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OCR for page 226
9
Dams and Mitigation of Their Effects
hi;
_~ L
INTRODUCTION
Dam construction in the Pacific Northwest began late in the 1800s when
small irrigation reservoirs were constructed on tributaries of the Snake River in
Idaho. Early in the twentieth century, the first hydropower dams were con-
structed on tributaries of the Columbia, such as the Spokane and Willamette
Rivers. During the early 1900s, dam construction moved more rapidly and most
of the reservoirs were relatively small (see Figures 3-9 and 3-10~. However,
beginning in the late 1930s with the initiation of the construction of Bonneville
and Grand Coulee, dam construction proceeded at a more rapid pace, as both the
number and storage volume of dams in Washington, Oregon, and Idaho increased.
During the 45-year period after the authorization of Bonneville (1933) and Grand
Coulee (1935) dams, 14 mainstem Columbia River and 13 mainstem Snake River
dams were completed.
By the late 1970s, potential sites and public support for major new dams had
been virtually exhausted, and the growth phase ended. Dams had been con-
structed across the migration routes of most Pacific Northwest salmon runs.
They range from irrigation diversions with a hydraulic head of only a few feet to
dams at Grand Coulee, Dworshak, and Hells Canyon that are several hundred feet
high and completely block upstream and downstream passage of anadromous
fish. Adult salmon can pass high dams with the aid of trap-and-haul arrange-
ments or even fishways, but the great depths, cross sections, and lengths of the
reservoir pools might hinder smolts from finding routes to the sea.
Figure 9-1 shows how the reservoir system has affected the average seasonal
226
OCR for page 227
227
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OCR for page 228
228
225
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can
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Z 125
LL
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75
50 _
25
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
DRAINAGE AREA = 103,200 SQ. Ml.
Observed Peak
Flowl82,100CFS
/1\
sol\
A alar`_, -A
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I N/ \
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Unregulated Peak
Flow 240, 170 CFS
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FIGURE 9-2 1993 Seals River hydrograph below Lower Granite Dam. Source: Colum-
bia River Water Report for 1993.
discharge at four points in the Columbia River system: the Columbia River at the
international border reflects the effect of Canadian storage, the sum of the
Okanogan and Methow rivers reflects regulation by the Lake Okanogan and
upstream reservoirs in Canada, the Snake River at its mouth reflects the effects of
all Snake River reservoirs, and the Columbia River at The Dalles reflects the
effects of all major storage facilities on the Columbia River system except those
on the Willamette, Cowlitz, and Lewis rivers. The average seasonal discharge of
the Columbia River mainstem has been drastically altered. However, the season-
ality of regulated flow of the Snake River has been much less affected (Figure 9-
2~. That is an important distinction for the discussion that follows because of two
common misperceptions: that there has been a major seasonal shift in the mean
discharge hydrograph of the Snake River, which Figure 9-2 shows is clearly not
the case; and that the reservoir storage in the Snake River is much less than on the
mainstem of the Columbia total storage in the Snake River system, expressed
as a fraction of the mean flow, actually is only slightly less than that of the
mainstem Columbia. Because there has not been a major shift in the Snake River
hydrograph, it is doubtful a priori that the declines in Snake River salmon stocks
are due to or reversible by changes in the seasonality of the flow regime of the
Snake River alone. These same salmon must traverse the Columbia River, whose
seasonal hydrograph has been substantially altered. Even if flow changes are
useful in rehabilitation efforts, they are likely to be insufficient without changes
in other human interventions in the salmon's life cycle and habitat.
OCR for page 229
DAMS AND MITIGATION OF THEIR EFFECTS
229
The major difference between the Snake River and the Columbia mainstem
is that much of the Snake River storage is used for irrigation rather than hydro-
power generation. The amount of hydrograph shaping (change in the natural
hydrography required to meet irrigation requirements is much less than that needed
for hydropower because the water-demand peak is in midsummer, typically only
1-2 months after the natural hydrograph peak. However, in contrast with hydro-
power, part of the water diverted for irrigation is consumptively used it does not
return to the river. In the case of the Snake River, the total consumptive use of
water by agriculture 4-5 million acre-feet (MAF) annually constitutes an ap-
preciable fraction of the natural flow of the river during the months of highest
agricultural demand (about 20% of the flow during the period May-September).
That is the basis for the argument that agriculture has greater effects on the
managed hydrology of the Snake River than does hydropower.
Although the flow regime of regulated rivers usually is less variable over the
course of the year than it was before dam construction, water storage in and
release from dams can result in large day-to-day or even day-to-night fluctuations
in flow and depth. The fluctuations can lead adult salmon to construct their nests
in unsuitable places and can strand juveniles. However, intensive studies below
Priest Rapids Dam during periods of peaking operation (large diurnal release
variations) and load operation (minimal diurnal variations) revealed little or no
effect on fall chinook spawning or abundance.
In addition to affecting seasonal hydrographs, the reservoir system has had
major effects on flow velocities, water chemistry (especially nitrogen supersatu-
ration downstream of dams), and stream temperatures. Supersaturation with
atmospheric gas, chiefly nitrogen, occurs when water is spilled over high dams.
Gas is absorbed into the bloodstream of fish during respiration, especially fish
that remain close to the surface. When the gas comes out of solution, bubbles
form and can subject the fish to a condition similar to the bends suffered by
divers. In some years before development of spill deflectors to prevent deep
entrainment of spilled water, gas supersaturation caused extensive mortality (Ebel
19691. River managers now coordinate spill at various projects to reduce risk of
serious losses. However, supersaturation can still exceed the high-risk levels
(125~o saturation) in years of high river discharge.
Reservoirs unintentionally provide thermal storage, as well as water storage,
so seasonal variations in stream temperature are reduced in much the same way as
seasonal variations in streamflow. In general, storage reservoirs tend to increase
winter temperatures and reduce downstream summer temperatures and to cause
maximum and minimum temperatures to occur later in the year than in the ab-
sence of damming. However, water below Bonneville Dam has shown longer
and warmer summer conditions over the last 40-50 years (Quinn and Adams, in
prep.~. Figure 9-3 shows the trend in the date when the spring water temperature
on the Columbia mainstem exceeded 15.5° since 1938. The upward trend in
spring water temperature is consistent with introduction of storage in upstream
OCR for page 230
230
(a Aug 1~-
LO
. .. . . . .. ................
May t
i I
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
l:5
a,
a,
x
AS
~ Jun 1
........ , ~.. ~_
...~....
July - r - ~
. ~. ~. ~J ~
....~.............................
-
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938 ~ 948 195;8 ~ 968 1978 ~ 988
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FIGURE 9-3 Spring water temperatures on Columbia mainstem since 1948.
reservoirs, particularly Canadian storage, which came on line in the 1970s (see
Appendix E'. Although the physical system has not changed since then, the trend
apparently continues. That might reflect long-term climatic change or variabil-
ity; Lettenmaier et al. (1994) found that air temperatures increased significantly
in much of the northwestern United States during 1948-1988, especially in late
winter and early spring. The effects of long-term changes in water temperature
on salmon depend on a complex interaction of early rearing conditions, emer-
gence date, and predator populations.
High dams can inundate substantial amounts of spawning and rearing habi-
tat. Some salmon, notably chinook, spawn in the mainstems of rivers and hence
lose usable area when rivers become reservoirs (e.g., John Day, Priest Rapids,
Coulee, and Wells dam pools). In addition, juveniles, particularly chinook, might
rear in large rivers or feed there during downstream migration (e.g., the Fraser,
Columbia, and Sacramento rivers) (Rich 1920, Levy and Northcote 1982,
Chapman et al. 19941. The reservoirs might constitute a reduction in desirable
habitat. In the Columbia River, however, reservoir rearing might substitute for
lost flowing-river habitat and make up for lost quality and quantity of estuarine
habitat (Rondorf et al. 1990, Chapman and Witty 1993~. The cumulative volume
of all Pacific Northwest (Idaho, Oregon, Washington, and northern California)
reservoir storage was over 50 MAF by 1980 and currently exceeds 65 MAF
(Figure 3-101.
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DAMS AND MITIGATION OF THEIR EFFECTS
231
The following discussion focuses on intermediate and large dams. Most
smaller dams were built and are operated primarily to generate hydroelectric
power. Their effects on salmon populations are discussed in Chapter 3.
EFFECTS OF DAMS ON SALMON
The effect of dams without fish-passage facilities on salmon is clear: the
upstream habitat is lost. Such dams block about one-third of the Columbia River
watershed to access by anadromous fishes; owing to natural passage barriers,
one-third was never accessible. One-third lost to anadromous fish is upstream
from Grand Coulee Dam on the Columbia River and the Hell's Canyon complex
of dams on the Snake River (Chief Joseph Dam, the reregulating dam for Grand
Coulee, also is impassable). Many dams on tributaries are also impassable, such
as Mayfield Dam on the Cowlitz River and Round Butte Dam on the Deschutes
River (tributaries to the Columbia), Detroit Dam on the North Santiam (tributary
to the Willamette), and Dworshak Dam on the North Fork of the Clearwater
(tributary to the Snake). The loss of spawning and rearing habitat because of
impassable dams is perhaps most acute on the Columbia River system but is by
no means restricted to it. Shasta Dam eliminated the upriver runs of salmon on
the Sacramento River, and rivers in Puget Sound (e.g., the Skagit River) and the
Strait of Juan de Fuca (the Elwha River) have impassable dams as well. On the
coast, access to the upper Klamath and Rogue rivers was blocked by Iron Gate
and Lost Creek dams, respectively. It is difficult to specify the magnitude of
losses attributable to such dams because record-keeping before construction was
often poor or nonexistent, and it might not be possible to survey the inundated
habitat to estimate potential production.
Not all the impassable dams were large hydroelectric dams. Small splash
dams, built to back up water and then float logs downriver, were often impassable
and sometimes remained in place long enough to obliterate major salmon runs,
e.g., in western Oregon and Washington (Sedell and Luchessa 1982) and in the
upper Adams River, British Columbia (Williams 19871. Relatively small irriga-
tion and hydroelectric dams blocked some salmon migrations early in western
development e.g., Black Canyon Dam on the Payette River in Idaho,
Grangeville Dam on the south fork of the Clearwater River, and Sunbeam Dam
on Idaho's Salmon River. More recently, a common practice at many fish hatch-
eries has been to block upstream migration at or near the hatchery to aid in
collecting returning adults or to isolate adults, possibly carrying diseases, from
the hatchery's water supply.
Dam-Related Mortality
Even when dams are constructed with fish ladders for upstream passage of
salmon, fish can still be delayed. Turbine discharge flows can disorient salmon
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232
UPSTREAM: SALMON AND SOCIETY IN THE PA CIFJC NORTHWEST
and make it difficult for them to find the small attraction flows that lead to the
ladder. Ladder designs have evolved greatly since the early l900s. Early facili-
ties often had excessive in-ladder flow rates and turbulence and lacked sufficient
resting areas; salmon either avoided them or found them impassable or too de-
manding of energy. Flood flows destroyed many wooden fish ladders, e.g., at
Condit Dam on the White Salmon River and Grangeville Dam on the south fork
of the Clearwater River. Poor concrete quality caused others to fail, such as
Sunbeam Dam on the Salmon River. Spillways close to fishway exits tended to
pull adults that left the ladders back over the dam and thus caused migration delay
(Bjornn and Peery 1992J. Delays might not kill fish, but salmon do not feed on
the upstream migration and must use stored energy as efficiently as possible to
migrate upstream, mature sexually, and spawn successfully (Gilhousen 19801.
Adult salmon can be killed if they drop back through turbine intakes, although the
rate of loss of fish that drop back is unknown. Counts at successive dams seem to
indicate that deaths occur between dams, although it is rare to observe dead
salmon there. Poaching might account for some of the loss. Interdam losses have
been estimated at up to 25% for the reach from Bonneville Dam to John Day
Dam, but current loss estimates are about 4-5% per project there and elsewhere in
the Columbia River system (Chapman et al. 19911.
Downstream passage of juveniles through bypass facilities comes at a bio-
logical price. These juveniles, hereafter referred to as guided fish, can make
contact with deflection-screen surfaces, gatewell walls, the vertical barrier screens
in gatewells, the orifice entrance, or portions of the bypass channel or downwell
(Figure 9-41. Such encounters can cause impingement, bruising, scale loss
(descaling), and stress (Chapman et al. 1991~. Because it is the most quantifiable
evidence of damage to fishes, descaling is evaluated by biologists quantitatively
and is used to indicate facility problems and fish viability.
Stress accompanies passage through bypasses. Fish that hold in currents to
resist passing downstream, that contact separators, or that otherwise experience
stressful contact (e.g., with equipment) become physiologically stressed. They
appear to recover when held for several to 48 hours. However, direct bypass to
the river delivers stressed fish to the outfall,1 where they can become prey for
birds and fish, especially northern squawfish (Ptychocheilus oregonensis).
Bypass systems also concentrate smells in a small area. Smolts from the
whole width of the Columbia and Snake rivers (which might flow at 250 and 90
thousand cubic feet per second [kcfs], respectively) for example, are gathered
into a bypass channel that flows at only 500 cubic feet per second (cfs). If
bypassed directly, many thousands of smelts per hour can be delivered in a small
volume of water to the dam tailrace, which provides a concentrated stream of
1The bypass outfall, the water just delivered from the bypass system to the river, delivers fish to
the tailrace or the tailrace edge; the tailrace is the entire river flow just downstream from the dam.
OCR for page 233
DAMS AND MITIGATION OF THEIR EFFECTS
,, C. ~
g.'
i`
q
Gatewell , Bypass channel
Vertical barrier screen - ~ ~ :~ tom Am\
{so i portion at top ~
Operating gate- ~ , ~ I '`: ,, -~'=~\ \\
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233
ll
FIGURE 9-4 Cross section of typical dam and bypass system Fyke nets are used to
estimate fish-guidance efficiencies Source: adapted from USCOE 1993:163.
prey for predators. At the Bonneville second powerhouse, extensive studies of
fall chinook passing through turbines and the bypass revealed that the survivals
through the two routes were not very different and that predators were keying on
the stream of prey from the bypass outfall (Ledgerwood et al. 1991~. Similar
studies have begun at the Bonneville first powerhouse. At other dams equipped
with bypasses, total bypass-related mortality has not been thoroughly investi
gated.
Some investigators consider bypass-caused deaths to include only those
which can be observed (carcasses) in the raceways and sampling facilities incor-
porated in bypass systems. But those do not provide data on impingement on
deflection screens, predation within the gatewell and bypass system, predation
caused by the bypass-concentrated stream of prey, stress-related deaths that occur
OCR for page 234
234
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
after smells leave the outfall area, or predation on stressed fish long after they
leave the outfall pipe. Carcass counts typically indicate a mortality of 1% in the
bypass system (Koski et al. 19851; some studies indicate that bypass-related
mortality averages perhaps 5-7% (Matthews et al. 1987~.
The location of the outfall is crucial to bypass-related mortality. Expanded
evaluations of bypass-related mortality include the first smolt encounters with
deflection screens to a point well downstream from the dam, where fish become
free of all physical effects of the bypass. Passage through the bypass system is
not immediate in most cases. The National Marine Fisheries Service annually
evaluates how quickly smelts pass out of the gatewell and through the orifice on
the basis of the fraction of smelts that enter the gatewell and leave it within 24
hours. Orifice-passage efficiency of 70~o is considered satisfactory; this means
that some smelts remain well over 24 h, some are delayed in the bypass channel,
and others are delayed beneath the separator (if the system incorporates one).
Seasonal races and individual populations of salmon migrate from rivers to
the ocean at specific times of the year. It is generally believed that photoperiod is
the primary cue that triggers migration, although flow, temperature, and social
interactions can also influence it (Godin 1982~. The rate at which smells migrate
downstream depends on both their swimming speed and their orientation to the
velocity of the flow. Smolts migrated downstream in the Columbia River more
rapidly before dams were constructed than they do now (Raymond 1979, Rieman
et al. 1991~. It also appears that migration is more rapid in years or times of the
year when river flow is greater. However, there is dispute over the validity and
interpretation of these data, as discussed in the next section.
There is some evidence that predators, such as northern squawfish, have
increased in abundance in the lower Columbia River. The most important fish
predators (squawfish) and birds, such as gulls, are native species. There is some
predation by nonnative fishes such as walleye (Stizostedion vitreum) and small-
mouth bass (Micropterus dolomieui). Perhaps more important, the reservoirs,
tailraces, and bypass outfalls might have improved the river as rearing habitat for
these species, and the tailraces and forebays of the dams might lead to an increase
in foraging efficiency over that in the undammed river. In addition to possible
increases in mortality en route to the ocean that might result from retarded migra-
tion, delayed arrival in the estuary or ocean might result in higher mortality or
reduced growth.
Storage in the upper Columbia River and Snake River has altered the main
Columbia River's hydrograph. Sherwood et al. (1990) analyzed monthly mean
flows of the Columbia River and found that large-scale manipulation of the flow
cycle began around 1969. Since then, monthly mean flow has varied less. Flow
damping has resulted in a reduction in average sediment supply to the estuary.
Except for times of major floods, residence time of water in the estuary has
increased with decreasing salinity. Detritus and nutrient residence has increased;
vertical mixing has decreased. Sherwood et al. ( 1990) noted that although hydro
OCR for page 235
DAMS AND MITIGATION OF THEIR EFFECTS
235
dynamic changes have probably enhanced the pelagic primary productivity in the
estuary, the costs have yet to be evaluated. The changes have enhanced estuarine
conditions for detritivorous epibenthic and pelagic copepods; the estuary has
been converted to a less-energetic microdetritus-based ecosystem with higher
organic sedimentation rates. Sherwood et al. (1990) concluded that
it is apparent that these changes, and other changes in the fluvial part of the
system, have contributed to the dramatic decline in salmon populations. The
implications such as have taken place in the Columbia River estuary and water-
shed need to be incorporated into contemporary estuarine and shorelands man-
agement strategies. In particular, proposals for comprehensive hydroelectric
and water withdrawal developments, shoreline modifications, and navigation
projects should all be evaluated in terms of potential consequences to the estua-
rine ecosystem and resulting effects on other resources, including fisheries,
which depend on a highly convolved and biologically diverse estuanne environ-
ment.
Sherwood et al. (1990) estimated that the estuary of the Columbia River lost
20,000 acres of tidal swamps, 10,000 acres of tidal marshes, and 3,000 acres of
tidal flats between 1870 and 1970. They further estimated an 80~o reduction in
emergent vegetation production and a l5% decline in benthic algal production.
Ebbesmeyer and Tangborn (1993) demonstrated that reservoir storage in the
Columbia River had altered the hydrograph by diverting summer flows to winter,
which altered coastal sea-surface salinities from California to Alaska. Coastal
ocean and estuarine dynamics have changed at various locations along 2,000 km
of North Pacific shoreline. The effects of those alterations on trophic dynamics,
loss to predators, and migration success are completely unknown. Ebbesmeyer
and Tangborn (1993) raised the question of whether homing behavior of salmon
might change in response to the altered salinities. If homing were affected, we
would expect to see steelhead and salmon from Columbia River hatcheries as
strays in coastal rivers of California, Oregon, and Washington. But no evidence
of extensive or unusual straying has been found.
Time of Travel
The effect of time of travel on subyearling chinook (i.e., ones that pass their
first winter in the sea, or ocean-type chinook) is less clear than the effects of
passing through dams. Ocean-type chinook pass their first winter of life (after
emerging from the read) at sea; stream-type chinook spend their. first winter of
life in the stream before going to sea. Some studies indicate that subyearlings
migrate downstream more quickly with higher flows (Rondorf and Miller 1994),
but other studies (e.g., Giorgi et al. 1990, Chapman et al. 1994) do not. Sub-
yearling chinook gradually move downstream as they grow-a rearing migration.
Rather than the rapid downstream passage of yearling chinook and steelhead,
which often reach an average speed of 20 miles/day or more, downstream pas
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236
UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
sage of subyearling chinook is perhaps 2 miles/day. Some subyearling summer-
fall chinook do not enter the sea in the first summer of life, as it is commonly
supposed; rather, they do not pass McNary Dam until late fall. Growth rates of
subyearlings in mainstem Columbia River reservoirs are very high (Chapman et
al. 19943. No information exists on whether subyearlings that reach large size in
reservoirs of the Columbia River have higher survival rates than small sub-
yearlings that go to sea in July.
The effect of migration speed on smolt survival has been inferred from data
on system survival and travel-time data acquired during the 1970s. The accuracy,
precision, and relevance of those historical estimates are questionable (Giorgi
1993), and for that reason, the data from the 1970s were abandoned in the early
1980s. Smolt survival through the same river reaches would differ today: the
smolt cohort in the Snake River has changed from almost 70% wild in the late
1960s to only limo wild today (Park 1993~. Hatchery steelhead and spring
chinook predominate in the main Columbia and Willamette rivers, and spring-
summer chinook in the Snake River. Wild fish still compose the majority of fall-
run and summer-run chinook of the mid-Columbia and in some tributaries that lie
in wilderness areas where hatcheries are not used (e.g., the middle fork of the
Salmon River and Chamberlain Creek in Idaho) or are considered refugia for
wild gene pools (John Day River in Oregon).
Not only the makeup of the smolt runs, but the river environment itself has
been altered. More turbines have been installed in mainstem dams. John Day
and other newer reservoirs have matured limnologically. Fish communities have
changed (e.g., exotic species have been introduced), and river management has
evolved in response to needs for fish conservation. Reach-specific or system
survival studies have not been completed that would allow managers to evaluate
modern conditions in the river; they are crucially important and should be pur-
sued with vigor and dispatch.
Recently, there has been sponsorship of new research, such as reach-specific
survival studies that began in 1993 (Snake River Salmon Recovery Team 1993~.
If such studies are continued, they would provide steady improvement in the
scientific knowledge available for the difficult challenges presented by salmon
rehabilitation. For instance, Williams and Matthews (1994) and Steward (1994)
critically reviewed the data obtained in the 1970s. They concluded that system
and average project mortalities in smelts migrating downstream were overesti-
mated and that the data should not be used to estimate current system and project
losses.
The lack of modern survival data that could be used to evaluate survival
through various dam-passage routes and through reservoirs has resulted in dis-
sension over the value of high flows for reducing mortality in downstream mi-
grants. Many investigators believe that the older data support a need to provide
flows of 80-90 kcfs in the Snake River and 200-220 kcfs in the main Columbia.
However, many of the same researchers question the gains in survival that the
OCR for page 243
DAMS AND MITIGATION OF THEIR EFFECTS
Flow Augmentation
243
Of the major effects of the dams on fish, the increase in time of passage
through the hydropower projects on the middle Columbia and lower Snake rivers,
particularly in the spring, has been identified as a key obstacle to survival of
juvenile salmon and steelhead. An attempt to reduce passage time was made
through a "water budget," which allocated some upstream storage in the Snake
(at Dworshak Reservoir) and in the mainstem Columbia (at Lake Roosevelt, the
reservoir of Grand Coulee Dam) to increase spring flows. It was anticipated that
most of the additional water released would pass quickly through the downstream
run-of-the-river dams (hydropower dams that primarily use riverflow rather than
steed water for power generation; see Appendix F) and therefore would not be
available for the generation of power during the peak-demand season. However,
the excess power produced by the increased flows at the time of the water-budget
releases, although of less value, could be sold to meet demands elsewhere (e.g., in
California, by use of the direct-current intertie); this compensated for some extent
for the loss due to the water-budget releases (Wood 1993) (see Box 9-11.
The purpose of flow augmentation is to reduce the travel time through the
reservoir system at key times in the salmon life cycle (most proposals are for the
period April 15-June 14, although some proposals would augment summer dis-
charge as well) to approximate more closely the pre-dam travel times. Flow
augmentation was originally termed the "water budget" by the Northwest Power
Planning Council (NPPC) in the middle 1980s. The idea was to reserve enough
storage at Dworshak Dam's reservoir and Lake Roosevelt (Grand Coulee Dam's
reservoir) to meet a flow target of 85 kcfs in the Snake River at Lower Granite in
May (about 53% of the mean May flow under pre-dam conditions and about 78~o
of mean May flow with regulation; see Chapman et al. 1991) and to meet a target
of 134 kcfs in the Columbia at Priest Rapids (about 87~o of the mean May flow
for the period 1971-1993, after further alteration of the natural hydrograph by
Canadian storages. The projects on the lower Snake and middle and lower
Columbia rivers were to operate near "full pool" (the maximum reservoir water
level authorized in the projects' design).
The storage required to meet the streamflow targets was determined by
adding the positive differences between the target monthly flows and the critical-
period-rule curve for hydropower targets. This resulted initially in a storage
requirement of about 4 MAF. NPPC later determined that sufficient unallocated
storage capacity was not available on the Snake; of the total reservoir storage of
11.7 MAF on the Snake, only about 2 MAF at Dworshak Dam is federally
controlled and thus potentially available. Therefore, the water-budget storage
was increased on the Columbia and decreased on the Snake so that the total was
4.65 MAF. The water budget was to be released during the peak small
outmigration period (April 15-June 14~.
OCR for page 244
244 UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
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Wood (1993) argued that the water budget has failed for four reasons. First,
the base power flows, which the water budget was designed to augment, are not
necessarily maintained during low-flow years on the Snake. Second, the man-
agement agencies, notably the U.S. Army Corps of Engineers, view the water
budget as "a cooperative arrangement," not a mandate, so water-budget volumes
are not guaranteed, especially in low-flow years, when they are most needed.
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DAMS AND MITIGATION OF THEIR EFFECTS
245
Third, other operating considerations have higher priority, e.g., for secondary
power release and in some cases refill of downstream reservoirs. Fourth, BPA
mitigates the cost of the water budget by selling the resulting excess spring
power, then buying it back in the summer from other sources, thus causing
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UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
abnormally low summer flows in the months after the period of spring enhance
ment.
More recently, the interagency Systems Operation Review (1992) consid-
ered a base-case flow-augmentation alternative, in which Dworshak Dam would
provide at least an additional 0.30 MAF release in the spring and 0.47 MAF
release in addition to 0.43 MAF release from the Upper Snake River Dam (note
that these are releases, and not storage allocations). In addition, up to 3 MAF of
spring flow augmentation would occur on the Columbia; this would be achieved
in part through winter flood-control shifts from the Snake River dams to Grand
Coulee Dam in some low-flow years.
The effectiveness of flow-augmentation alternatives has not been demon-
strated. On the basis of modeling studies that evaluated the effectiveness of
several flow-augmentation alternatives for improvement of juvenile survival, the
Systems Operation Review (1992) found that "flow augmentation alternatives
produce similar results in all three drainages (Snake, Lower Columbia, Upper
Columbia), providing negligible survival benefits for inriver migrants." Flow
augmentation might be useful if it provides sufficient water to reduce dam-related
mortality (e.g., by spill). However, it is unable to reduce the water-particle travel
times through the pools in average-flow years by more than a few days prob-
ably biologically insignificant beyond the levels already achieved by NPPC's
85-kcfs Lower Granite Dam target. It might well be important that the system
operating policy treats the NPPC "targets" of 85 kcfs at Lower Granite Dam and
134 kcfs at Priest Rapids Dam as operating constraints, rather than operating
targets; i.e., these targets are given precedence over power production. Flow
augmentation should be implemented in such a way that targets are met in all
years when storage is available, not just in average and above-average years.
Merely focusing on average years is insufficient, as was shown in the years after
1986. In some of those years, spring flows were very low; for example, average
May discharge at Lower Granite Dam in 1990 was only 68.2 kcfs. Before Snake
River sockeye and chinook salmon were listed under the Endangered Species Act
(ESA), the Fish Passage Center allocated the water budget on the basis of num-
bers of fish passing through the system. It concentrated mitigative efforts on
hatchery fish, which tend to move in a relatively narrow period. Wild fish from
some tributaries move through the Snake River over a much longer period-
some from as early as mid-April, others as late as the end of June (Chapman et al.
1991).
Reservoir Drawdown
A different approach to increasing survival has been suggested in response to
the ESA listing of the Snake River sockeye and spring-summer and fall chinook
and the general perception among fishery managers that the water budget has
failed to stop the decline of Snake River salmon runs. Although the water-budget
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DAMS AND MITIGATION OF THEIR EFFECTS
247
approach can reduce the time of travel through the reservoir system (in particular,
the lower Snake and middle Columbia River reservoirs, all of which are run-of-
the-river Esee Appendix Fly, the effect is at best modest a few days at mean
springtime flows. Average travel time through a reservoir is roughly equal to
discharge (volume per unit time) divided by the average cross-sectional area.
Therefore, drawdown would reduce travel times by lowering reservoir levels
(subject to control of dissolved gases; see the discussion of spill) to decrease the
cross-sectional area, hence increasing the water-particle velocities through the
run-of-the-river reservoirs. As an extreme case, drawdown could be made to the
pre-dam or original river channel.
For example, a drawdown of zero corresponds to full operating pool (essen-
tially the present operating condition), and, for a particular dam, 110 ft of draw-
down corresponds to the undammed river channel. Harza and Associates (1994),
in reviewing a range of drawdown options considered by the interagency Systems
Operation Review (BPA et al. 1992), recommended that only three be considered
in detail: natural river, deep drawdown to some level below minimum operating
pool, and maximum drawdown (to spill crest). Harza found that
for the maximum drawdown option [to the] natural river ~channel], the maxi
mum travel time advantage (that is, the decrease in travel time through the
Lower Snake River reservoir system for the drawn down pools relative to travel
time with present pool levels) is 7 days at 100 kcfs, 14 days at 50 kcfs, and 29
days at 25 kcfs. The latter condition (25 kcfs flow) applies less than 2% of the
time in the spring migration season (Apnl 15 - June 15), although it is not clear
whether this figure includes the effect of the water budget.
The System Operation Review found that the natural river options "decrease
travel time (from the mouth of the Salmon River to Bonneville Dam) between 7-
10 days, depending on the stock." However, effects are limited to Snake River
populations since the action is restricted to the Snake River Basin.
Most of the studies, including Harza's (1994), that have considered a draw-
down option have concluded that drawdown to any elevation greater than natural
river level is not likely to have biologically significant benefits, in comparison
with other alternatives for improving juvenile survival (such as flow augmenta-
tion and barging). However, reduction of the reservoir only to minimum operat-
ing pool would allow hydropower turbines to continue to operate and would not
require reconstruction of fish-passage facilities, such as fish ladders.
In addition to the economic costs associated with elimination of hydropower
production, navigation, and recreation during the natural-river drawdown peri-
ods, a number of complications accompany drawdown. These include at least the
following:
· Loss of spawning habitat for nonsalmon in the reservoir pools and tribu-
taries that directly enter the pools.
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UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
Concentration of predators in the relatively small channel volume.
· Necessity to reconstruct passage facilities for use during the drawdown
and refill period, which would otherwise be unusable at reservoir levels below
. . .
minimum operating pool.
· Increased mortality due to operation of turbines at less than maximum
efficiency during the drawdown and refill periods.
· Loss of rearing habitat for subyearling fall chinook.
Direct economic costs are also associated with reconstruction of fish-passage
facilities, irrigation withdrawal intakes, and channel stabilization.
The Snake River Salmon Recovery Team (1993) found, on the basis of
model predictions, that the natural-river drawdown option would produce the
highest inriver survival of yearling migrants in the Snake River basin. This
option also would have the potential to increase survival over that predicted for
transportation. However, the team added that "the tother] drawdown alternatives
are highly uncertain, and even the most optimistic juvenile passage assumptions
associated with a four pool drawdown fail to improve survival values of Snake
River stock beyond what is achievable with juvenile transportation."
Models that incorporate all life stages of salmon and their responses to
various sources of mortality have the potential to be helpful in comparing alterna-
tive management and environmental scenarios and as guides to research. The
committee has not evaluated any of the models that have been developed on these
lines (examples include models produced by the Columbia River Salmon Passage
(CRISP) project (Center for Quantitative Science, University of Washington,
undated), but it encourages their development and especially the collection of
reliable data for them.
Dam Removal
Although dams are seemingly permanent (albeit recentJ features of the North-
west riverine environment, like all artificial structures, they have a finite engi-
neering and economic life expectancy. Structural criteria for dam safety have
changed greatly in the 85 or so years since construction of the first high dams in
the region, notably to include passage of extreme floods and resistance to earth-
quakes. Some smaller dams have already undergone significant modifications
for these reasons. In addition, dams trap sediment, which significantly reduces
their active storage capacity and economic value. Although sedimentation in
most Columbia River reservoirs is minor compared to dams on rivers elsewhere,
which carry higher sediment loads under natural conditions, the economic life of
all reservoirs is ultimately affected by sedimentation. A number of older, low
dams elsewhere in the U.S. (notably in the East and Midwest) have been removed
because of sedimentation.
Where dams are a significant contributor to the decline of salmon runs, dam
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DAMS AND MITIGATION OF THEIR EFFECTS
249
removal is an obvious rehabilitation alternative. Like the construction process
that created the dams, dam removal would be a major engineering undertaking
with major environmental consequences. It would be naive to expect that re-
moval of a dam would allow a stream quickly to revert to its natural state. After
removing the structure, in most cases, there would be major and long-lasting
downstream effects due to movement of sediment stored behind the structure,
and engineered rechannelization of the former reservoir bed would almost cer-
tainly be necessary.
The Elwha River Proposal
The Elwha River dams provide a useful case study that gives some idea of
the magnitude of the dam removal problem. The Elwha River drains 831 km2 of
the Olympic Mountains, WA, and discharges to the Strait of Juan de Fuca. The
Elwha Dam, about 8 km from the mouth of the river, was constructed from 1910
to 1913, and the Glines Canyon Dam, about 21 km from the mouth of the river,
was constructed from 1925 to 1927. The entire firm energy productions of about
19 MW is presently used at a pulp mill in nearby Port Angeles. Glines Canyon
Dam lies in the Olympic National Park, which was created in 1937, as does 83~o
of the Elwha River drainage. Neither dam has fish-passage facilities, and the
lower dam (Elwha) has never had a federal license to operate.
In 1992, Congress authorized the secretary of the interior to acquire the dams
and remove them if he determined that their removal was necessary to "the full
restoration of the Elwha River ecosystem and native anadromous fisheries" (PL
102-495, Elwha River Ecosystem and Fisheries Restoration Act). A report pur-
suant to the act (USDI 1994) found that "The removal of the Elwha and Glines
Canyon Dams is the only alternative that would result . . . fin meeting the goals of
the act]". The report also conducted a preliminary engineering analysis of dam
removal alternatives, reviewed briefly here.
The major issues in the Elwha River project are removal of the structures,
management of sediment and erosion during and following dam removal, and re-
establishing and protecting a channel within the presently inundated area. Al-
though various combinations of dam removal (e.g., only Glines Canyon) and
provision of fish passage facilities (at Elwha Dam) were considered, only the
option of removal of both dams is reviewed here, because the report concluded
that it is the only option that would meet the goals of the Elwha Act.
Removal of the dams is complicated by the necessity to provide a stream
channel during the dam removal process, which is estimated to require about 18
months. The channel and temporary discharge structure must be sufficient to
3Firm energy production is the amount of energy that could be produced during the worst-case
historical conditions (known as the critical period), with all other demands on the system (e.g.,
irrigation) fixed.
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UPSTREAM: SALMON AND SOCIETY IN THE PACIFIC NORTHWEST
pass floods safely during this period. The options considered for diversion of the
river were a) construction of a diversion tunnel, b) construction of a surface
diversion channel, c) construction of a low-level diversion through the dam struc-
ture; and d) progressive construction of notches through the dam along with top-
down removal of the structure. The report did not conclude which option was
preferable (although comparative costs were estimated), since the removal op-
tions affect sediment management and channel reconstruction as well. Further,
site-specific constraints preclude application of some of the options at both sites.
For instance, the Elwha Dam was back-filled with large rock and gravel as a
result of a structural failure of the dam foundation during the initial filling.
Therefore, options c and d are not feasible at this dam. Construction of a surface
diversion channel at Glines Canyon Dam is considered infeasible due to con-
struction staging problems and high cost associated with the need to excavate
much of the channel in bedrock.
The lowest-cost alternative is a combination of low-level diversion through
the dam, or progressive notching of the dam and top-down removal at Glines
Canyon, and construction of a surface diversion channel at Elwha Dam. If top-
down removal were used at Glines Canyon, it would be accomplished by con-
struction of multiple notches 15.2 m (50 It) deep by 5.2 m (17 It) wide and
temporary gates, in nine increments of 3.8 m (12.5 ft) each. Dam material above
the notch would be removed by barge. At the last step, the dam base at the
bottom of the gorge would be removed by cableway or boom crane during sum-
mer low-flow conditions. The low-level diversion alternative would make use of
a small outlet valve that was installed at the base of the dam during construction.
This outlet would be enlarged, and the reservoir level lowered to its level to allow
"dry" removal of most of the dam. Removal of the remaining portion of the dam
structure would be the same as with the top-down approach. At Elwha Dam, the
surface channel alternative would isolate the north spillway structure, which
would be removed "dry," then the stream channel would be diverted in a second
stage to a channel excavated through the existing north spillway site, and the
main concrete dam section as well as power plants and south spillway would be
removed.
Perhaps the most difficult aspect of the project would be sediment removal
and management. An estimated 8.6 million m3 of sediment has been deposited
behind Glines Canyon Dam in the 70 years since the reservoir was filled, and 2-
3 million m3 has been deposited behind the Elwha Dam (mostly before comple-
tion of the upstream dam). A delta has been formed at the head of Lake Mills
(formed by Glines Canyon Dam), which is about 21 m deep. The delta consists
mostly of coarse materials (small sand and larger); fine sediment is more gener-
ally distributed throughout the reservoirs. Three alternatives, and combinations
thereof, have been evaluated for sediment management: removal, erosion, and
retention. Removal would be accomplished either after draining of the reservoirs
by trucking to an upland site or to disposal in salt water, or (before draining of the
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DAMS AND MITIGATION OF THEIR EFFECTS
251
reservoirs) by pipe transport of dredging spoils as a slurry to salt water or to an
upland site.
Both of the removal approaches are comparatively costly, require a long time
for completion (as much as 9 yearsJ, and would have significant ancillary envi-
ronmental effects (e.g., to the disposal site). They have the advantage that the
final topography of the presently inundated areas could be made to resemble pre-
dam conditions closely.
The erosion alternative would allow the river to transport the accumulated
material downstream. Resuspension of the materials would be augmented by
dredging or other mechanical means. Fine and smaller coarse material would be
transported by the river to salt water, and larger materials would likely be rede-
posited downstream. Although this alternative has the advantage of low cost, it
would result in high levels of suspended sediment downstream, and almost cer-
tainly would extensively damage downstream fish habitat. Further, it was esti-
mated that approximately 20 years would be required to remove all of the mate-
rial from Lake Mills. An alternative of dredging the Lake Mills delta material
and disposing of the dredged material over Glines Canyon Dam before removal
was also investigated. This alternative would effectively store the coarse materi-
als in Lake Aldwell (formed by Elwha Dam), and allow the river to transport the
fine sediments downstream. This alternative would reduce the period necessary
for stabilization of the channel system with respect to sediment movement and
would eliminate the need for an upland disposal site. Because there is insuffi-
cient space in Lake Aldwell for disposal of all of the Lake Mills delta material,
the Lake Mills bed would not be completely restored to natural topography.
The retention approach would relocate materials deposited in the old (pre-
dam) river channel elsewhere within the lake beds, but would otherwise leave the
accumulated sediment in its present location. Two retention alternatives were
considered. The first would use hydraulic dredging before dam removal to re-
store the original channel through the Lake Mills delta. The channel would be
dredged to the original river-bed elevation. The second alternative would remove
less material by dredging, and would allow the river to erode the delta to form a
new channel after removal of the dams. However, unlike the erosion alternatives,
no attempt would be made to remove sediment away from the original channel.
The hydraulic dredging alternative is expected to have the least downstream
effect, as much of the fine sediment disturbed during channel excavation would
settle out in the reservoirs, rather than being transported downstream. Resettling
would be enhanced through use of impermeable silt curtains, and by using the
coarser dredged material to form containment cells, that would aid in dewatering
of fine sediments. Subsequent to dewatering, the sediment would be graded,
compacted, and revegetated.
The estimated total costs of dam removal, sediment management, and reveg-
etation (but excluding the cost of the hydropower loss) was estimated to range
from about $70 million to $240 million. The lowest cost alternatives were those
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UPSTREAM: SALMON AND SOCIETY IN THE PA CIFIC NORTHWEST
that would remove sediment only from the former river channel, use either the
notch or low level diversion at Glines Canyon Dam, and a surface diversion
channel at Elwha Dam.
Most of the cost of the restoration project (about 75% for the lowest-cost
alternative, and well over 90% for the higher-cost alternatives) would be associ-
ated with sediment management. Although the least-cost alternatives do not
restore the topography of the inundated areas to their original contours, and might
result in less spawning habitat in these areas, they have the advantage (in addition
to cost) of greatly reducing the movement of fine sediment downstream. Further-
more, only a relatively small amount of the spawning habitat that would be re-
opened as a result of dam removal is in the inundated area.
The dam removal project would itself be a significant engineering project.
In addition to the removal of the dam structure, it would involve (depending on
the alternative selected) construction of temporary roads, channels, and diversion
dams, as well as extensive dredging and channel work. The greatest uncertainty
in performance of the different alternatives (and their cost) is undoubtedly asso-
ciated with sediment management, and this uncertainty is in turn greatest for
alternatives that rely on channel transport of sediment (as opposed to removal or
retention). Although there has been some experience with rechannelization via
dredging in a manner similar to one of the sediment retention alternatives by
British Columbia Hydro in the case of a reconstructed dam, there is essentially no
comparable experience with any of the other approaches. In addition, there
remains, after project completion, the potential problem of sediment movement
due to slope failures induced by channel migration, which might well require
permanent channel structures, such as levees, at least for the sediment-retention
alternatives.
Applicability of Experience
How applicable is the Elwha River experience to potential removal of dams
in the Columbia River system? At this point, the strongest candidates for dam
removal in the Columbia system appear to be the run-of-the-river dams in the
middle Columbia and lower Snake, which are the targets of current flow-aug-
mentation studies. The most obvious differences between the Elwha River and
middle Columbia and Snake dams are the size of the structures and the climate.
Annual precipitation in the Elwha River Basin is about 1,700 mm/yr; in the
middle Columbia and lower Snake it is in the range 200-400 mm. The humid
climate in the Elwha River Basin would aid revegetation efforts; it is quite likely
that a native forest could ultimately be restored in the presently inundated areas
(albeit with soil enhancement), whether or not the original contours were re-
tained. In the arid middle Columbia and lower Snake region, revegetation (and
hence erosion control) in the presently inundated areas would pose a much greater
challenge due to the probable necessity to irrigate, at least in the short term. In
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DAMS AND MITIGATION OF THEIR EFFECTS
253
addition, the channel slopes and river gradients in the case of the much larger
middle Columbia and lower Snake River dams are lower than the Elwha, which
is a steep mountain stream, especially at Glines Canyon Dam. Therefore, the
inundated area and length of inundated channel is much larger for the middle
Columbia and lower Snake River dams relative to the Elwha dams, with expected
restoration costs for the channel and inundated areas proportionately larger, as
well. On the other hand, the amount of sediment deposited behind the middle
Columbia and lower Snake dams is likely to be less, because these dams were
constructed after upstream dams had already trapped much of the sediment that
would otherwise have been deposited. Nonetheless, the costs of removing a
middle Columbia or lower Snake River dam would be much larger than those
estimated for the Elwha River dams.
Selection of Mitigation Alternatives
Many entities including the Fish Passage Center, CBFWA, the Idaho De-
partment of Fish and Game, the Oregon Department of Fish and Wildlife, and the
U.S. Fish and Wildlife Service have recommended spill of water at Snake
River dams, elimination of transportation, flow increases, and drawdown of the
Snake River reservoirs to increase water velocities. Opposing that position have
been the Corps of Engineers, NMFS, utilities, and others.
The Snake River Salmon Recovery Team (1993) extensively modeled miti-
gation alternatives and their effect on the proportion of Snake River smelts that
would arrive at a point downstream from Bonneville Dam. The team concluded
that no combination of spill, flow augmentation, and drawdown within the limits
imposed by present dam structures would produce survivals close to those ob-
tained by transporting smolts from collector dams to the Bonneville tailrace.
The team modeled a transport-benefit ratio (TBR) of 2:1. That ratio was based
on an average of TBRs obtained in transport tests in 1986 (1.6:1) and 1989
(2.5:1 J. The study years represented modern river management and community
structure. The team found that for all flow regimes, including high discharges
(151 kcfs in the Snake and 401 kcfs in the Columbia), a mitigation program using
transportation as the main tool was most effective. Only drawdown of the Snake
River reservoirs to river grade, which would take years to design and build,
potentially offers higher survival than transportation.
No investigator to date has provided the Columbia River region with experi-
mental results that demonstrate higher survival of inriver migrants than trans-
ported migrants at any discharge level. Until such experimental data become
available, transportation should continue to be used. However, it is essential that
managers use an adaptive (experimental) management approach and avoid tak-
ing any action that jeopardizes all of the fish in a stream. For example, if some
fish in a stream are transported downstream, the action should be designed so its
effectiveness can be assessed and compared with other alternatives, such as spill.
Representative terms from entire chapter:
columbia river
