Chapter I: Purpose and Need
History of Cascades Diversion Dam
Cascades Diversion Dam (dam) is located on the main stem
of the Merced Wild and Scenic River
in Yosemite National
Park at an elevation of 3,800 feet near the
intersection of El Portal Road
and Big Oak Flat Road (figure
I-1). Cascades Diversion Dam was constructed in 1917 to divert the flow of the
main stem of the Merced River into a hydroelectric
generating facility. The dam itself consists of “cribs” formed by logs or
square timbers spiked together. These cribs were then filled with boulders and
rocks and were anchored by concrete facing roughly 18 feet below the crest
of the dam. The cribs were sheathed with rough-hewn redwood boards, which are
visible during periods of low water. The dam spans 184 feet across, with a
crest height of about 17 feet, flanked by 30-foot-high concrete abutments.
While in operation, the hydroelectric generating facility provided a maximum
output of 2,000 kilowatts of electricity. The hydroelectric facility was
taken off line in 1985 and 1986 because it was deteriorated and outdated.
Turbines and other equipment were removed from the powerhouse, and the majority
of the penstock was removed.
Some elements of the former hydroelectric generating
facility still remain, including abutments that flank the dam, an intake
structure, a screenhouse, 300 feet of concrete penstock (which historically
conveyed water from the dam to the powerhouse), the powerhouse, and the
transmission lines. Today the powerhouse is used as a transfer facility from
Pacific Gas and Electric Company transmission lines to National Park Service
transmission lines. The dam, abutments, intake structure, and screenhouse
remain as they were when decommissioned in 1986.
As part of the decommissioning of the hydroelectric
facility, the National Park Service determined that Cascades Diversion Dam and
ancillary facilities would be removed. Dam removal was to occur in 1999 but was
halted until an approved comprehensive management plan for the Merced Wild and Scenic
River was completed. A record of
decision on the Merced Wild and Scenic
River Comprehensive Management Plan and its Final Environmental Impact Statement was signed in August 2000 and
later revised in November 2000.
Cascades Diversion Dam and intake structure with screens
were determined to be eligible for nomination to the National Register of
Historic Places in 1982 (NPS 1982). The National Park Service has completed all
cultural resource compliance procedures required under the National Historic
Preservation Act for removal of the dam and associated features (NPS 1987b).
Refer to Chapter VI, Consultation and Coordination, for a history of
environmental compliance related to Cascades Diversion Dam.
FIGURE I-1 – PROJECT LOCATION
Purpose Of the Cascades Diversion Dam Removal Project
The purpose of the Cascades Diversion Dam Removal Project
is to remove an unnatural obstruction on the Merced River
and to restore the river’s natural free-flowing condition. This removal project
is consistent with the Wild and Scenic River guidance provided in the Merced
Wild and Scenic River Comprehensive Management Plan (Merced River Plan) (NPS
2001a) and will meet the direction of the Yosemite Valley Plan (NPS 2000a),
which calls for the dam’s removal.
Need For the Cascades Diversion Dam Removal Project
Cascades Diversion Dam is classified as a high-hazard
structure (USBR 1997). It is in unsatisfactory condition due to flood damage sustained
in 1997 and to continuing deterioration associated with age. In addition, the
dam no longer serves a useful purpose – water is not diverted from the site to
generate electricity or for other uses, and the impoundment does not regulate high
water. Removal of the existing dam structure is necessary to prevent possible
uncontrolled and sudden failure, which could result in a release of impounded
water and the deposition of concrete and timber debris, grouted rockfill, and
impounded sediment along the downstream channel. Such an occurrence could pose
a considerable threat to valued resources (such as aquatic life, scenic vistas,
and recreational opportunities), infrastructure (El Portal
Road, wastewater, telephone, and electrical lines),
and human life.
In addition, the National Park Service is entrusted with
conserving and restoring park values. This responsibility includes protecting
the biological and physical processes that created the park, along with scenic
features, natural landscapes, and native plants and wildlife. The removal of the
dam would work toward fulfilling this mandate by restoring this segment of the Merced
River.
Relationship to Yosemite National
Park Plans
Planning in Yosemite
National Park takes two different
forms: general management planning and implementation planning. General
management plans are required for national parks by the National Park and
Recreation Act of 1978.
The purpose of a general management plan is to set a
“clearly defined direction for resource preservation and visitor use” (NPS 1998) and provide
general directions and policies to guide planning and management in the park.
The General Management Plan is the overall planning document for Yosemite
National Park.
The Merced River Plan is a general management plan that guides
management of the Merced Wild and Scenic Corridor. In designating the Merced
River as a Wild and Scenic River, Congress authorized the National Park Service
to prepare a management plan for the river by making appropriate revisions to
the park’s General Management Plan
(16 United States Code 1274[a][62]). The Merced River Plan, which amended
the General Management Plan in
certain respects, provides a framework for decision-making on future management
actions within the Merced Wild and Scenic
River corridor. The Cascades
Diversion Dam Removal Project complies with conditions outlined in the Merced
River Plan.
Implementation plans, which tier off of the General Management Plan and Merced River
Plan, focus on “how to implement
an activity or project needed to achieve a long-term goal” (NPS 1998).
Implementation plans may direct specific projects as well as ongoing management
activities or programs, and provide a more extensive level of detail and
analysis. The Yosemite Valley Plan is
an implementation plan that presents a comprehensive management plan for
Yosemite Valley. The Yosemite Valley Plan
guides protection of natural and cultural resources, opportunities for
high-quality resource-based visitor experience, reduction of traffic
congestion, and effective park operations. The Cascades Diversion Dam Removal
Project would implement an action called for in the Yosemite Valley Plan.
Regulations and Policies
The Cascades Diversion Dam Removal Project Environmental
Assessment is written within a complex set of regulations, policies, and
approved plans. The project must not only comply with requirements of the
National Environmental Policy Act, but it must do so within the parameters of
other legislation (see Appendix A, Regulations and Policies) that govern land
use within Yosemite National
Park.
National Park Service Organic Act
In 1916, the Organic Act established the National Park
Service in order to “promote and regulate the use of parks…” and defined the
purpose of the national parks as “to conserve the scenery and natural and
historic objects and wild life therein and to provide for the enjoyment of the
same in such manner and by such means as will leave them unimpaired for the
enjoyment of future generations.” This legislation provides overall guidance
for the management of Yosemite National
Park.
The Prohibition on Impairment of Park Resources and
Values
The Organic Act establishes the management responsibilities
of the National Park Service. While Congress has given the National Park
Service the management discretion to allow certain impacts within parks, that
discretion is limited by the statutory requirement that park resources and
values be left unimpaired, unless a particular law directly and specifically
provides otherwise. This cornerstone of the Organic Act establishes the primary
responsibility of the National Park Service. It ensures that park resources and
values will continue to exist in a condition that allows the American people to
have present and future opportunities to enjoy them. National Park Service Management Policies 2001 provides additional
guidance on impairment of park resources and values.
Management goals identify long-range direction for Yosemite
National Park. Any proposed project
must carefully balance multiple goals, especially in a park as large and
complex as Yosemite National
Park. This section presents the goals set forth
in Yosemite’s General
Management Plan, the Merced River Plan, and Yosemite Valley Plan.
General Management Plan Goals
The General
Management Plan for Yosemite
National Park sets forth five broad
goals for management of the park as a whole. These goals include:
-
Reclaim priceless natural beauty
-
Allow natural processes to prevail
-
Promote visitor understanding and enjoyment
-
Markedly reduce traffic congestion
-
Reduce crowding
Merced River Plan Goals
While the Merced River Plan adopts the five broad goals
from the General Management Plan, it
also outlines an additional set of goals for management of the Merced Wild and Scenic
River. The main stem and South Fork
of the Merced River were designated by Congress in 1987
for protection under the Wild and Scenic Rivers Act. The National Park Service
developed the five goals of the Merced River Plan to further the policy
established by the Wild and Scenic Rivers Act, namely to preserve designated
rivers in their free-flowing condition, and protect and enhance the river’s
Outstandingly Remarkable Values. The Merced River Plan goals include:
-
Protect and enhance river-related natural
resources
-
Protect and restore natural hydrological and
geomorphic processes
-
Protect and enhance river-related cultural
resources
-
Provide diverse river-related recreational and
educational experiences
-
Provide appropriate land uses
The Merced River Plan
applies seven management elements to prescribe desired future conditions,
typical visitor activities and experiences, and park facilities and management
activities allowed in the river corridor. The management elements as they
relate to Cascades Diversion Dam are discussed in Chapter V, Merced Wild and Scenic
River. The Wild and Scenic Rivers
Act Section 7 determination is included
in Appendix B, Merced Wild and Scenic River Section 7 Determination.
Yosemite Valley Plan Goals
The Yosemite Valley
Plan carries forward the goals of the General
Management Plan. In addition, the Yosemite
Valley Plan identifies four specific purposes of the plan. These include:
-
Restore, protect, and enhance the resources of Yosemite
Valley
-
Provide opportunities for high-quality,
resource-based visitor experiences
-
Reduce traffic congestion
-
Provide effective park operations, including
employee housing, to meet the mission of the National Park Service
The Yosemite Valley
Plan also identifies criteria for accomplishing the broad goals of the General Management Plan in Yosemite
Valley and the specific purposes of the Yosemite Valley Plan. The criteria include:
-
Protect and enhance natural and cultural
resources
-
Enhance visitor experience
-
Provide effective operations
-
Provide appropriate land uses
The following issues were raised during the public scoping
process conducted for this project (see Chapter VI, Consultation and
Coordination) and by National Park Service staff. These issues are addressed in
the analysis presented in Chapter IV, Environmental Consequences. Chapter II,
Alternatives includes a description of issues and concerns that are addressed
in other park plans as well as those that are beyond the scope of this project.
Natural Resources
Geomorphology/Hydrology
Cascades Diversion Dam has affected the natural flow, sediment
deposition, and natural processes of the Merced River
for over 80 years, altering the shape and nature of the river. The dam, in
concert with other facilities, has disrupted the natural river processes and
the functional relationship between the river and upland communities. These
issues are addressed in the natural resources analyses presented in Chapter IV,
Environmental Consequences.
Sedimentation, both during removal and over the long term
after dam removal, has the potential to affect (either beneficially or
negatively) downstream resources, including riparian habitats, fish and
wildlife, swimming holes, beaches, and sandbars. Chapter II, Alternatives,
and the natural resources analyses presented in Chapter IV, Environmental
Consequences, address the question of whether to completely remove sediments
behind the dam or to let natural river processes prevail. Public comment
indicated that the type and percentages of sediment impounded by the dam should
be discussed and potential disposal sites disclosed. These issues are addressed
in Chapter II, Alternatives and in the natural resources analyses presented in
Chapter IV, Environmental Consequences.
Concerns were raised that dam removal would have the
potential to increase turbidity and decrease water quality of the Merced
River, and that the project should include best management
practices to avoid or minimize such effects. This issue is addressed in Chapter
II, Alternatives and in the natural resources analysis presented in Chapter IV,
Environmental Consequences.
Fish, Wildlife, and Plant Habitat
The Merced River corridor supports
diverse aquatic and riparian habitats for plant and wildlife species. Natural habitats at the dam site have
been altered by facilities and decades of human use. Concerns were expressed
that dam removal should be designed to improve the environment and to avoid long-term adverse effects to the Merced River. Concern was also raised that uncontrolled
dam failure could affect riparian habitat. These issues are addressed in Chapter
II, Alternatives, and in the natural resources analyses presented in Chapter
IV, Environmental Consequences.
The upstream impoundment created by the dam may contain
species specialized to this slow-moving water environment. Dam removal would
alter the existing local ecosystem, returning the area to a more natural state.
Concerns were raised that species adapted to the impoundment could be adversely
affected by the proposed action. This issue is addressed in the natural
resources analyses presented in Chapter IV, Environmental Consequences.
Air Quality
Yosemite National
Park is a Class 1 airshed (under the Clean Air
Act) and therefore must maintain the highest standard of air quality. Dam-removal-related vehicle
trips may temporarily affect local air quality. This issue is addressed in the
air quality analysis presented in Chapter IV, Environmental Consequences.
Cultural Resources
Archeological Sites
As home to American Indians for thousands of years, the Merced
River corridor is rich with archeological sites, both historic and
prehistoric. Prehistoric sites are important for their research value and as a
tangible link to the heritage of culturally associated American Indian people.
Historic sites can provide information important to understanding past land use and management. Over the years, some of these
sites have been eroded by river meandering or covered by river sediments.
Facilities along the riverbanks have affected many of
these sites. Some historic archeological sites, such as dumps, have been
exposed; while these sites have scientific and interpretive value, they also
pose environmental and public health hazards. This issue is addressed in the
cultural resources analyses presented in Chapter IV, Environmental
Consequences.
Ethnographic Resources
Yosemite National
Park is part of a living tradition for local
American Indian groups. Many places along the river corridor
are important for traditional cultural uses and practices, including the
gathering of plant materials for food, basketry, and other uses, and the
conduct of traditional ceremonies. Many of these places and access to them have
been affected by visitor use and park development. Ethnographic resources
are addressed in Chapter III, Affected Environment.
Historic Structures and Cultural Landscapes
Historically significant structures and landscapes exist
throughout the Merced River corridor. Cascades Diversion
Dam is a historic structure, but one that restricts the free-flowing condition
of the Merced River. Historic structures and cultural
landscapes are addressed in Chapter III, Affected Environment.
Social Resources
Transportation
Cascades Diversion Dam is situated at one of the busiest
intersections in the park, at El Portal Road
and Big Oak Flat Road.
Roadside parking provides informal recreational access to areas
used for such activities as sightseeing, photography, and fishing. Measures may be necessary to
manage transportation and access during dam removal. Concern was
also raised that uncontrolled dam failure could affect El
Portal Road. These issues are addressed in the
transportation analysis presented in Chapter IV, Environmental Consequences.
Scenic Resources
The Merced River offers beautiful
views of the
river itself and of the surrounding landscape. Although unnatural, the dam provides
an interesting view of the calm impoundment upstream and the downstream rushing
cascades of the gorge. This issue is addressed in the scenic resources analysis
presented in Chapter IV, Environmental Consequences.
Recreation
Sightseeing, photography, and nature study are among the
recreational activities available within the area of Cascades Diversion Dam.
These activities may be temporarily affected during dam removal. Concern was
also raised that uncontrolled dam failure could affect downstream recreational
uses. These issues are addressed in the recreation analysis presented in
Chapter IV, Environmental Consequences.
Other Issues: Planning Processes and Management
Plans and Policies
The Merced River Plan (NPS 2001a) identifies management
zoning and management elements specific to the corridor of the Merced
River. Implementation of the proposed action must conform with
adopted goals and elements. This issue is addressed in Chapter V, Merced
Wild and Scenic River.
Conflicts Between Protection of Natural Landscapes and Cultural Landscapes
The protection of one type of resource can sometimes involve
actions that may be detrimental to other resources. For example, removal of
Cascades Diversion Dam would affect the historic landscape of Yosemite
Valley’s hydroelectric generating facility, but would restore the
free-flowing condition of the Merced River.
Relationship between the Cascades Diversion Dam Removal Project and the
El Portal Road Improvement Project – Cascades Diversion Dam to Pohono
Bridge
There has been considerable public interest in the
possible future realignment of El Portal Road
between the El Portal Road/Big Oak Flat Road
intersection and Pohono Bridge.
The National Park Service intends to remove Cascades Diversion Dam, let natural
processes prevail through this reach of the Merced River,
and allow the river to stabilize prior to planning, designing, or implementing
any proposed future improvement to this segment of El
Portal Road. Potential future designs would comply
with the Wild and Scenic Rivers Act, the National Environmental Policy Act, as
well as other legislation and park planning documents and would be subject to
public input and review. Although the Cascades Diversion Dam Removal Project is
completely distinct from this proposed future action, the road improvement
project is included in the cumulative impact analysis in this document (see
Appendix E, Projects Included in the Cumulative Impact Analysis).
The preferred and alternative plans for removal of Cascades
Diversion Dam and the evaluation of potential impacts of these alternatives are
integrated in this document and will be referred to collectively as the Cascades
Diversion Dam Removal Project Environmental Assessment. The contents of this
document are as follows:
-
Chapter I, Purpose and Need – The first chapter
includes a discussion of the project’s purpose and need, planning context,
relationship to management goals and objectives, and the scope of this
environmental assessment.
-
Chapter II, Alternatives – This chapter discusses
the No Action Alternative and action alternatives under consideration by the
National Park Service for Cascades Diversion Dam and provides a set of summary
tables comparing the alternatives.
-
Chapter III, Affected Environment – This chapter
provides an overview of the affected environment, or the existing condition of
Cascades Diversion Dam and its surroundings, including natural resources, cultural
resources, social resources, and
facilities.
-
Chapter IV, Environmental Consequences – This
chapter presents the analysis of the potential impacts of each alternative.
Supporting data are included as appendices.
-
Chapter V, Merced Wild and Scenic
River – This chapter analyzes the
consistency of the proposed action with the management elements of the Merced
River Plan.
-
Chapter VI, Consultation and Coordination – This
chapter summarizes the process of preparing and reviewing this document.
-
Chapter VII, List of Preparers – This chapter
lists the names and qualifications of the persons who are primarily responsible
for preparing and reviewing the document.
-
Chapter VIII, Glossary – This chapter defines
the technical terms and acronyms used in this document.
-
Chapter IX, Bibliography – This chapter lists
the references cited in this document.
