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Alternative Dam Heights
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Attachment F.39.1
Environmental Impacts of Alternative Dam Heights
Exhibit E of the License Application considers all aspects of project
construct:1.on and operation in relations to probable impacts on fish,
vegetation, wildlife and other resources of the project area. That
discussion is based on the Watana reservoir elevation 2185 alternative
combined with subsequent construction of the Devil Canyon dam and reservoir.
The following discussion generally compares the differences in impacts if a
lower max1mum normal water surface elevation were to be used at the Watana
site.
WATANA ALTERNATIVES
The majority of the anticipated impacts on terrestrial and aquatic resources
resulting from the construction and operation of the two dam project, as
described in the License Application, are related to the first phase of
development, the Watana 2185 dam and reservoir. The relative impacts of the
proposed Watana alternatives are therefore compared to those for the base
case Watana 2.185 development. Dams with lower reservoir normal maximum
water surface elevations (2100, 2000 or 1900 feet) would result in:
1) less area inundated,
2) less borrow material needed,
3) shorter construction periods
4) less inherent capacity for flood control and less regulation
of downs·::ream flows, and
5) greater reservoir drawdown.
These changes, in turn, would modify the ' :;a.:!ts that are described in many
sections of Exhibit E. Lower dam heights would be matched by a reduction 1n
installed capacity (but not the turbine discharge capacity). Modified
project operation schedules would, in turn, result in alterations in
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seasonal, and potentially weekly and daily, release patterns and therefore
in downstream flow regimes.
Area of Inundation
Table F.39a.l shows that at reservoir elevations of 2100, 2000 or 1900 feet,
the length of the reservoir would be 5, 10 and 15 miles shorter, respec-
tively, than at elevation 2185. Also the area inundated is 26, 48 and 62
percent less, respectively than for the reservoir at elevation 2185.
Exhibit E identifies the major impact issues directly related to the amount
of area inundated by the Watana development as:
Loss of grayling spawn1ng and rear1ng habitat
Removal of vegetation
Loss of winter/spring moose and spring bear habitat
Interference with big game movements and potential for accidents
Inun~ation of Jay Creek mineral lick
Inundation of raptor nests
Impacts on other wildlife
Impacts on existing archaeologic and aesthetic resources
Loss of grayling spawning and rear1ng habitat. The Watana 2185 reservo1r
will flood 54 miles of Susitna River mainstem habitat and 28 miles of
tributary habitat, including ten miles along Watana Creek, as well as
portions of other tributaries. The primary long-term impact is the
reduction of clear water spawning habitat in the tributaries that currently
supports a substantial population of grayling (estimated to be at least
15,000 in 1982). Future aquatic habitats within the reservoir area are not
expected to support a significant grayling population (page E-3-121).!/ In
addition, some reduction of burbot and whitefish spawning area is expected
in mainstream habitats.
-/ Unless otherwise identified, page references are to Exhibit E of the
License Application as filed, February 28, 1983.
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TABLE F.39a.l
SUSITNA HYDROELECTRIC PROJECT
ALTERNATIVE WATANA DEVELOPMENTS
Characteristic 2,185 2,100
Dam Height (ft) 885 BOO
Reservoir Area (Acres) 38,000 28,300
Total Reservoir Volume (103AF) 9,470 6s645
Active Reservoir Volume (103AF) 3:740 3,315
River Length Inundated (miles) 54 49
Inundated of Principal Clear-23.9 18.1
water Tributaries (stream miles)
Maximum Drawdown 120 150
Volume of Dam (106cy) 62 41
1/ License Application
2,000 1,900
700 600
19,800 14,500
4,248 2,546
2,370 1,675
44 39
14.4 10.7
150 150
25 16
Measures to minimize impoundment impacts would be to "substantially lower
the surface elevation of the reservoir or to maintain surface level during
the embryo incubation period" (page E-3-171). It will not be feasible to
maintain constant reservoir elevations during the grayling incubation period
(May and June) because of the need to refil the reservoir, but the
alternative Watana developments would have substantially lower r2servoir
surface elevations and therefore the reservoir would inundate
correspondingly fewer stream miles of tributary habitat than the 28 miles
inundated by the elevation 2185 development (Table F.39a.2). Deadman,
Watana, Kosina and Jay Creeks would be impacted by a reservoir at elevation
1900, but to a considerably smaller extent than by reservoirs with higher
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maximum water surface elevations. The falls on Deadman Creek, with crest
elevation of 1800 feet, would be inundated under all aternatives. The mouth
of Goose Creek is at an elevation of approximately 2060 feet at its
confluence with the Susitna River and would not be adversely affected by the
two lower alternatives. The mouth of the Oshetna River would be inundated
only by the Watana 2185 development.
Table F.30a.2
PRINCIPAL TRIBUTARY STREAMSa/
INUNDATED BY WATANA RESERVOIR
Location Length (miles) Inundated
River Elevation
Stream Mile at Confluence
(feet)
Deadman Creek 186.7 1,513
Watana Creek 194.1 1,552
Kosina Creek 106.9 1,670
Jay Creek 208.6 1' 700
Goose Greek 231.2 2.060
Oshetna River 233.5 2,110
Total for six tributaries
Other minor tributaries inundated
by Watana 2185 Development
by Reservoir El.
1900 2000 2100
0.7 1.2 1.7
6.1 7.7 9.2
2.2 3.2 3.9
1.7 2.3 3.0
0.3
10.7 14.4 18.1
b/ b/ b/
2185
2.3
10.4
4.6
3.6
1.1
1.9
23.9
4.1
28.0
a/ In addition, the lower portions o.f 39 smaller, unnamed tributaries will
be inundated, for 0.1 to 3.9 miles, by all four alternatives with an
additional 4, 12 and 13 tributaries inundated by the elevation 2000, 2100
and 2185 alternatives respectively.
b/ Not determined at this time for smaller tributaries but expected to be
proportionately less as reservoir elevation is lowered.
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Removal of Vegetation. "Construction of the Watana Devlopment will result
in the direct removal of vegetation within an area of approximately 35,605
acres (14,409 ha) covering a range of elevations from 1400 to 2400 feet (430
to 730 m). In addition, about 5,258 acres (2128 ha) of unvegetated areas
will be inundated or developed" (page E-3-225 as revised by supplemental
information filled with FERC on July 11, 1983). The total reservoir areas
associated with the smaller projects will require correspondingly less
removal of vegetation. Table F.39a.3 shows the total reservoir area and the
vegetation area of the reservoir for each of the Watana alternatives. The
Watana 2100, 2000 and 1900 al~ernatives would result in preservation of
about 9,000, 17,000 and 22,000 acres of natural vegetation, respectively,
with corr1:!sponding reductions in impacts to wildlife resources. Natural
vegetation that would be preserved by lower Watana dam heights primarily
consists of black spruce, white spruce, and mixed forest types.
Table F.39.3
RESERVOIR AREA AND REQUIRED
CLEARING FOR WATANA ALTERNATIVES
Retiervoir elevation (feet, msl) 2,185 2,100
Reservojr area (acres) 38,000a/ 28,300
River length inundated (miles) 54 49
Unvegetated area 5,400b/ 4,900E_/
Vegetated area 32,600 23,400
Percent reduction 1n required
clearing for reservoir 28
2,000 1,900
19,800 14,500
44 39
4,400b/ 3,900b/
15,400 10,600
53 67
a/ From Exhibit A, page A-2-1, and Exhibit E, Chapter 2, page E-2-55.
b/ Assumed to be proportional to river length inundated since this is
predominately open water.
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Loss of Moose and Bear Habitat. Removal of vegetation and filling of the
reservoir for the Watana 2185 development will reduce the carrying capacity
of the moose winter range. Also, the impoundment zones, particularly the
south-facing slopes, are important as a source of early spring foods and as
calving areas for moose. These zones also contain several large areas of
river valley bottomland with mixed spruce deciduous woodlands that may
provide critical ,...~oose habitat during years with severe winters. Brown
bears likewise make heavy spring use of the riparian vegetation and south-
facing slope habitat where they prey on moose calves and forage on new
spring vegetation and overwintered berries. The permanent loss of habitat
and early spring foods in the impoundment area may cause a decrease in the
carrying capacity of the aLea for brown bears. Loss of habitat will be most
significant for black bears. A large proportion of the acceptable black
bear habitat in the middle basin will be eliminated. Whereas no known brown
bear denning habitat will be affected by the Watana 2185 reservoir, 15 of 26
identified black bear den sites in the Watana impoundment area will
apparently be flooded (S. Miller, ADF&G, unpub. data). Lower reservoir
surface elevations would impact moose and ~ears to a correspondingly lesser
extent for each of the smaller reservoir alternatives. For example, 10 of
the 15 black bear dens potentially flooded by Watana 2185 occur within an
elevation range of 1900 to 2200 feet. Therefore, the number of den sites
actually flooded could vary from 5 to 15 depending on the dam height and the
exact elevation of the dens.
Interference with Big Game Movementso Reduction of reservo1r area;
particularly in the length of mainstem and tributary streams inundated and
the narrower reservoir width associated with the lower Watana developments,
will reduce the magnitude of impacts on the carrying capacity of the area
for big game species. Such a reduction would also reduce the potential for
interference with movements and the possibility for big game fatalitis
during river crossing attemptso Moose, caribou, brown and black bears, and
possibly Dall sheep cross the river in the project area. Barriers and
potential for accidents would be less at lower Watana ele~ations, and would
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not be present at some key cross1ng areas due tQ the shorter reservoir
lengths fe.g., in the vicinity of Goose Creek, the Oshetna River, and along
a portion of Watana Creek).
Inundation of Jay Creek Mineral Lick. Partial inundation of the Jay Creek
mineral lick may negatively impact the Watana Hills Dall sheep population.
With the reservoir at elevation 2185, up to 42 percent of the surface area
of the mineral lick would be inundated by the Watana impoundment (page E-3-
512). This lick app~ars to be an important nutrient source for the Watana
Hills Dall Sheep population. The lick extends from elevation 2000 to 2450,
so lower elevations of the reservoir will inundate less of the lick area or
may totally avoid it (e.g., at elevation 1900).
Inundation of Raptor Nests. Reduction of reservoir elevation may also be
significant for raptors. Lowering the elevation of the Watana reservoir
would reduce or eliminate impacts to two bald eagle nests, one golden eagle
nest, one gyrfalcon nest, and six raven nests, depending on the alternative
selected and the exact nest elevations. Two bald eagle, five golden eagle,
one goshawk, and five raven nests would be inundated regardless of the
alternative selected.
Impacts on Other Wildlife. Reservoir clearing and general ground
disturbance associated with the Watana development will have adverse impacts
on the other species of wildlife present in the area (pages E-3-512 to 517
and Tables E.3.149 to 158). Lower reservoir elevations with less needed
clearing and general ground disturbance would reduce construction and
inundation impacts on all wildlife species in the area, espectially forest-
inhabiting species such as many birds, small mammals, and certain
furbearers. The impact reduction would be especially significant for marten
which is dependent on forest habitat and is the most economically important
furbearer in the reservoir vicinity. A reservoir elevation of 1900 feet
would reduce marten impacts substantially because only about half of the
forest habitat lost with the Watana 2185 project would be inundated by the
lower dam height. Areas of stream habitat utilized by mink and otter (both
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moderately abundant furbearers in the Watana reservoir vicinity) would also
be signi~icantly less-affected by lower reservoir elevations.
Other Impacts of Inundation. A total of 16i' historic and archaeological
sites are discussed in the License t~plication. Of these, 30 are identified
:1s being directly affected by the Watana Dam and impoundment (at E. 2185).
Three additional sites may be affected (one site directly and two
potentially) by borrow area activities. The remaining 134 sites would be
unaffected by possible changes in normal maximum reservoir elevations at the
Watana development. Since pr~paration of Exhibit E, 26 additional sites
have been identified from the Watana area~
Three of these sites appear to be in the construction area and will likely
be impacted regardless of elevation selected. One ~ite is located upstream
of the Oshetna River and would only be affected by the elevation 2185
development. The relative elevations of the remaining 52 sites are shown in
Table F.39a.4.
Elevation (ft.)
1540 -1900
1920 -2000
2050 -2100
2133 -2185
2200 -2300
Table F.39a.4
ELEVATIONS OF IDENTIFIED ARCHAEOLOGICAL
SITES IN THE WATANA RESERVOIR AREA
No. of Sites
20
6
10
2
14
52
Thus, lowering the normal maximum reservoir elevation from 2185 to 2100,
2000 or 1900 would reduce the number of sites directly affected by 3, 13,
and 19 respectively. These sites would remain subject to indirect impacts
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during both pro}ect construction and operation, however, as discussed in
Exhibit E of the License Application.
The License Application also indicates that the Watana 2185 dam and
reservoir will inundate six structures, of which one, a lean-to for hunting
and fishing, is presently maintained for temporary use. These six
structures are located close to the river adn will be affected by the
Watana Development, regardless of selected reservoir elevation.
Since the lower alternative i'cservoir elevations would inundate
significantly fewer acres and stream miles than the reservoir as described
in the License Application, the lower elevation developments would
progressively reduce the total magnitude of changes in land use and related
land use activities. Although development would increase the potential for
access to the area, the lower alternatives would result in larger areas
remaining in primitive nbefore project" condition for recreational
activities i eluding boating, fishing, hunting, and biking. It is not
aniticipated that changes in the dam height or reservoir level would result
1n any significant modifications to the project related facilities proposed
in the Recreation Plan.
Differences in alternative Watana developments will not change impacts to
the exceptional Natural Features in the project area as identified in
Chapter 8 of the License Application. In terms of the aesthetic quality of
the reservoir and the adjacent area, as the reservoir is lowered and the
total number of river and tributary miles are reduced and the total size of
the reservoir and borrow sites decr~a.ses, increasingly larger areas will
retain their natural landscape characteristics. For example, Deadman Creek
Falls, which is located approximately 0.5 miles north of the Susitna. River-
Deadman. Creek confluence and rises to 1800 feet in elevation, will still be
inundated. Sally Lake near the mouth of Watana Creek has a surface
elevation of approximately 2050 feet and would be affected only by the 2100
and 2185 dam alternatives •
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Borrow Material. A project at elevation 2100 reduces the total volume of
the dam by 34 perce~t as compared with the El. 2185 development (See Table
F.39a.l). A develo1ment at elevation 2000 reduces the volume by 60 percent
and a development at ~levation 1900 reduces the volume by 73 percent as
compared with the El. 2185 development. These reductions would
correspondingly reduce impacts to aquat-ic and terrestrial habitats.
Borrow areas for the Watana dam are shown in Figure E.2.131 of the License
Application. Borrow area E 1s a large alluvial fan deposit at the
confluence of Tsusena Creek with the surface of the deposit ranging
in elevation from a low of 1410 feet near the river to 1700 feet against the
valley walls. Although the mined area will be rehibilitated to provide
feeding and overwintering fish habitat following construction, some
increased turbidity will occur from the mining activities. The reduced
volume of material needed from borrow area E will tend to reduce the extent
and duration of turbidity and sedimentation in the river downstream duzing
construction. Also, reducing the volume needed from this area may reduce
impacts on the existing riparian habitat for moose and other species.
Borrow areas A and D are located in upland areas away from the reservoir.
The volume of material needed for impervious fill (area D) is progressively
less at lower dam heights than that for the dam as described in the License
Application.
The volume of material needed from the rock quarry (area A) is also
considerably less at lower dam heights but, except for the El. 1900
alternative, is greater than that .for the project described in the License
Application. This is due to a redistribution in the proportion of the types
of materials used in dam construction under the modified design. The
project modificatoins result in a reduction in material extracted from the
river (area E) and a smaller increasee in material excavated from the rock
quarry (area A). This results in a trade-off between less dis tt\rbance to
aquatic and riparian habitats through removal of the sand and gravel
sustrate and less turbidity downstream and increased disturbance to the
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area around the rock quarry with increased blasting and resultant dust and
increased aesthetic impact in the quarry area"
Shorter Construction Period. Many project impacts discussed in Exhibit E
are essentially time dependent in that the shorter the construction period,
the less the cumulative impact. Of particular concern is increased hunting
and fishing pressure and the general disturbance to the environment that
will occur throughout the construction period. The lower dams, with less
placement of fill materials, will require less time for construction. This,
in turn, will result in a reduction of cumula.tive impact. Although
completion of construction would not totally eliminate some sources of
impact (e.g. access to the area), impacts due to other factors may be
reduced by shorter cons true tion· times. Such factors include:
Erosion
Potential for Oil and Hazardous Material Spills
Blasting
River Diversions
Reservoir Filling
Water Quality Changes
Maintenance of Access and Temporary Camps
Aircraft Disturbance
Flood Control. The Watana 2185 project as described in the FERC License
Application is designed so that the powerhous~ and outlet facilities, plus
reservoir storage, will have sufficient capacity to pass floods with
recurrence intervals up to one in fifty years without operating the main
spillwayo During floods of this magnitude, the reservoir will be allowed to
surcharge to elevation 2193 if necessary. By contining the fifty year flood
without use of the spillway structure, problems related to nitrogen
supersat11ration. If a lower elevation for the Watana project is considered
(2100 to 1900), project facilities will be modifi~d (e.g. larger outlet
works capacity) so that flows up to the 1 in 50 year flood will continue to
be passed wittout operati~u of the main spillway
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Because of the increaseed active storage capacity of the Watana 2185
reservoir, there is a greater capacity to contain flood flows without need
to use the outlet facilities. As the active storage capacity of the
reservoir decreases at lower elevations, the reservoirs will tend to fill
earlier in the summer high flow season and result in a greater need to use
~he outlet facilities, resulting inless flow regulation downstream'
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