pier (Photo 1). Historic bridge pier footings remain in the channel bed about 40 feet upstream of the existing structure. The bridge and highway are aligned at about 40 degrees to the floodplain corridor and active channel. Likewise, the center pier is at about a 45-degree angle to the direction of the thalweg, impinging flows on the upstream, broadside of the oval pier (Photo 1).

AGI and DTM Bitterroot River Geomorphic Summary 62

Photo 1. Silver Bridge looking downstream at the middle pier, October 2002.

The new bridge is much longer than the older bridge. Four spans are supported by 3 sets of cylindrical columns (Photo 2). The right 3 spans of the bridge spans over a large point bar feature (Photo 3), and there is a training dike on the right floodplain between the two structures. At the old bridge, the right bank abutment encroaches into the riparian corridor, where as the new bridge broadly spans a wide swath of riparian vegetation.

Photo 2. View downstream of new bridge to replace Silver Bridge, November 2007.

AGI and DTM Bitterroot River Geomorphic Summary 63

Photo 3. View northwest towards Bitterroot River showing bridge span over point bar; river channel is at far end of bridge, November 2007.

Table 12. Summary of geomorphic conditions at Silver Bridge. Silver Bridge Site Summary Approach Angle Sub-parallel (~ zero degree) angle of approach in 1937 followed (Original Bridge) by a channel shift to the west and increased angle of approach to pier. By 1972, the channel follows eastern road embankment at ~70 degree angle to pier; since 1972, point bar excavation/removal upstream of the bridge improved approach angle. By 1995, however renewed bar formation upstream has increased the angle through 2005. Approach Angle The new bridge has a sub-parallel angle of approach as the (Replacement Bridge) channel follows the riprapped west bank below the older bridge structure. Floodplain Width (ft) 887 (56 % encroachment) Floodway Width (ft) 887 (56% encroachment) Local Channel Width 750 ft (47% encroachment) Stability Index 52.4 Fair (Good if shear stress stability indicator is very low) Bank Armor Extent 13% (Reach Average = 11%) Reach Character Narrow corridor bound by alluvial terraces Comment Original bridge has been replaced and is slated for removal. New bridge has broad span, cylindrical piers, and low angle approach.

Approximately 1 mile upstream of Silver Bridge, the belt width of the Bitterroot River channel corridor begins to taper in the downstream direction (Figure 6-21). This tapering reflects increasing confinement by 15-20 feet high alluvial terraces as the (1940) bridge is approached from the upstream (Weber, 1972). Both the floodway and floodplain are less than 1000 feet wide at the bridge, which is markedly narrow for the Bitterroot River

AGI and DTM Bitterroot River Geomorphic Summary 64

corridor (Figure 6-21). Although the belt width and floodway are relatively narrow features, the river supports multiple channel threads immediately upstream of the bridge. In this area the active channel reaches 750 feet in width. Appendix C, Sheets 1 and 2 show the presence of multiple channels immediately upstream of the bridge in all of the time frames evaluated. In 1937, the primary channel upstream of the bridge followed the west edge of the active channel corridor, flowing relatively straight along the base of the terrace towards the bridge. At this point in time, the river’s approach to Silver Bridge was almost parallel to the current orientation of the bridge piers, and the bridge was constructed to this general alignment in 1940. By 1972, the angle of approach was more than 70 degrees as the approaching river flowed along the right highway embankment for several hundred feet before hitting the bridge pier at a high angle. This high angle approach in 1972 was associated with the formation of a large bar upstream of the bridge that deflected flows eastward. By 1995, the alignment angle of the river to the piers was reduced to a more effective configuration, either due to natural migration of the channel westward, or potentially due to excavation of the 1972 bar feature. The 2002 air photos show that a new bar upstream of the bridge is beginning to form, once again deflecting the channel to the west and increasing the angle of approach to the pier. This trend continues from 2002 to 2005.

Photo 4. Looking downstream from Silver Bridge deck with rock riprap on left bank; the new bridge has been constructed at this location, October 2002.

As early as 1955, the left bank downstream of Silver Bridge was protected with rock riprap. The existing bank protection is about 764 feet in length (Photo 2) and serves the Woodside Ditch, stabilizing the bank and channel alignment upstream of the head gate diversion structure. The existing riprap appears relatively new and functional. The new bridge has been constructed in this area. For a distance of approximately ½ mile

AGI and DTM Bitterroot River Geomorphic Summary 65

upstream and ½ mile downstream of the bridge, a total of 13% of the bank line is armored, which is slightly greater than the average armor extent for the reach (11%). The mapped extent of eroding bank within this 1 mile long reach is less than average conditions throughout the geomorphic zone (Figure 6-19).

The Silver Bridge has had continual changes in its approach angle since its construction in 1940. It appears that deposition and channel shifting into secondary threads has caused the change in alignment. The bar formation upstream of the bridge may be in part due to the encroachment of the structure into the active channel corridor. There is no evidence, however that these impacts extend beyond the immediate area of the bridge. The replacement of Silver Bridge with a new bridge that has a broader span and is supported by cylindrical piers will result in a higher angle of approach and improved sediment conveyance capacity.

6.8.2 Woodside (RM 52.9) The Woodside Bridge was constructed in 1954 and consists of steel beams with three bridge piers. A time series of historic aerial photos near Woodside Bridge is presented in Appendix C, Sheets 3 and 4. No historic photo coverage was available for 1937. The historic 1955 aerial photo shows that when the bridge was built, it was on the apex of the bend. Flow paths were aligned parallel to the piers on the bendway apex. Since 1955, northward translation of the bend has resulted in migration of the bendway apex downstream of the structure. The main channel is currently aligned to the bridge piers at about a 60-degree approach angle (Photo 5).

The most eastern two spans on the right side of Woodside Bridge channel carry all of the Bitterroot River base flow. The western span is partially obstructed by the upstream end of a point bar (Photo 5). From the bridge to a location approximately 4000 feet upstream, the left bank of the Bitterroot River is protected with full bank rock riprap (RM 53.7 to 52.9; Appendix B, Plates 10-11) that protects both the highway and private property. At the upper extent of the riprap, there is a flow split that extends approximately ½ mile downstream towards the bridge. The channels converge at RM 53.2, forming a large vegetated floodplain island that supports mature cottonwood and woody shrubs. Woody vegetation on the island has increased from 1972 to present.

AGI and DTM Bitterroot River Geomorphic Summary 66

Photo 5. View downstream of Woodside Bridge, October 2002.

In the vicinity of Woodside Bridge, the Bitterroot River has been described as having a dynamic planform that is prone to lateral shift (Gaeuman, 1997). Significant bedload movement through the reach is evidenced by gravel bars both upstream and downstream of the centermost pier. The floodplain is over a mile wide, and dissected by secondary channels upstream of the bridge. In 1955, a large island had formed upstream of the bridge; the channels that defined the eastern margin of the island have since been abandoned, and by 2002 it appears that these abandoned primary channels behave as capillary channels that are fed by groundwater.

Table 13. Summary of conditions at Woodside Bridge. Site Summary Approach Angle 1955: Low approach angle to piers as bridge occupies bendway apex. Subsequent migration of bendway downstream has resulted in increased angle of approach Floodplain Width (ft) 6043 (89% encroachment) Floodway Width (ft) 2271 (51% encroachment) Local Channel Width 700 ft (66% encroachment) Stability Index 54.6 Fair Bank Armor Extent 48% (Reach average = 11% Reach Character Dynamic reach with active channel migration, island formation, and capillary channel activity Comment Althrough reach is dynamic, bridge span is large and significant effects on local sediment transport/geomorphology are not evident. Left bank riprap has arrested downstream bendway translation past bridge.

AGI and DTM Bitterroot River Geomorphic Summary 67

The Woodside Bridge is located within a series of distinct meander bends and point bar features. These bendways are associated with bank erosion on their outer banks and downstream limbs. This type of erosion pattern results in migration of bendway features in both a cross-valley and down-valley direction. At Woodside Bridge, extensive left bank armor upstream of the bridge has stopped down valley migration of a bendway towards the structure. The armor has kept the approach angle at approximately 60 degrees; additional left bank erosion would have further increased the angle of approach. As the river approaches the bridge, it enters another bendway and point bar complex. The upstream extent of the point bar forms the left bank under the bridge.

Since 1955, the most significant lateral channel migration in the area occurred approximately 2,000 feet upstream of Woodside Bridge. Between 1972 ands 1995, the main channel avulsed nearly 1,000 feet to the east creating an island. This avulsion has created a split flow condition, and the primary channel has jumped back and forth around the island. Currently, most flow is in the west channel which is riprapped. About one mile upstream of Woodside (RM 53.8; Appendix B, Plate 11), a meander bend is compressed and appears prone to cutoff. The implications of a cutoff upstream include a redirection of most or all flow into the east channel, which may place substantial stress on the riprap bank at RM 53.1. This process is a natural occurrence for this type of river system.

For a distance of ½ mile upstream and ½ mile downstream of Woodside Bridge, approximately 48% of the bank line of the Bitterroot River is armored. This is significantly higher than the 11% average armor extent for the reach. Of this 48% armor, however, it has been estimated that 17% effectively protects the bridge structure, whereas 31% is protecting private property.

6.8.3 Victor (RM 44.3) Victor Crossing transverses a broad floodplain that is almost 8000 feet wide (Table 14). As the Bitterroot River traverses Victor Crossing, it flows through three major channels, each of which have bridges, as well as several culverts that convey secondary channels through the road embankment. This report focused primarily on the Victor Bridge on the west branch, which was reconstructed in 2000. This bridge was built for Ravalli County and does not belong to the Montana Department of Transportation. It is constructed of concrete beams with three cylindrical piers with hammerhead pier caps on the top that laterally support the beams (Photo 6).

Victor Bridge is similar to Woodside Bridge in that it is located on the upstream end of a bendway and associated point bar. In 2005, the approach angle of the river to the bridge was about 30 degrees. Deposition has occurred under the western-most 2 spans that form the upstream end of the point bar (Photo 6). This deposition extends upstream of the structure and has potentially impacted conveyance conditions for woody debris and additional sediment through the spans, forcing base flows to pass under the two eastern spans (Photo 7). Just upstream of the bridge, a backwater area is located along the right bank, adjacent to the highway embankment. On the left bank, three additional overflow

AGI and DTM Bitterroot River Geomorphic Summary 68 and side channel culverts (one concrete box and two corrugated metal pipes) are located on the left floodplain, and offer additional relief and hydraulic connectivity during high flows (Photo 8).

Table 14. Summary of conditions at Victor Bridge. Site Summary Approach Angle 30 Degrees Floodplain Width (ft) 7838 (68% encroachment) Floodway Width (ft) 6835 (78% encroachment) Local Channel Width 1030 ft (57% encroachment) Stability Index 58 Fair Bank Armor Extent 16% (Reach average = 11%) Reach Character Very wide floodplain with 3 widely separated active channels Comment Cross sections suggest that west branch thalweg is at higher elevation than east branch hence future shifts in primary flow path are likely

Photo 6. Looking downstream at Victor Bridge (west branch Bitterroot River) , October 2002.

AGI and DTM Bitterroot River Geomorphic Summary 69

Photo 7. View downstream of Victor Bridge showing left bank bar formation, October 2002.

Photo 8. Culvert on left side of Victor Bridge, October 2002.

Approximately 4 miles upstream from Victor Bridge, the Bitterroot River splits into two major channels known as the east and west branches. These branches have remained spatially isolated by a broad floodplain area that is over ½ mile in width and over 6 river miles long. In 1937, the west branch conveyed only a small fraction of the total river

AGI and DTM Bitterroot River Geomorphic Summary 70

flow. Flow was likely augmented from groundwater discharge into capillary channels that are common on the floodplain. Gaeuman (1997) describes the west branch as having maintained a relatively deep and narrow geometry through time. The eastern branch resembles a more typical braided stream morphology including a wide shallow cross- section, and more transient sparsely vegetated bars (Appendix C, Sheets 5 and 6).

Since 1937, the west branch of the Bitterroot River has grown in dimensions and captured increasing proportions of the total Bitterroot River discharge. According to Gaueman (1997), the west branch had captured the majority of river flow by 1987. In October 2002, the west branch appeared to have the largest capacity. A third channel flows between the east and west branches, re-entering the east branch about 3,500 feet downstream of Victor Crossing. Each of these channels crosses Victor Crossing through a bridge.

As the west branch has captured more flow through time, the east branch has commensurately lost flow volume. In response to this trend, water right holders on the east branch have attempted to increase flows into the east branch via the construction of two structures at the point of flow split. These structures consist of a W-weir and vortex weir that extend across the main channel. This project raised the channel bed elevation on the west branch by several feet, thereby diverting more flow into the east branch. Since construction, a mid-channel bar appears to have prograded downstream over the weirs, and the flow split into the east branch has been maintained via channel excavation through the bar.

The USDA-NRCS (1995) cross-section data at Tucker Crossing indicates that in 1995, the thalweg of the east branch was approximately 2 feet lower than that of the west branch by about 2 feet (Figure 6-23). Gaeuman (1997) also observed this topographic pattern, and concluded that the possibility exists for a major avulsion to the east in the central valley near Victor.

Lateral channel shifting immediately upstream and downstream of the Victor Crossing has established an active channel width of about 600 feet. In 1995, left bank erosion upstream of the structure resulted in a very high angle of approach, as the river was flowing essentially parallel to the road and bridge. This erosion reflects downstream translation of a bendway into the bridge. Since then, the channel has been relocated eastward to reduce that angle of approach, and overflow structures installed to maintain side channel connectivity through the embankment. This re-alignment upstream of the bridge along the left bank margin consists of over 1000 feet of rock toe revetment. This revetment creates a “funnel effect” on the left side of the braid belt that should effectively taper the river corridor as it approaches the bridge.

At Victor Bridge, the Bitterroot River is characterized by a very dynamic channel that supports persistent, large scale split flow conditions. The local impacts of the bridge structure on channel morphology include reduced conveyance for woody debris and potentially sediment, although the bridge is located on the upstream end of a naturally occurring point bar and hence is prone to some deposition on the left side of the cross

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section. Because of the complexities associated with multiple active channels, it is likely that the flows conveyed through the bridge during a given hydrologic event will change through time.

Tucker Crossing NRCS (1995)

3409 West Branch East Branch 3407

3405

3403

3401

3399 Elevation (ft) NAVD-88 3397

3395 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Distance (ft)

Figure 6-23. Valley bottom cross section from Tucker Crossing showing higher elevation of bed of west branch channel relative to east branch channel.

6.8.4 Bell Crossing (RM 41.4) The existing bridge structure at Bell Crossing was built in 1974, constructed with concrete beams and three sharp-nose piers and four spans (Photo 9). An older bridge was located approximately 100 feet upstream of the existing bridge from at least 1937 to 1974. On the old structure, the bridge trend was at an angle to the east-west road axis. Construction of the new bridge and road included a realignment of the crossing so that the bridge and approaches ran straight across the river corridor rather than including bends in the road where it met the angled bridge. As the main channel trends about 30 degrees to the northeast at the bridge site, the realignment resulted in an increased approach angle of the Bitterroot River to the new bridge (Appendix C; Sheets 7 and 8). The piers are skewed northeast at a 35-degree angle to the bridge deck to help improve alignment with the main channel. Both bridge abutments are heavily protected with concrete and boulder riprap.

At Bell Crossing, the Bitterroot River flows through a relatively wide active braid belt; the modeled floodway just upstream of the bridge is over 2000 feet wide (Table 15). For a distance 1.5 miles upstream and 2.5 miles downstream of the bridge, the active channel corridor contains extensive unvegetated point bar deposits, indicating that the reach is characterized by substantial bedload transport and storage. Due to the active bedload

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movement and point bar growth through the reach, bank erosion is common, and rates of planform change have been high since 1937 (Appendix C). Upstream of the bridge, the active channel width exceeds 1000 feet.

The cross section at the bridge is asymmetric in shape. A depositional bar is present under the left two spans, deflecting all low flows through the eastern span. It is interesting to note that this cross section shape has changed significantly since the 1993 cross section surveyed at the bridge, when low flows were conveyed through the left spans (Figure 6-14).

Table 15. Summary of conditions at Bell Crossing. Site Summary Approach Angle 45 degrees (western primary channel approach to skewed piers)

Floodplain Width (ft) 9052 (85% encroachment) Floodway Width (ft) 2060 (78% encroachment) Local Channel Width (ft) 1030 ft (57% encroachment) Bank Armor Extent 21% (Reach average = 11%) Stability Index 72.2 Poor Reach Character Highly depositional reach with extensive bedload transport and storage within broad point bars and mid-channel bars. Comment Mid-channel bar deposition upstream of bridge is due to sediment pulse generated by cutoff just upstream of the bridge that occurred between 1995 and 2002

Since 1937, extensive deposition of open gravel bars immediately upstream of the bridge reflects a potential exacerbation of overall reach-scale depositional trends by the bridge structure itself. Approximately 1000 feet upstream of the structure, a large mid-channel gravel bar splits the river flow, resulting in a high angle of bridge pier approach for both channels (Appendix C; Sheets 7 and 8). The bar formation has also created moderate to severe erosive pressure on the banks approaching the bridge. The left bank is armored by full bank riprap and cabled logs for a distance of approximately 1700 feet upstream of the bridge. As this armored channel margin approaches the bridge, it hits the road embankment which is protected by full bank riprap nearly perpendicular to its trend (Photo 9). Within ½ mile upstream and downstream of the bridge, over 20% of the banks of the Bitterroot River are armored, and approximately one-half of that armor is protecting non-bridge related private property. The average extent of armor within the geomorphic zone is 11% (Section 6.7).

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Photo 9. View downstream of Bell Crossing Bridge, October 2002.

Approximately ½ mile upstream of the bridge, the right bank is armored by 1,085 feet of full bank rock riprap that is protecting a private residence constructed since 1995. Approximately 500 feet upstream of that armor, the left bank is armored to protect a left bank diversion structure. Downstream of the bridge, armoring is common as well. Approximately 1500 feet downstream on the right bank, 1,116 feet of root wad revetment was installed adjacent to floodplain pasture immediately upstream of a diversion at RM 40.8. This protection was constructed after 1995 in response to about 400 feet of eastward bank retreat between 1995 and 2002.

The approach angle of the Bitterroot River to the Bell Crossing bridge piers has increased significantly since 1995. Overall, the historic alignment has been good, but the current bar configuration upstream of the structure has resulted in a high angle of approach in both low flow channels. The composite suite of air photos in Appendix C clearly show that this bar configuration developed between 1995 and 2002, after a large bendway located 1500 feet upstream was cut off. In 1995, a very large bendway with a broad, unvegetated point bar had formed upstream of the bridge. By 2002, that bendway had cut off through the excavation of a new, shorter channel through the point bar. This channel excavation through the point bar resulted in the delivery of a sediment pulse downstream to the bridge. This process had occurred in the same area previously; between 1937 and 1955, the river avulsed several thousand feet eastward which caused new channel formation and accelerated rates of sediment delivery downstream.

This corridor segment upstream of Bell Crossing serves as a good example of typical geomorphic processes of the Bitterroot River system, which include channel relocation, bendway cutoff, and production of sediment pulses due to new channel excavation. The

AGI and DTM Bitterroot River Geomorphic Summary 74 significant changes in planform in the vicinity of Bell Crossing between 1995 and 2002 suggest that the reach responded to high flow events of 1996 and 1997 with massive bedload transport and planform adjustment. It is also evident from bar formation just upstream of the bridge that the conveyance of such pulses through the crossing is impacted at bridge, likely due to backwatering effects at high flows that locally reduce bedload transport rates.

6.8.5 Stevensville (RM 34.4) The Stevensville bridge was built in 1951, constructed with steel beams, three sharp-nose piers, and four spans. In spring 2002, the bridge deck was replaced and a pedestrian walkway and railing were added to the structure. Round, cylindrical concrete piers and abutments were placed immediately upstream and downstream from the existing columns. These piers extend 3 to 10 feet deeper than the existing columns (Photo 10). The bridge abutments are protected by gabion structures on both banks.

The Stevensville Bridge and roadway span a relatively narrow floodplain corridor that is less than 2,000 feet wide immediately upstream of the highway (Table 16). The bridge is located within a narrow section of river corridor due to lateral confinement by a Quaternary-age outwash terrace formation (Qafy) referred to as the Burnt Fork fan. The tapering of the river corridor towards the bridge controls the routes of two tributaries, McCalla and Kootenai Creek, that enter the Bitterroot River just upstream of the bridge from the west. The span length at Stevensville bridge is longest of all bridges relative to the active channel width measured just upstream (29% encroachment; Table 16).

Three permeable training dikes on the right bank guide the river towards the bridge opening. The training structures appear to have been installed between 1972 and 1995, and effectively form a funnel along the right bank margin. A major willow shrub plant community and wetland complex is located on the backside (east) of the permeable training dikes, an example of how bank protection measures may still allow floodwater access to the floodplain and help mitigate for encroachment (Photo 11).

The geomorphic stability at the bridge is rated as good. A large shadow bar has formed immediately downstream of the second pier, which possibly indicates some local pier scour. Active bank erosion is evident along a fish access site and riverside park downstream of the bridge on the right bank. A high terrace scarp confines the downstream left channel margin. The left bank is typically stable with good cottonwood regeneration, stable sod grass banks, and mature ponderosa pine on the terrace (Photo 12). Approximately 11% of the bank line is armored in the 1 mile reach containing Stevensville Bridge, and the vast majority of the armor is attributable to bridge protection. This extent of armor is slightly higher than the average for the geomorphic zone, which is 8% (Table 16).

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Table 16. Summary of conditions at Stevensville Bridge. Site Summary Approach Angle 15 degrees Floodplain Width (ft) 1935 (72 percent encroachment) Floodway Width (ft) 1119 (63 percent encroachment) Local Channel Width (ft) 450 (29% encroachment) Bank Armor Extent 11% (Reach average = 8%) Stability Index 40.8 Good Reach Character Narrow corridor confined by Quaternary-age terrace. Generally stable banks. Low approach angle to bridge piers. Comment Historic air photos indicate reach upstream of bridge has experienced significant channel migration since 1937.

Photo 10. View upstream to southwest of Stevensville Bridge, October 2002.

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Photo 11. View upstream from Stevensville Bridge showing permeable training dikes and vigorous willow community, October 2002.

Photo 12. View downstream from Stevensville Bridge deck, October 2002.

Upstream of Stevensville Bridge, the Bitterroot River is straight, providing a low angle alignment of the river to the bridge piers. Historically, however, the river segment that extends approximately 0.7 miles upstream of the bridge has experienced substantial

AGI and DTM Bitterroot River Geomorphic Summary 77

lateral shift. The majority of this change happened between 1937 and 1955, when the channel migrated approximately 600 feet westward (Appendix C; Sheets 9 and 10). Between 1955 and 1995, the channel had migrated another 300 feet to the west, but since then it has maintained a relatively stable configuration and narrow belt width. There is no evidence that Stevensville Bridge has caused significant change in overall geomorphic character of the Bitterroot River. Bank armor extents are similar to the average for the zone, and there are no discernable changes in channel cross section and depositional patterns that would be considered attributable to the structure.

6.8.6 Florence (RM 23.8) Florence Bridge, completed in 1956, consists of steel beams, two square-nosed piers, and three spans (Photo 13). The bridge site is located at the upstream end of a major valley constriction. The river channel is confined on the right (east) bank by late Pleistocene alluvial outwash terrace and fan complex deposits (Qafy), which provided a natural elevated surface (about 25 feet) for the right bridge abutment. The bridge slopes westward to a lower floodplain surface on the left bank (Photo 14). The bridge piers are skewed approximately 20 degrees such that the river channel approach is almost parallel to the piers (Table 17).

Prior to 1956, the historic Florence Bridge span crossed the main channel perpendicular to the axis of the river channel. This alignment required a slight, ~15- degree curve along the west and east roadway approach to the structure (Appendix C; Sheets 11 and 12). The realignment of the bridge during its reconstruction is evident in the 1955 aerial photograph, which was taken when new bridge construction was underway. The highway was straightened, thus the angle of the bridge to the river channel approach was slightly increased. Currently, the old road grade provides a fishing and boat access site on the left bank upstream of the bridge. There are a series of isolated wetlands adjacent to the old road grade that are likely original borrow pits. The original pre-1956 concrete bridge abutments are prominent features on both banks upstream of the existing bridge.

Table 17. Summary of conditions at Florence Bridge. Site Summary Approach Angle Sub-parallel due to skewed bridge piers Floodplain Width (ft) 3743 (92% encroachment) Floodway Width (ft) 1199 (58% encroachment) Local Channel Width (ft) 800 (59% encroachment) Bank Armor Extent 11% (Reach average = 8%) Stability Index 28.4 Good (Excellent if shear stress stability indicator is very low) Reach Character Upstream end of valley constriction Comment High elevation right bank approach; encroachment is entirely on west side of channel

AGI and DTM Bitterroot River Geomorphic Summary 78

Photo 13. View downstream of Florence Bridge, October 2002.

Photo 14. View west across Bitterroot River on downstream side of Florence Bridge, October 2002.

At Florence Bridge, the 100-year floodplain corridor is over 3700 feet wide immediately upstream of the crossing (Table 17). Over 90% of this floodplain width has been encroached upon by the road and bridge complex, and this encroachment is entirely on

AGI and DTM Bitterroot River Geomorphic Summary 79

the west side of the bridge. At the bridge, the Bitterroot River is characterized by a relatively low width to depth ratio, a single thread, an armored cobble bed, and a straight planform. All of these indicators suggest that bedload is effectively transported through the reach.

The Bitterroot River has experienced significant lateral shift immediately upstream of Florence Bridge since 1937. Between 1937 and 1972, at a location approximately 1500 feet upstream of the bridge, the river migrated westward approximately 1000 feet as it formed a very large meander bend and point bar complex. As the bendway developed, it increased in meander amplitude and bend length and decreased in radius of curvature, resulting in an overlengthened channel segment with diminishing sediment transport capacities (Appendix C; Sheets 11 and 12). By 1995, the channel had avulsed, forming a chute cut- off, or conversely, may have been channelized to achieve the existing straight configuration and improved alignment. Inflows into the oxbow cutoff are limited to tributary and capillary channel inflow and over bank flood flows from the main Bitterroot River channel.

Bank armor extents in the vicinity of the bridge (11%) are slightly higher than the average protection mapped in the geomorphic zone (8%). However, there is no evidence that Florence bridge impacts local geomorphic conditions related to bank erosion and sediment transport capacity. Any sediment pulse that was created in response to the upstream meander cutoff between 1972 and 1995 did not result in any significant deposition upstream of the bridge structure, indicating that sediment transport capacities through the bridge are typically sufficient to accommodate such pulses.

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7.0 Conceptual Strategies to Minimize Geomorphic Impacts at Bridges On a dynamic, wide river corridor such as the Bitterroot River in Ravalli County, it is infeasible to completely avoid any geomorphic impacts at transportation crossings. However, field mapping results indicate that currently, the bridges exhibit a range of applied strategies that effectively reduce the impacts of the bridges on river form and process. These strategies, as well as additional opportunities, include the following:

Minimized active channel encroachment; Minimized floodplain isolation; Maintenance of secondary channel continuity; · Minimized habitat loss on armored banks; · Tapering (gradually narrowing) of the active channel corridor at bridge approaches; · Accomodation of downstream passage of large woody debris · Maintenance of fish passage through bridges and secondary channel culverts; and, · Removal of derelict features from active corridor and floodplain.

7.1 Minimizing Encroachment There are two basic types of roadway encroachments on the Bitterroot River; transverse and longitudinal encroachment. Encroachment may occur into the floodplain, flood way, or active channel braid belt. The results of this bridge assessment indicate that all of the bridges result in some degree of transverse encroachment into these areas. Although totally avoiding any encroachment is infeasible in a wide, dynamic system such as the Bitterroot River, the minimization of encroachment, especially into the active channel, would be an effective means of minimizing geomorphic impacts.

7.1.1 Active Channel Encroachment The length of a bridge is commonly a point of contention in bridge design, primarily because of the dramatic cost increase associated with the increasing length of a bridge. However, the design and construction of bridge structures over alluvial rivers that carry a high bed load commonly do not fully consider sediment transport conditions under a wide range of flows. As such, the structures commonly cause a disruption of sediment transport processes due to the creation of backwatered conditions. This condition is especially challenging on the Bitterroot River between Hamilton and Stevensville, where avulsions and cutoffs create sediment pulses that are delivered downstream. At Bell Crossing, for example, the delivery of a sediment pulse following an upstream cutoff resulted in bar formation and alignment degradation at the structure. Further downstream at Florence Bridge, a similar cutoff event caused no deposition or change in flow alignment, indicating inherently better transport conditions at Florence Bridge. Field observations indicate that in the most dynamic reach of river, between Hamilton and Stevensville, there are opportunities to improve sediment transport continuity with longer bridges that can accommodate such sediment pulses.

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If a larger bridge span is adopted to minimize geomorphic impacts, one fundamental strategy to apply is to minimize encroachment into the active channel. The extent of encroachment of the existing bridges into the active channel is discussed in Section 6.3. This corridor is that area in which the channel frequently shifts course, actively eroding and depositing new materials. As the channel width is dynamic, and reflective of recent hydrologic events, the bridge design should consider belt width with regard to high flow periods as well as recent hydrologic events.

7.1.2 Floodplain Isolation Floodplain isolation is defined as a condition of ecological, physical, and/or hydrological disconnect between the floodplain and river channel. Connectivity refers to the capacity of a landscape to support the movement of water, organisms, materials, or energy (Peck, 1988). Channel constriction, confinement, channelization, and roadway encroachment may cause floodplain isolation, and may represent a total or partial barrier to the movement of water, sediment, plants, animals, and food. Floodplain isolation observed as caused by transportation infrastructure includes a section of system U.S. 93 north of Darby (RM 75.8; Appendix B, Plate 17), and a section of railroad north of Stevensville near the National Wildlife Refuge. Highway relocation, which in most cases is not a viable option, may reduce floodplain isolation and encroachment. Removal of derelict levees and berms, thereby reconnecting overflows to isolated areas for flood storage and slackwater habitat should be considered where feasible. However, it is important that secondary impacts associated with berm removal be fully evaluated prior to any action.

7.1.3 Secondary Channel Continuity Avulsion is the diversion of a river channel to a new course at a lower elevation on its floodplain as a result of channel aggradation. It causes established channel segments to become abandoned and new ones to form. One strategy for reducing the geomorphic impacts of bridges is to integrate the assessed potential for avulsion at a given site into bridge design strategies. For example, the potential for an eastward avulsion near Victor and Tucker Crossing, in combination with ongoing efforts by stakeholders to increase flows to the East Branch is noteworthy. Prior to 1955, the main flow of the Bitterroot River crossed Victor Crossing through the east branch, hence the existing bridge over the east channel was constructed with relatively high conveyance capacity. Having relatively high conveyance capacities over multiple channels that are prone to avulsion at Victor Crossing is an effective means of minimizing geomorphic impacts of the entire crossing. Gaeuman (1997) suggests the potential for major eastward channel avulsions near Victor, Tucker Crossing, and Willow Creek.

Another strategy to minimize the effects bridges on channel process is to maintain the conveyance and continuity of smaller secondary channels at bridge crossings (Figure 7-1). This typically requires the conveyance of capillary and secondary channels through culverts or bridges in the roadway. Currently, numerous transverse roadway crossings have culverts to provide continuity to these secondary and capillary channels. Continued

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construction of these culverts or smaller bridges is recommended as a strategy to minimize the impacts of the infrastructure the secondary channel environment, which may extend beyond the margins of the active channel.

Large-scale avulsion into floodplain capillary channels has not been observed in the project reach, as major channel realignments tend to involve two established channels. However, capillary channels should be considered with respect to their potential for main channel capture, and their locations considered in bridge and roadway design.

Figure 7-1. Conceptual strategy for maintaining capillary and secondary channel connectivity on floodplain by installing culverts at road embankments in locations of existing channels.

7.2 Maintaining alignment at bridges The results presented in this report indicate that one of the most challenging aspects of designing and maintaining transportation infrastructure in the Bitterroot River corridor is accommodating the high rates of channel change and bedload transport that are typical of the system. Channel shifts in the vicinity of bridges result in altered angles of approach to the structures, which can cause bank erosion against the road embankment, reduce flow conveyance through the bridge, and further diminish sediment transport capacities

AGI and DTM Bitterroot River Geomorphic Summary 83

due to backwatering caused by the high angle of the piers to the main current. Bell Crossing provides an excellent example of a dramatically increased angle of approach to the bridge piers following deposition upstream of the structure. At Victor Bridge, left bank erosion between 1972 and 1995 resulted in erosion against the base of the road embankment; the channel was subsequently realigned away from the embankment to improve its angle of approach (Appendix C; Sheets 5 and 6). It is clear from the historic air photo assessment that the angle of approach of the river to the bridges is an important aspect of the geomorphic condition of the channel, in that where the angle of approach is low, sediment transport continuity is less affected. Although an optimal means of addressing this issue is the construction of bridge spans that extend across the entire braid belt, this solution is typically financially infeasible. As such, it is important to consider other means of addressing the issue of channel encroachment and resulting shifts in channel alignment to the bridge piers.

7.2.1 Tapering the Braid Belt to Improve Long-Term Angle of Approach One of the challenging aspects of addressing angle of approach at bridges is that the right-of-way associated with bridges is typically limited to the immediate vicinity of the structure. As such, it is commonly infeasible to extend treatments beyond the immediate bridge area. However, one strategy for improving the angle of approach and long-term maintenance of sediment transport continuity through the structure is to taper the active channel corridor as it approaches the bridge. Currently, the meander belt axis typically doglegs at bridges, narrowing abruptly at the bridge abutments. This configuration commonly results in the downstream translation of bendways into the approach embankment, causing the channel to approach the bridge piers at right angles. Victor Bridge in 1995 shows an excellent example of this condition (Appendix C; Sheet 6).

A tapering of the active channel corridor as it approaches a bridge requires the extension of treatments upstream of the structure. An excellent example of this tapering concept is present at Stevensville Bridge, where a permeable training dike has been constructed on the right (east) floodplain upstream of the structure. This structure gradually narrows the Bitterroot River channel as it approaches the bridge. Channel alignment through Stevensville Bridge has remained almost parallel to the bridge piers, and there are no discernable effects of the structure on large-scale sediment transport patterns. The use of permeable dikes, such as those at Stevensville Bridge, offers several advantages over traditional riprap banks or impermeable dikes. Permeable training dikes (Figure 7-2) provide a predictive management tool to control the channel approach upstream of a bridge and road embankment. They control lateral bank erosion and channel migration while still allowing overbank flows to access the floodplain behind the structures and sustain wetlands (Figure 7-3). A staggered alignment allows floodwater to access and drain from the backside of these structures, promotes sedimentation on the floodplain, and creates wetland and riparian habitat zones that remain connected to the hydrology of the main channel. Where stakeholder collaboration is feasible such that treatments can extend beyond the bridge right-of-way, permeable training dikes are an effective means of reducing geomorphic instability as well as long-term alignment maintence requirements at the bridges.

AGI and DTM Bitterroot River Geomorphic Summary 84

Figure 7-2. Plan view of permeable training dike and fish habitat structures.

(Figure 7-2)

Figure 7-3. Schematic section showing permeable dike concept (Cross section A-A’ from Figure 7-2).

AGI and DTM Bitterroot River Geomorphic Summary 85

At each bridge, tapering the channel on both banks can allow some level of channel shift within a controlled belt width and maintain proper alignment with the bridge piers (Figure 7-4). Any erosion resistant features on the margin of the floodplain, such as bedrock or terraces could be incorporated as a tie-in for the upstream end of the training dikes.

Currently, several of the assessed bridges have partially tapered the active channel corridor via existing bank protection measures. Where new bridges are constructed, or where existing bridge alignments require modification due to accelerated bank erosion or reduced conveyance, it would be appropriate to consider the concept of corridor tapering with training dikes to minimize geomorphic impacts as well as long-term maintenance requirements. It is critical to recognize, however, any extension of treatments upstream of the structure will require the involvement and cooperation of landowners in the affected area.

Figure 7-4. Permeable training dikes to control river approach angle upstream of bridge.

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7.3 Minimizing Habitat Loss on Armored Banks The extent of armoring in the vicinity of bridges on the Bitterroot River is not markedly higher than the average extent of armoring throughout the project reach (with the exception of Woodside Bridge, Figure 6-20). Over 50% of the armor inventoried through the project reach can be attributed to the protection of private property, primarily residences located in close proximity to the riverbank. Hence bank armoring is a phenomenon on the Bitterroot River that extends well beyond the bridge crossings. Armored bridge abutments provide a critical element of long-term bridge stability. Hence, it may prove that in the immediate proximity of the bridges, conservatively- designed rock riprap is the most appropriate means of maintaining the integrity of the bridge abutments. However, where the bank protection extends away from the bridge, either upstream or downstream, such that the intent of the armor is to improve the alignment towards the crossing, there are opportunities to apply bank protection techniques other than full bank riprap that will minimize habitat loss on the affected bank line.

A fairly small fraction of the bank armor inventoried along the length of the Bitterroot River has incorporated bioengineering techniques such as rootwad emplacements to minimize the impact of full bank riprap on bank line habitat (Table 7). Rootwads improve fish habitat along the bank by providing cover and hydraulic variability along the armored bank. Although rootwads have been shown to improve habitat conditions on numerous rivers, their level of performance as bank protection measures on the Bitterroot River has been mixed. Several installations of rootwads have reportedly failed on the Bitterroot River, prompting questions regarding their applicability on this large dynamic river system. Their application in the vicinity of transportation infrastructure should therefore a be preceded by a rigorous assessment of anticipated stability.

Other strategies to reduce degradation of bank line habitat include the construction of a bankfull bench, commonly referred to as an inset floodplain surface, above rock toe protection that can support upper bank vegetation treatments (Figure 7-5). The construction of an inset floodplain surface over a rock toe is often an integral component of successful revegetation as part of a bank stabilization project. The incorporation of such a feature requires an analysis of the hydraulic conditions associated with the surface in terms of shear stress and frequency of inundation. The height of the inset floodplain surface can then be designed to ensure that the frequency and duration of flooding corresponds with the biological and physical requirements of the site. The construction of an inset floodplain often helps re-establish channel and floodplain connectivity where the historic floodplain (terrace) is no longer accessible to the channel due to channel incision or downcutting. An inset floodplain is generally built at a height that corresponds with bankfull conditions (normal high water) under the present climate regime.

Interplantings of woody shrubs within bank armor can improve both aesthetics and riparian habitat quality (Figure 7-5). The vegetation provides fish with shade, protection from predators, and insects for feeding. Planting of woody plants such as willow and cottonwood using multiple techniques such as pole cuttings, potted containers, bare root

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stocks, re-located mature plants, and re-seeding are all viable plant propagation techniques to use at these sites. This may require re-sloping and reshaping the toe of the channel and bank. Such an approach requires an understanding of the proximity of revegetation treatments to the water table and seasonal soil moisture conditions, as different plants have specific requirements and tolerances to survive and reproduce.

Figure 7-5. Strategies to minimize habitat loss on armored banks include: A) Construction of an inset floodplain bench surface at bankfull discharge, or B) Inter-planting riprap with riparian vegetation.

7.4 Large Woody Debris Passage The geomorphology of the Bitterroot River, which is characterized by active lateral migration, avulsion, and bendway cutoff, results in episodic delivery of Large Woody Debris (LWD) to the channel. During the field investigation, areas below recent avulsions typically contained a high concentration of debris jams due to new channel excavation upstream. Invariably, these areas had a high level of in-stream habitat quality and complexity. As a result, it is clear that LWD contributes to the fishery through the project reach. In order to prevent depletion of LWD through the project reach, bridge design and maintenance practices should make every attempt to pass debris. Where feasible, it would be appropriate to maintain sufficient clearance at a high flow event to pass a typical root wad through the structure without getting snagged on the bridge deck. With regard to maintenance practices, MDT personnel have indicated that when LWD collects against a bridge, all attempts are made to dislodge the debris and send it downstream rather than removing it from the system. This strategy is an appropriate

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approach towards minimizing the impacts of the bridges on habitat complexity downstream.

7.5 Maintaining Fish Passage Impeded fish passage has not been identified as a limiting factor on the main stem of the Bitterroot River. Nonetheless, effective fish passage through secondary and capillary channels flowing through culverts at road embankments should be ensured. Fish passage through secondary overflow channels may be inhibited due to channel downcutting and perching of a culvert (Photo 15), hence these structures should be monitored for such conditions. Native fish of the Bitterroot drainage include the Bull trout, a threatened species listed under the Endangered Species Act, and the Westslope Cutthroat trout, a species of special concern. As such, culverts should be constructed and maintained so as to secure fish passage for native cold-water salmonids.

Photo 15. A new culvert at Victor Crossing provides secondary channel and floodplain connectivity, but may impede fish passage, October 2002.

7.6 Removal of Derelict Features Along a few stream segments of the Bitterroot River, old dikes and levees exist that could be removed to reduce impacts on the floodplain. The purpose of removing unnecessary or remnant dikes and levees is to re-activate a larger historic floodplain area, and thus reduce the impacts of flood flows that are otherwise confined to an artificially narrower corridor. By doing so, flow velocities may be reduced and recharge rates on the floodplain increased. When bridges are replaced, the removal of derelict embankments should be considered to reduce long term braid belt encroachme nt. At Silver Bridge, the

AGI and DTM Bitterroot River Geomorphic Summary 89 imminent removal of the old bridge will include removal of the encroaching abutment, which is an effective means of limiting geomorphic impacts of the derelict structure.

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8.0 References Alt, D. and K.D. Cartier. 1984. The bewildering Bitterroot, the river that won’t behave. Montana Bureau of Mines Special Publication 89: Profiles of Montana Geology, p 29- 31.Alt, David D. 1984. Profiles of

Cartier, K.D. 1984. Sediment, channel morphology, and streamflow characteristics of the Bitterroot River drainage basin, southwestern, MT. M.S. Thesis. University of Montana, Missoula, Montana. 191 p.

Gaeuman, D. 1997. Historical channel changes and processes of the central Bitterroot River, Ravalli County, Montana. M.A. Thesis. University of Montana, Missoula, Montana, 78 p.

Goudie, A., B.W. Atkinson, K.J. Gregory, I.G. Simmons, D.R. Stoddart, and D. Sugden (editors). 1994. The Encyclopedic Dictionary of Physical Geography, Second Edition. Basil Blackwell Inc., Cambridge, Mass. USA.

Holnbeck, S.R.1999. Analysis of scour potential for bridge structure No. S00370 000+0.5361 Crossing Bitterroot River at Secondary Route 370, 2 miles Northeast of Victor, Montana. WRD. U.S. Geological Survey, Helena, Montana.

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Appendix A. Glossary of Terms

Anastomosing - comprised of multiple channels that maintain low width depth ratios and well-vegetated, wide floodplain islands between channels. Anastomosing is a type of stable multi-channel system developed under aggrading conditions often with levees and backwater environments. Anastomosing channels are those that have major distributaries that branch and then rejoin the main channel and these branches may be meandering or braided.

Avulsion - the rapid or episodic change in a river channel’s course, usually caused by sediment transport-water discharge imbalances. The result is often the formation of a straighter channel pattern characterized by an increase in channel bed slope and decrease in channel length.

Backwatering - where water backs up from a decrease in channel flow capacity or change in hydraulics downstream, such as a bridge crossing; or at a channel confluence and/or convergence zone that causes back flow and raises water surface elevations upstream, creating no current or low flow velocity conditions.

Bankfull Depth - refers to the maximum depth of flow measured from the channel thalwag to the estimated bankfull elevation.

Bankfull Discharge - the discharge corresponding to the stage at which flow is contained within the limits of the river channel, and does not spill out onto the floodplain. The stage just before over bank flow begins.

Basal springs - shallow groundwater upwelling that supplies surface flow into secondary or capillary channels on the floodplain.

Belt width - the linear distance, perpendicular to the valley’s axis, within the floodplain, and including the width of the channel(s), where channel shifting and/or lateral migration forms a bare or sparsely vegetated depositional or eroded surface that may be estimated from aerial photographs.

Belt width encroachment - the act of encroaching or advancing beyond natural limits; for example the impact of a road or irrigation ditch, in the area adjacent the active channel estimated to be the braid or meander belt; the area where the channel frequently shifts its course, building and eroding new surfaces (see Belt width).

Bendway translation - a geomorphic process where a river channel bend migrates down-valley.

Bifurcation - the division or forking of the active channel into two branches.

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Bimodal - in statistics, having two modes within a distribution. In this context, bimodal refers to two modes of bed material size where the majority of bed particles fall within two distinct size classes (i.e., sand and cobbles).

Convex profile - refers to a shape that is curved or rounded outward.

Debris jam potential - the potential for an open channel or bridge structure spanning a channel to accumulate debris such as large woody materials, which may also impact water flow and/or sediment conveyance.

Floodplain swales - depressions in the floodplain, which are often remnant channels that have partially filled in with sediment and/or vegetation.

Fluvial - formed or produced by the action of flowing water; of, pertaining to, or inhabiting a river or stream.

Funnel - of or pertaining to a conical form, for example, river bank revetment used to channel flows under a narrow opening, the apex of the cone; in this case, a bridge or culvert crossing along a road embankment.

Geomorphic threshold - the threshold or sudden change of landform stability that is exceeded either by intrinsic change of the landform itself, or by a progressive change of an external variable. In this context, threshold refers to the point where episodic change in river course, form, or pattern occurs.

Geomorphology - the study of landscape evolution including shape, form and process through space and over time. It is the earth science that focuses on understanding the processes of erosion, weathering, transport, and deposition, with measuring the rates at which such processes operate, and with quantitative analysis of the forms of the ground surface and the materials of which they are composed (Goudie et. al. 1994).

Glacio-lacustrine - of or pertaining to lakes formed by a glacier.

Lacustrine - of or pertaining to lakes.

Large Woody Debris (LWD) - Functional wood in streams is called large woody debris. The definition of large woody debris has evolved in the scientific, regulatory and political arenas to include wood as small as four inches in diameter and six feet in length. However, the typical size of LWD are 18-36 inches in diameter and 12 – 32 feet in length.

Maximum Belt Width - the maximum braid or meander belt width for a given channel segment or reach.

Morphology - of or pertaining to shape.

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Permeable Training Dikes - bank protection structures with openings that permit surface flow through but at reduced velocities. These structures do not isolate large segments of floodplain from flood flows like more traditional riprap or impermeable dike structures. Their purpose is to deflect flows back towards the center of the channel to prevent and/or control lateral migration and channel movement.

Perturbation - a disturbance or variation, usually of small amplitude, about some well-defined, usually basic or typical state.

Planform - the configuration of a river channel system as viewed from above.

Prograding - the advancing or growth of a bar deposit.

Rosgen classification - a system of river channel classification developed by Rosgen (1994) that uses a letter and number system (i.e., B4, C3) as nomenclature to describe the geomorphic character of the stream channel, floodplain, and surrounding valley. Physical variables used in a morphological description Level II Rosgen classification include channel gradient, bed material type and size, channel pattern, and channel geometry.

Sediment continuity - where sediment input equals sediment output.

Seral-stage - of or pertaining to plant succession its relation to disturbance mechanisms such as floods or fires over time. A particular plant community type or dominant species may represent a seral-stage along a temporal scale; a climax stage represents the most mature and stable state prior to disturbance.

Sinuosity - the measurement of a channel’s relative straightness or curving configuration. It is the ratio of channel length to downward valley length; for example, a value of one 1.0 is a straight channel pattern, whereas a sinuosity of 1.5 is considered meandering.

Stream competency - the ability of a stream to mobilize its sediment load; refers to the maximum size of particles of given specific gravity, which, at a given velocity, the stream will move.

Stream power - a concept that relates fluvial energy to sediment transport. To transport sediment, work (defined as the product of force and distance) must be performed. Power is the rate of doing that work, and stream power per unit length of stream. It is expressed as the product of the specific weight of water, discharge, and water surface slope.

Subaqueous return flow - existing or situated under water, as in the movement of shallow groundwater through riverbank materials to open channel flow.

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UTM Coordinate (Universal Transverse Mercator) - a projection where the globe is divided into 60 north and south zones, each spanning six degrees of longitude. Each zone has its own central meridian, with the division between north and south zones occurring at the equator. The X-coordinate is latitude and the Y-coordinate is longitude.

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Appendix B. Project Reach Maps (Plates)

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Appendix C. Historic Aerial Photo Sheets at Bridges

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