Appendix C

Define the Proposed Activity

Project proponent

National Park Service, Yosemite National Park

Purpose and need for the project

The purpose of the Happy Isles Gauging Station Bridge Removal Project is to:

·          Protect visitor health and safety by eliminating the threat posed by the failing concrete bridge.

·          Protect park resources from localized flooding that could result from uncontrolled bridge collapse during a high water period.

·          Prevent the difficult and potentially dangerous removal of bridge debris from the river that would be required if the bridge collapsed.

·          Protect park infrastructure (e.g., the historic Happy Isles Gauging Station and Happy Isles Vehicle Bridge, as well as water supply lines) from possible damage due to uncontrolled bridge collapse by removing the bridge before it collapses of its own accord.

·          Protect Happy Isles Gauging Station from gauge calibration distortion caused by an uncontrolled bridge collapse.

·          Enhance the free-flowing condition of the river.

The need for the proposed project arose as a result of the 1997 flood damage to the Happy Isles Gauging Station Bridge. Structural damage sustained by the bridge during the 1997 flood, as well as subsequent deterioration and structural failure (including a large sinkhole on the river-left abutment), have resulted in the need to remove the failing bridge before it collapses of its own accord. The National Park Service intends to remove the failing structure, which has been closed to the public since 1997.

Initial and subsequent evaluations by the Federal Highway Administration in 1997, 1998, and 2001 (letters on file at Yosemite National Park) indicate that the bridge is in danger of collapse due to severe flood damage, and total collapse of the river-left side of the structure is imminent. The undermined river-left abutment and subsequent settlement have rendered the Happy Isles Gauging Station Bridge unsafe.

Geographic location of the project

The Happy Isles Gauging Station Bridge is located on the main stem of the Merced River in Yosemite National Park, at the far east end of Yosemite Valley (Latitude 37°43'54", Longitude 119°33'28" NAD27). Refer to Figure I-1 of the Happy Isles Gauging Station Bridge Removal Project Environmental Assessment.

Project description

The proposed action is the Controlled Demolition Alternative, which involves partially removing the Happy Isles Gauging Station Bridge. The river-right abutment, which is located on a large boulder on the riverbank, will be retained to protect the historic Happy Isles Gauging Station. The Controlled Demolition Alternative will involve separating the bridge into liftable segments and removing the segments using equipment located on the riverbank. A temporary containment system (e.g., a reinforced tarp, netting,  cage, or floating barge) will be installed beneath the Happy Isles Gauging Station Bridge to catch small amounts of debris during partial bridge removal. This containment system will prevent slurry from concrete saws, as well as small debris, from falling into the Merced River. However, not all demolition debris would be prevented from falling into the river; masonry debris greater than 2 inches in any dimension and all metal debris that inadvertently fall into the river would be removed. A temporary structural support system (e.g., scaffolding, jacks, or mechanical lifts) may be installed, if necessary, to prevent uncontrolled collapse of the bridge structure during demolition or to anchor the containment system.

Once the river-left abutment has been removed, the existing void in the riverbank will be filled and shaped to match adjacent bank composition (boulders, cobbles, and gravels) and contour. The large size of substrate materials along this stretch of the river provides for bank stability. Riparian vegetation will be planted to match conditions of adjacent portions of the riverbank, but would not be relied upon to provide for bank stability along this steep gradient section of the river.  A viewing platform will be established on the river-right abutment of the bridge. The platform will allow visitors views of the Merced River.

Duration of the proposed activities

The U.S. Army Corps of Engineers requires that demolition activities occur during low water months. In-channel activities therefore will take place during the fall of 2001, when flow of the Merced River is expected to be less than 200 cubic feet per second (approximately 95% of the time). Bridge demolition will be avoided during higher flow periods.

Magnitude and/or extent of the proposed activities

Refer to the Happy Isles Gauging Station Bridge Removal Project Environmental Assessment, Chapter IV, Environmental Consequences for detailed data concerning potential impacts of the proposed action.

Mitigation

The proposed action will protect Outstandingly Remarkable Values from possible damage from uncontrolled collapse of the bridge, make a small improvement to the free-flowing condition of the river, and restore natural fluvial processes in the vicinity of the river-left abutment of the bridge. Mitigation (e.g., Best Management Practices, and resource-specific measures) is incorporated into the proposed action. Refer to the Happy Isles Gauging Station Bridge Removal Project Environmental Assessment, Chapter II, Alternatives for mitigation measures incorporated into the proposed action.

Relationship to past and future management activities

The Yosemite Valley Plan calls for the replacement of the Happy Isles Gauging Station Bridge. The Happy Isles Gauging Station Bridge Removal Project is being executed consistent with the Merced Wild and Scenic River Comprehensive Management Plan and General Management Plan. Implementation of the proposed action will not alter management of this river segment.

Describe How the Proposed Activity Will Directly Alter Within-Channel Conditions

The position of the proposed activity relative to the streambed and streambanks

The Happy Isles Gauging Station Bridge is located within the bed and banks of the Merced River. The demolition activities to remove the bridge will occur within the bed and banks of the Merced River as well.

Any likely resulting changes in:

Active channel location

Removal of the Happy Isles Gauging Station Bridge will eliminate an existing constriction to the natural flow of the Merced River. Currently, the bridge is causing localized bank erosion, stream widening, and can result in storm-related flooding that adversely impacts areas beyond the riverbanks and threatens park facilities. Once removed, the river channel is expected to return to a more natural flow condition, similar to what existed prior to bridge construction. The river-right abutment, which will be left in place, will not impede river flows during normal flow conditions because the abutment is anchored on a large boulder that controls the majority of flow around the abutment. The active channel location will not be altered. The bridge removal action will improve the active channel by returning it to more natural conditions.

Channel geometry (cross-sectional shape, width, depth characteristics)

The project will remove the river-left abutment and bridge deck leaving the river-right abutment intact to protect the Happy Isles Gauging Station from flood damage and excessive scour. The bridge removal will eliminate a constriction to natural river flow and removing the river-left abutment will reduce the occurrence of stream bank erosion and scour that form downstream eddies adjacent to the abutment. The river reach in the immediate vicinity of the bridge will be returned to flow conditions similar to those that existed prior to the bridge construction. Overall channel geometry, both in the project vicinity and along the entire reach will be unaffected.

Channel slope (rate or nature of vertical drop)

The current configuration of the bridge does not alter the slope of the river channel and the overall channel slope through this segment will remain unaffected by the bridge removal.

Channel form (straight, meandering, or braided)

The Merced River through the project area is a straight river channel underlain by boulders and cobbles. Removal of the bridge will not affect the channel form.

Relevant water quality parameters (turbidity, temperature, nutrient availability)

The removal of the bridge will not result in a long-term net increase or decrease of turbidity, temperature, nutrient availability, or other pollutant loads (sediment, bacteria) within the Merced River.

Navigation of the river

River navigation is not applicable to this section of river.

Describe How the Proposed Activity Will Directly Alter Riparian and/or Floodplain Conditions

The position of the proposed activity relative to the riparian area and floodplain

The Happy Isles Gauging Station Bridge is located within the bed and banks of the Merced River – below ordinary high water and within the 100-year floodplain.

Any likely resulting changes in:

Vegetation composition, age structure, quantity, or vigor

Project implementation could result in the possible removal of one mature tree. The tree would be removed only if deemed necessary. Should the tree be removed, the National Park Service would either cut and remove the tree from the site, cut and retain the tree to contribute woody debris to the river, or destabilize and control the fall of the tree to retain woody debris and a natural-appearing fallen tree with the root ball attached. Such activities would occur subsequent to bridge demolition. Removal of the river-left abutment will somewhat improve free-flow of the Merced River and return this portion of the river to a more natural state, thereby enhancing its biological integrity. Minor regrading and revegetation will increase bank and vegetation integrity in this area.

Relevant soil properties such as compaction or percent bare ground

No long-term adverse impact to soil resources is anticipated.  The project may decrease concentrated visitor use on the riverbank in the river-left abutment area thereby reducing soil compaction and improving riverbank soil conditions.

Relevant floodplain properties such as width, roughness, bank stability, or susceptibility to erosion

Currently, the Happy Isles Gauging Station Bridge constricts river flow. As was observed during the January 1997 flood, the bridge can act as a debris dam, forcing flood waters to leave the riverbanks and flood areas adjacent to the river. Because the project will remove the constriction and the river has ample capacity, the river is not expected to overflow its banks at flood stage. The project restores the river to more natural flow conditions and therefore, will reduce the potential for flooding due to river constriction. The project will not affect the natural floodplain properties within this reach.

Once the river-left abutment has been removed, the existing void in the riverbank will be filled and shaped to match adjacent bank composition (boulders, cobbles, and gravels) and contour. The large size of substrate materials along this stretch of the river provides for bank stability. Riparian vegetation will be planted to match conditions of adjacent portions of the riverbank, but would not be relied upon to provide for bank stability along this steep gradient section of the river. Because this reach of the Merced River is underlain by large granitic boulders and these boulders naturally armor the riverbank, the placement of boulders in the vicinity of the river-left abutment would essentially replace boulders that were removed when the bridge was built. This will return the bank to a more natural condition, as can be seen immediately upstream and downstream of the bridge.

Describe How the Proposed Activity Will Directly Alter Upland Conditions

The position of the proposed activity relative to the uplands

The proposed action is located within the bed and banks of the Merced River. Uplands will not be affected by the proposed action.

Any likely resulting changes in:

Vegetation composition, age structure, quantity, or vigor

No measurable effect to age, vigor, quantity, structure or composition of upland vegetation. Minor regrading and revegetation would increase local bank and vegetation integrity. Implementation of these measures under the proposed action would result in localized beneficial effects on vegetation. The extent and quality of vegetation throughout the remainder of the Yosemite Valley segment of the river would be unaffected.

Relevant soil properties such as compaction or percent bare ground

No measurable effect to upland soil resources. Site restoration and stabilization would repair eroded areas and increase the protection of riverbanks, adjacent trails, and Yosemite Valley’s water supply line, resulting in a localized beneficial impact on soils. Native soils throughout the remainder of the Yosemite Valley segment of the river would be unaffected.

Relevant hydrologic properties such as drainage patterns or the character of surface and subsurface flows

No measurable effect to upland hydrologic properties. No portion of the proposed action, including equipment staging, demolition activities, or materials storage, would be located within or otherwise affect surface or subsurface drainage patterns from the uplands to the Merced River.

Potential changes in upland conditions that would influence archeological, cultural, or other identified significant resource values

All actions would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement. The proposed action would have no measurable effect to archeological, cultural, or other identified significant resource values in upland areas.

Evaluate and Describe How Changes in On-Site Conditions Can or Will Alter Existing Hydrologic or Biologic Processes

The ability of the channel to change course, re-occupy former segments, or inundate its floodplain

Currently, the Happy Isles Gauging Station Bridge constricts river flow. As was observed during the January 1997 flood, the bridge can act as a debris dam, forcing flood waters to leave the riverbanks and flood areas adjacent to the river. Because the project will remove the constriction and the river has ample capacity, the river is not expected to overflow its banks at flood stage. The project restores the natural river flow conditions and therefore, will reduce the potential for storm-stage flooding due to river constriction.

Streambank erosion potential, sediment routing and deposition, or debris loading

Streambank erosion potential is highest when river flow is constricted by an in-stream structure such as a bridge. The proposed action will remove the bridge, thereby removing the constriction, restoring flows to more natural conditions, and restoring natural erosion, sedimentation, and depositional processes. The presence of the river-right abutment will not initiate or increase streambank scour caused by upstream eddies because the abutment is anchored on a large boulder that controls the majority of flow around the abutment.

Once the river-left abutment has been removed, the existing void in the riverbank will be filled and shaped to match adjacent bank composition (boulders, cobbles, and gravels) and contour. The large size of substrate materials along this stretch of the river provides for bank stability. Riparian vegetation will be planted to match conditions of adjacent portions of the riverbank, but would not be relied upon to provide for bank stability along this steep gradient section of the river. Because this reach of the Merced River is underlain by large granitic boulders and these boulders naturally armor the riverbank, the placement of boulders in the vicinity of the river-left abutment would essentially replace boulders that were removed when the bridge was built. This will return the bank to a more natural condition, as can be seen immediately upstream and downstream of the bridge.

The amount or timing of flow in the channel

Removal of the bridge deck and river-left abutment will not affect flow rates or discharge of the river.

Existing flow patterns

Removal of the bridge deck and river-left abutment will eliminate an existing constriction to the natural flow of the Merced River. Once removed, the river channel is expected to return to a more natural flow condition, similar to what existed prior to bridge construction. By removing the bridge deck and river-left abutment, constriction of flood waters behind the bridge will be eliminated and the river should remain within its banks during flood stage. Retaining the river-right abutment will not impede river flows during normal flow conditions because the abutment is anchored on a large boulder that controls the majority of flow around the abutment.

Surface and subsurface flow characteristics

The proposed action will have no measurable effect on surface or subsurface flow characteristics. No portion of the proposed action, including equipment staging, demolition activities, or materials storage, would be located within or otherwise affect surface or subsurface drainage patterns from the uplands to the Merced River.

Flood storage (detention storage)

By removing the bridge deck and river-left abutment, constriction of flood waters behind the bridge will be eliminated and the river will remain within its banks during flood stage. Otherwise, there will be no measurable effect to flood storage as a result of the proposed action.

Aggregation and/or degradation of the channel

No measurable effect to aggregation and/or degradation of the channel. The proposed action will remove the bridge deck and river-left abutment, thereby removing the river-constricting bridge, restoring natural flows, and restoring natural erosion, sedimentation, and depositional processes.

Biological processes such as:

Reproduction, vigor, growth and/or succession of streamside vegetation

Removal of the river-left abutment will somewhat improve the free-flow of the Merced River and return this portion of the river to a more natural state, thereby enhancing its biological integrity. Minor regrading and revegetation in this area will improve bank and vegetation integrity.

Nutrient cycling

No measurable effect is anticipated. The proposed action will not adversely affect riparian vegetation, woody debris or free-flowing characteristics (major contributing components of riverine nutrient cycling) of the Merced River. Local nutrient availability and cycling may be temporarily affected during the 2-month demolition period due to an increased amount of fine sediment released in the river. However, the sediment dislodged by construction associated with the proposed action is anticipated to be less than would occur under the No Action Alternative, as construction activities would be centralized in one location along the riverbank. In the long-term, nutrient availability will be enhanced because the minor regrading and revegetation will improve bank and vegetation integrity.

Fish spawning and/or rearing success

No measurable effect is anticipated. The river in the vicinity of Happy Isles Gauging Station Bridge is swift and lacks fish spawning or rearing habitat (e.g., riffles, pools, gravel substrate). Minor regrading and revegetation would increase bank integrity, somewhat improving fish habitat. The extent and quality of fish habitat throughout the remainder of the Yosemite Valley segment of the river would be unaffected.

Riparian dependent avian species needs

No measurable effect is anticipated. The river in the vicinity of Happy Isles Gauging Station Bridge supports no riparian vegetation. No riparian vegetation would be removed under the proposed action. The extent and quality of avian habitats throughout the remainder of the Yosemite Valley segment of the river would be unaffected.

Amphibian/mollusk needs

No measurable effect is anticipated. The river in the vicinity of Happy Isles Gauging Station Bridge is swift and lacks amphibian or mollusk habitat. The proposed action would not have an adverse effect on amphibian or mollusk needs. The extent and quality of amphibian and mollusk habitats throughout the remainder of the Yosemite Valley segment of the river would be unaffected.

Species composition (diversity)

No measurable effect is anticipated. Upon project completion, the biological integrity of the site would be enhanced.

Estimate the Magnitude and Spatial Extent of Potential Off-Site Changes

Consider and document:

Changes that influence other parts of the river system

The effects of the proposed action are localized and will not result in changes that will influence other portions of the Merced River system.

The range of circumstances under which off-site changes might occur (for example, as may be related to flow frequency)

Once the bridge is removed, river flow will no longer be constricted in this area, and natural flow processes will largely prevail. There are no obvious circumstances under which offsite change will occur.

The likelihood that predicted changes will be realized

The predicted change for this project is that more natural, unconstricted flows will be restored under the proposed action. There is every indication that these predicted changes will be realized.

Specify processes involved, such as water and sediment, and the movement of nutrients

Natural fluvial processes such as free-flow and sediment transport would be enhanced upon completion of the proposed action.

Define the Time Scale Over Which Steps 3-6 are Likely to Occur

Review steps 3-6, looking independently at the element of time.  Define and document the time scale over which the effects will occur

Demolition will occur over a period of about 2 months. Riverine system adjustment to a natural hydrologic regime will be immediate.

Scientific – The entire river corridor constitutes a highly significant scientific resource because the river watershed is largely within designated Wilderness in Yosemite National Park. Scientific Outstandingly Remarkable Values relate to the Merced River’s value for research. This Outstandingly Remarkable Value applies to all the Merced River segments.

The proposed action would remove the bridge using controlled demolition methods. Controlled demolition would avoid catastrophic collapse of the bridge, which could damage the historic Happy Isles Gauging Station and alter the gauging station pool. Maintenance of the gauging station and pool is important for scientific research purposes on a segment-wide level in Yosemite Valley. The gauging station has recorded hydrologic data since 1915, and has produced the longest continuous record of streamflow data available anywhere in California and is one of the 49 benchmark gauges in the United States. The gauging station also acts as an early warning system for floods in Yosemite Valley. The proposed action would retain the river-right abutment to protect the Happy Isles Gauging Station from damage and excessive scouring in high flow conditions. Segment-wide the proposed action would contribute to the protection of the scientific Outstandingly Remarkable Value. The proposed action could have a beneficial effect on the scientific Outstandingly Remarkable Value on a segment-wide basis.

Scenic – This segment provides magnificent views from the river and its banks of waterfalls (Nevada, Vernal, Illilouette, Yosemite, Sentinel, Ribbon, Bridalveil, and Silver Strand), rock cliffs (Half Dome, North Dome/Washington Column, Glacier Point, Yosemite Point/Lost Arrow Spire, Sentinel Rock, Three Brothers, Cathedral Rock, and El Capitan), and meadows (Stoneman, Ahwahnee, Cook’s, Sentinel, Leidig, El Capitan, and Bridalveil). There is a scenic interface of river, rock, meadow, and forest throughout the segment.

The proposed action would provide a river-viewing platform on top of the river-right abutment, which would provide opportunities from this location for views from the river and its banks of waterfalls, rock cliffs, and the scenic interface of river, rock, meadow, and forest. As compared to the No Action Alternative, there are no such viewing opportunities from this location. The proposed action would protect the scenic Outstandingly Remarkable Value on a localized level by providing a river-viewing platform on the river-right abutment. On a segment-wide level, however, the proposed action would contribute only negligibly to the enhancement of the scenic Outstandingly Remarkable Value. Therefore, the proposed action would have no effect on the scenic Outstandingly Remarkable Value on a segment-wide level.

Geologic Processes/Conditions – This segment contains a classic, glaciated, U-shaped valley, providing important examples of a mature meandering river; hanging valleys such as Yosemite and Bridalveil Creeks; and evidence of glaciation (e.g., moraines below El Capitan and Bridalveil Meadows).

The proposed action would have no effect on the geologic processes/conditions Outstandingly Remarkable Value. The controlled demolition of the Happy Isles Gauging Station Bridge would not affect the classic U-shaped valley, hanging valleys, or evidence of glaciation.

Recreation – This segment offers opportunities to experience a spectrum of river-related recreational activities, from nature study and sightseeing to hiking. Yosemite Valley is one of the premier outdoor recreation areas in the world.

The proposed action would provide a river-viewing platform on top of the river-right abutment, which would allow opportunities to experience a spectrum of river-related recreational activities, including nature study, sightseeing, photography, and enjoying solitude and natural sounds along the river over the long term.

Recreation (cont.)

Provision of a river-viewing platform would negligibly enhance the recreation Outstandingly Remarkable Value on a localized level. The effects of the proposed action would be limited to the immediate vicinity of the Happy Isles Gauging Station Bridge, and would have no effect on the spectrum of river-related recreational activities throughout the remainder of Yosemite Valley. Although the proposed action would have localized beneficial effects, on a segment-wide level the proposed action would have no effect on the recreation Outstandingly Remarkable Value.

Biological – Riparian areas and low-elevation meadows are the most productive communities in Yosemite Valley. The high-quality, vast riparian, wetland, and other riverine areas provide rich habitat for a diversity of river-related species, including special-status species, neotropical migrant songbirds, and numerous bat species.

The proposed action would involve minor regrading and revegetation of the river banks in the immediate vicinity of the Happy Isles Gauging Station Bridge, which would have site-specific, long-term, beneficial effects on bank and vegetation integrity. The proposed action also would somewhat improve riparian, wetland, and other riverine areas that provide rich habitat for a diversity of river-related species, including special status species.

Catastrophic collapse of the bridge under the No Action Alternative could result in extensive erosion, as well as an uncontrolled release of bridge debris into the Merced River that could temporarily affect downstream riparian and aquatic resources and river-related special-status species. The proposed action would avoid these impacts to biological resources.

The effects of the proposed action would be largely limited to the Happy Isles Gauging Station Bridge area, and would have no effects on river-related biological resources throughout the remainder of Yosemite Valley. Although the proposed action would locally enhance this Outstandingly Remarkable Value, on a segment-wide level the proposed action would have no effect on the biological Outstandingly Remarkable Value.

Cultural – This segment contains evidence of thousands of years of human occupation, reflected in the large number of archeological sites and continuing traditional use today. Nationally significant historic resources are found here, such as designed landscapes and developed areas, historic buildings, and circulation systems (trails, roads, and bridges) that provide visitor access to the sublime views of natural features that are culturally valuable.

Happy Isles Gauging Station Bridge is a component of the Valley segment cultural Outstandingly Remarkable Value. As part of the National Historic Preservation Act Section 106 documentation of the Yosemite Valley Cultural Landscape, Happy Isles Gauging Station Bridge is listed as a contributing resource related to Yosemite Valley’s circulation system.

Happy Isles Gauging Station Bridge has sustained extensive damage in recent years, the bridge was damaged by felled trees during the massive rockfall and related windblast in the Happy Isles area in July 1996, and was severely damaged in a major flood in January 1997. Subsequent to the flood, Happy Isles

Cultural (cont.)

Gauging Station Bridge was deemed unsafe by representatives of the Federal Highway Administration. The National Park Service closed the bridge for safety reasons in July 1997. The cultural integrity of the Happy Isles Gauging Station Bridge has been undermined due to its inability to function in the manner for which it was designed.

Continued deterioration of the bridge would likely result in its uncontrolled collapse. Since the bridge is failing, controlled removal under the proposed action would be beneficial for other components of the Yosemite Valley Cultural Landscape, such as Happy Isles Vehicle Bridge and Happy Isles Gauging Station, both of which could be damaged should the Happy Isles Gauging Station Bridge catastrophically collapse. The proposed action would be undertaken by the National Park Service in accordance with stipulations in the park’s 1999 Programmatic Agreement.

Although the controlled removal of the Happy Isles Gauging Station Bridge would result in the loss of a contributing element of the Yosemite Valley Cultural Landscape and a component of the Valley segment cultural Outstandingly Remarkable Value, the effects would be localized. The Valley segment cultural Outstandingly Remarkable Value includes circulation systems (trails, roads, and bridges) in their entirety. Although the loss of an individual component of such a circulation system is locally adverse, the loss of individual elements (particularly an element that is in danger of uncontrolled collapse and whose cultural integrity has been undermined due to its inability to function in the manner for which it was designed) would not degrade the cultural Outstandingly Remarkable Value on a segment-wide basis. In addition, controlled removal of the bridge would avoid potential damages to other components of the cultural Outstandingly Remarkable Value, including Happy Isles Vehicle Bridge and the Happy Isles Gauging Station. The adverse effects of the proposed action would be limited to Happy Isles Gauging Station Bridge, and would have no effect on significant historic resources throughout the remainder of Yosemite Valley. Although the proposed action would have localized adverse effects, on a segment-wide level the proposed action would have no effect on the cultural Outstandingly Remarkable Value.

Cultural (cont.)

Controlled removal of the Happy Isles Gauging Station Bridge could have an adverse impact to historic archeological resources due to ground-disturbing activities. The adverse effects of the proposed action would be limited to the immediate vicinity of the Happy Isles Gauging Station Bridge, and would have no effect on historic archeological resources throughout the remainder of Yosemite Valley. Although the proposed action would have localized adverse effects, on a segment-wide level the proposed action would have no effect on the cultural Outstandingly Remarkable Value.

Ethnographic resources, including traditional use areas, would not be affected under the proposed action.

Hydrologic Processes – This segment is characterized by a meandering river, world-renowned waterfalls, an active flood regime, oxbows, unique wetlands, and fluvial processes.

Controlled removal of Happy Isles Gauging Station Bridge would help restore the active flood regime and fluvial processes in the immediate vicinity of the bridge. The removal of the Happy Isles Gauging Station Bridge would eliminate constriction of river flow and improve the local, natural hydrologic regime. The proposed action would have a localized, beneficial effect on the flood regime and fluvial processes.

In addition, the proposed action would avoid localized, catastrophic adverse effects to fluvial processes in the Happy Isles area associated with collapse of the Happy Isles Gauging Station Bridge under the No Action Alternative. Controlled removal of the bridge would avoid potential bank erosion and localized flooding associated with catastrophic bridge collapse. The avoidance of these adverse effects would have a localized, beneficial effect on fluvial processes.

The effects of the proposed action would be largely limited to the Happy Isles Gauging Station Bridge area, and would have no effects on fluvial processes or the flood regime throughout the remainder of Yosemite Valley. Although the proposed action would locally enhance this Outstandingly Remarkable Value, on a segment-wide level the proposed action would have no effect on the hydrological Outstandingly Remarkable Value.