U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Marine Fisheries Service

NOAA Technical Memorandum NMFS-NWFSC-64

Passage of Adult and Juvenile

Salmonids through

Federal Columbia River

Power System Dams

March 2005
NOAA Technical Memorandum

NMFS Series

The Northwest Fisheries Science Center of the National

Marine Fisheries Service, NOAA, uses the NOAA

Technical Memorandum NMFS series to issue informal scientific and technical publications when complete formal review and editorial processing are not

appropriate or feasible due to time constraints. Documents published in this series may be referenced in the

scientific and technical literature.

The NMFS-NWFSC Technical Memorandum series of

the Northwest Fisheries Science Center continues the

NMFS-F/NWC series established in 1970 by the

Northwest & Alaska Fisheries Science Center, which

has since been split into the Northwest Fisheries

Science Center and the Alaska Fisheries Science

Center. The NMFS-AFSC Technical Memorandum

series is now being used by the Alaska Fisheries

Science Center.

Reference throughout this document to trade names

does not imply endorsement by the National Marine

Fisheries Service, NOAA.

This document should be cited as follows:

Ferguson, J.W., G.M. Matthews, R.L. McComas, R.F.

Absolon, D.A. Brege, M.H. Gessel, and L.G.

Gilbreath. 2005. Passage of adult and juvenile

salmonids through federal Columbia River power

system dams. U.S. Dept. Commer., NOAA Tech.

Memo. NMFS-NWFSC-64, 160 p.
NOAA Technical Memorandum NMFS-NWFSC-64

Passage of Adult and Juvenile

Salmonids through

Federal Columbia River

Power System Dams

John W. Ferguson, Gene M. Matthews,

R. Lynn McComas, Randall F. Absolon,

Dean A. Brege, Michael H. Gessel,

and Lyle G. Gilbreath

Northwest Fisheries Science Center

Fish Ecology Division

2725 Montlake Boulevard East

Seattle, Washington 98112

March 2005

U.S. DEPARTMENT OF COMMERCE

Carlos M. Gutierrez, Secretary

National Oceanic and Atmospheric Administration

Vice Admiral Conrad C. Lautenbacher, Jr. USN (Ret), Administrator

National Marine Fisheries Service

William T. Hogarth, Assistant Administrator for Fisheries
Most NOAA Technical Memorandums NMFS-NWFSC

are available online at the Northwest Fisheries Science

Center web site (http://www.nwfsc.noaa.gov)

Copies are also available from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone orders (1-800-553-6847)

e-mail orders (orders@ntis.fedworld.gov)

ii
Table of Contents 

List of Figures.............................................................................................................................................vii 

List of Tables ...............................................................................................................................................ix 

Executive Summary .....................................................................................................................................xi 

Acknowledgments.....................................................................................................................................xvii 

Acronyms and Abbreviations ....................................................................................................................xix 

Introduction...................................................................................................................................................1 

Juvenile Salmon Passage Through Spillways...............................................................................................3 

Background...............................................................................................................................................3 

Research Results and Discussion .............................................................................................................4 

Spill Efficiency and Effectiveness .......................................................................................................4 

Spill Survival........................................................................................................................................8 

Causal Mechanisms Affecting Spill Survival.....................................................................................13 

Forebay Passage and Predation ..........................................................................................................17 

Tailrace Passage and Predation ..........................................................................................................17 

Daily Spill Timing..............................................................................................................................19 

Seasonal Spill Timing ........................................................................................................................21 

Summer Spill......................................................................................................................................21 

Dissolved Gas/Gas Bubble Disease ...................................................................................................23 

Spill Passage Conclusions......................................................................................................................28 

Juvenile Salmon Passage Through Mechanical Screen Bypass Systems ...................................................31 

Background.............................................................................................................................................31 

Fish Guidance Efficiency .......................................................................................................................33 

Definition and Methods......................................................................................................................33 

Results................................................................................................................................................35 

Conclusions........................................................................................................................................39 

Orifice Passage Efficiency......................................................................................................................39 

Definition and Methods......................................................................................................................39 

Results................................................................................................................................................39 

Discussion ..........................................................................................................................................40 

Conclusions........................................................................................................................................41 

Separators and Separation Efficiency.....................................................................................................41 

Background and Methods...................................................................................................................41 

Results................................................................................................................................................42 

Conclusions........................................................................................................................................44 

Water Temperature Effects.....................................................................................................................44 

Background ........................................................................................................................................44 

Results................................................................................................................................................44 

Conclusions........................................................................................................................................48 

iii 
Effects of Bypass Systems on Smolt Condition and Survival................................................................49 

Background ........................................................................................................................................49 

Results................................................................................................................................................49 

Discussion ..........................................................................................................................................52 

Conclusions........................................................................................................................................53 

Effects of Bypass Systems on Blood Chemistry ....................................................................................54 

Background ........................................................................................................................................54 

Results................................................................................................................................................54 

Discussion ..........................................................................................................................................55 

Conclusions........................................................................................................................................59 

Diel Passage and Timing ............................................................................................................................60 

Background.............................................................................................................................................60 

Lower Granite Dam................................................................................................................................60 

Little Goose Dam ...................................................................................................................................62 

Lower Monumental Dam .......................................................................................................................64 

Ice Harbor Dam......................................................................................................................................64 

McNary Dam..........................................................................................................................................65 

John Day Dam........................................................................................................................................65 

The Dalles Dam......................................................................................................................................67 

Bonneville Dam......................................................................................................................................68 

First Powerhouse ................................................................................................................................68 

Second Powerhouse............................................................................................................................69 

Spillway..............................................................................................................................................70 

Diel Passage Conclusions.......................................................................................................................70 

Juvenile Salmon Passage Through Sluiceways and Surface Bypass Systems............................................71 

Prototype Surface Bypass Systems.........................................................................................................73 

Discussion ..............................................................................................................................................77 

Passage Through Surface Bypass Systems Conclusions........................................................................80 

Juvenile Salmon Passage Through Turbines ..............................................................................................82 

Methods..................................................................................................................................................82 

Turbine Survival Estimates Based on Juvenile vs. Adult Returns.....................................................82 

Turbine Survival Estimates Based on Balloon-Tag, Radiotelemetry, and PIT-Tag Methodologies..83 

Single- and Paired-Release Protocols for Turbine Survival Estimates ..............................................85 

Results of Survival Estimates for Juvenile Fish Passing Turbines.........................................................86 

Operation of Existing Turbines ..............................................................................................................86 

Studies Related to New Turbine Designs...............................................................................................95 

Discussion ..............................................................................................................................................97 

Passage Through Turbines Conclusions.................................................................................................98 

iv 
Adult Passage............................................................................................................................................100 

Background...........................................................................................................................................100 

Research Results and Discussion .........................................................................................................101 

Dam Passage ....................................................................................................................................101 

Migration Rates................................................................................................................................103 

Fallback ............................................................................................................................................104 

Survival ............................................................................................................................................105 

Marine Mammal Predation...............................................................................................................107 

Zero Flow Operations.......................................................................................................................108 

Water Temperature...........................................................................................................................108 

Dissolved Gas Supersaturation.........................................................................................................110 

Kelts .................................................................................................................................................112 

Other Adult Passage Issues ..............................................................................................................113 

Adult Passage Conclusions...................................................................................................................118 

References.................................................................................................................................................121 

v 
vi 
List of Figures 

Figure 1. Map of the federal Columbia River power system....................................................................2 

Figure 2. Cross section of the mechanical screen bypass system at the Bonneville Dam 

Second Powerhouse.................................................................................................................32 

Figure 3. Diel distributions for surface bypass collector, pier nose, spillway, and within-turbine 

sample locations at Lower Granite Dam, 1998, based on fixed-location hydroacoustics.......61 

Figure 4. Diel distribution of fish passage for surface bypass collector, powerhouse, and spillway 

with fixed-location hydroacoustics at Lower Granite Dam, 1999...........................................61 

Figure 5. Total number of detections over the entire study period vs. mean hourly discharge 

at Little Goose Dam, 11 July–24 August 1994, 17 July–24 August 1995, 

17 July–26 August 1996, and 15 July–23 August 1997..........................................................63 

Figure 6. Cross-sectional view of a Columbia River dam showing turbine passageway components...93 

vii 
viii 
List of Tables 

Table 1. Location, species and run type, study year, fish-marking method, spillbay, 

test conditions, and survival estimates for spillway passage at dams on the 

lower Snake and Columbia rivers............................................................................................10 

Table 2. Estimated spill caps.................................................................................................................25 

Table 3. Fish guidance efficiency at Columbia and Snake River dams for 1999 configuration ...........37 

Table 4. Daily estimates of fish guidance efficiency for existing conditions........................................38 

Table 5. Most recent orifice passage efficiency estimates for existing conditions 

(and prototype testing) at Snake and Columbia River dams. ..................................................40 

Table 6. Average descaling and mortality rates for juvenile salmonids passing through 

fish bypass systems at Snake River dams from 1998 to 2002.................................................51 

Table 7. Average descaling and mortality rates for juvenile salmonids passing through 

fish bypass systems at lower Columbia River dams from 1998 to 2000.................................51 

Table 8. Recent juvenile Chinook salmon and steelhead mean blood plasma stress indicator 

concentrations by juvenile fish facility sample location for COE-operated hydroelectric 

projects on the Columbia and Lower Snake rivers..................................................................56 

Table 9. Routes of passage in spring 2002, via radiotelemetry.............................................................62 

Table 10. Percent night passage for each season at John Day Dam, 1985 to 1997.................................65 

Table 11. Percent night passage for 1992–1995 at Bonneville Dam First Powerhouse..........................69 

Table 12. Turbine passage survival estimates for Snake and Columbia River dams..............................87 

Table 13. Ranges in estimated survival for 3 turbine release locations; each range represents 

the 4 turbine loads tested. ........................................................................................................96 

Table 14. Project kelt passage, guidance, sluiceway, and spillway efficiencies, sluiceway and 

spillway effectiveness, at John Day, The Dalles, and Bonneville dams, 2001–2002............113

ix 
x 
Executive Summary 

Over the past 60 years, a large amount of information has been amassed from studies 

conducted on the behavior and survival of juvenile and adult Pacific salmon (Oncorhynchus 

spp.) during passage through eight hydroelectric dams operated by the U.S. Army Corps of 

Engineers on the lower Snake and Columbia rivers. The eight dams are Lower Granite, Little 

Goose, Lower Monumental, and Ice Harbor Dams on the lower Snake River, and McNary, John 

Day, The Dalles, and Bonneville Dams on the lower Columbia River. These dams and their 

reservoirs are the mainstem component of the Federal Columbia River Power System (FCRPS). 

This technical memorandum focuses primarily on passage data associated with the dams

as they have been configured recently, and not on effects on salmon that might accrue from

major changes such as dam removal. The effects of the FCRPS on evolutionarily significant 

units of salmon listed under the Endangered Species Act and potential benefits to these 

populations from actions undertaken in the hydropower system are being addressed elsewhere, 

such as through deliberations of the Technical Recovery Teams, which establish biologically 

based recovery goals, and Biological Opinions. Here we present a synthesis of the most recent 

and current information and summary conclusions on the following topics: 1) juvenile salmonid 

passage through spillways, 2) juvenile passage through mechanical screen bypass systems, 3) 

juvenile passage through turbines, 4) juvenile passage through sluiceways and surface bypass 

systems, 5) juvenile diel passage and timing past dams, and 6) adult salmonid passage past dams

and through the eight-dam reach. 

Spill increases fish passage efficiency (proportion of juvenile fish passing through 

nonturbine routes) and reduces migration time through forebays and tailraces. In general, when 

daytime spill is provided, spillway passage of juvenile salmonids occurs at a fairly constant diel 

rate, and migration timing may be delayed if spill is managed based on passage patterns at the 

powerhouse. Spill has the potential to reduce the exposure of juvenile salmonids to predation, 

high water temperatures, and disease. Tailrace residence time for juvenile salmonids passing 

through spillways is influenced by spill volume and pattern. Generally, spillway passage results 

in more rapid tailrace egress than bypass system passage. Because piscine predators prefer 

hydraulic conditions provided in eddies and backwaters, spill patterns that facilitate the egress of 

juvenile salmonids from the spillway stilling basin and through the tailrace likely increase 

survival. 

Fish survival through spillways can be very high (near 1.00) and is often higher than 

turbine or bypass system survival when spill passage conditions are optimal. However, survival 

through spillways with deflectors or shallow stilling basins can be negatively influenced by 

stilling basin depth and turbulence, hydraulic patterns in the basin, spillbay location within the 

spillway relative to the spill pattern being used, deflector elevation relative to tailwater elevation 

and thus total river flow, gate opening, and fish location when passing through the spillbay and 

under the control gate. Survival through spillways with deflectors or shallow basins can be 

xi 
considerably less than 0.98, a value typically used in hydropower system modeling exercises. 

Observations and lessons learned from spillway survival studies at The Dalles, Ice Harbor, and 

Lower Monumental Dams should be applied to other spillways with deflectors to ensure that safe

spillway passage conditions are being provided at these locations. 

Estimates of spill efficiency (the proportion of fish approaching a project that pass via the 

spillway) and spill effectiveness (spill efficiency divided by the proportion of total river flow 

passing over the spillway) for juvenile salmonids vary by dam, the percentage of river flow 

spilled, spill pattern, time of day, species/run, and dam configuration. Spill effectiveness is 

generally between 1.0 and 2.0, but can range considerably higher at certain dams. There is a 

strong indication that above a certain spill level there is no gain and very likely a reduction in 

spill effectiveness, although spill efficiency will continue to increase with the percentage of total 

river flow spilled. 

Relatively few studies have been conducted on subyearling Chinook salmon 

(Oncorhynchus tshawytscha) survival through spillways during the summer migration period 

under contemporary conditions. Estimates of mean spillway passage survival at Ice Harbor and 

The Dalles Dams since 1997 using traditional spill patterns ranged from 0.75 to 1.00. However, 

due to problems associated with stilling basin conditions at these dams, this wide range in 

spillway survival estimates may not be representative of other Snake and Columbia River dams. 

For example, estimated mean survival through the spillway for John Day Dam in 2002 and 2003 

ranged from 0.96 to greater than 1.00 when 30% of river flow was spilled throughout a 24-hour 

period. In addition, estimated mean survival through the Ice Harbor Dam spillway was 0.96 in 

2003 under a “bulk” spill pattern, where higher flow was concentrated in fewer spillbays. 

Results of gas bubble disease (GBD) monitoring and research on juvenile salmonids 

indicate that incidents of high prevalence and severity of GBD signs and probable related 

mortality are associated with exceptionally high river flows or unusual dam operation problems 

where powerhouse flows are limited. Under total dissolved gas cap conditions of 115% 

(forebay) and 120% (tailrace), signs of GBD are minimal or nonexistent, with no apparent GBDrelated mortality. 

Estimates of how many fish are guided into mechanical screen bypass systems at dams 

vary by location (dam, turbine), time (season, day, time of day), species/run, and method used 

(physical netting, hydroacoustics, radiotelemetry, and estimates based on detection of fish tagged 

with passive integrated transponder [PIT] tags). These variables make it difficult to confidently 

develop accurate and precise point estimates of fish guidance efficiency. Even so, we present 

our best estimates by dam. Extended-length screens appear to guide higher percentages of 

smolts without concomitant increases in descaling and injury rates, but only when appropriately 

designed vertical barrier screens are installed that can manage increased flows into turbine 

gatewells. Once fish have been guided into gatewells, estimates of passage efficiency into the 

bypass system have generally exceeded 70%, the regionally acceptable value, and estimates are 

higher for extended-length than for standard-length fish guidance screens. Contemporary 

mechanical screen bypass systems are vastly improved compared to the original systems that 

operated during the 1970s and early 1980s, based on recent low rates of descaling, injury, and 

system mortality. However, outfall location and configuration are important considerations for 

maximizing the survival of juvenile salmonids that are bypassed back to the river below dams. 

xii 
Mechanical screen systems have the potential to be selective for species/run, life history types, 

and other attributes such as size. Operations that provide multiple routes to pass fish around 

dams presumably minimize the risk of reducing biodiversity associated with mechanical screen 

bypass systems. 

Smolt-to-adult return rates for PIT-tagged in-river migrants are related to bypass history. 

Smolts detected at dams other than Lower Granite Dam often return as adults at lower rates than 

fish not detected at any dams. For hatchery (but not wild) smolts, detection frequency is 

inversely related to return rates. A confounding factor is that smaller smolts with lower return 

rates tend to be more easily guided into bypass systems than larger smolts with higher return 

rates. 

Passage through components of mechanical screen bypass systems induces physiological 

changes related to the general adaptation syndrome, or stress, in juvenile anadromous salmonids. 

However, the physiological effects of passage through fish bypass facilities are nominal for 

juvenile Chinook salmon and steelhead (O. mykiss), as measured by blood plasma stress indices, 

and follow a typical sequence for fish subjected to an acute stressor followed by acclimation to 

or removal of the stressor. Unless exhaustion or physical injury are involved, the majority of 

bypassed fish are probably minimally affected by a single, and possibly several, stressful 

experiences. There is some evidence that multiple stress experiences can cumulatively diminish 

survivability, and post-stress survival likely depends on a combination of the type of stressor 

involved and each fish’s overall condition and health, and thus, sensitivity to that stressor at the 

time. 

At bypass facilities all fish sizes and species are mixed and small yearling Chinook 

salmon smolts can become stressed when in the presence of larger steelhead smolts. Separation 

will be necessary to reduce extended holding of small fish with larger fish, especially in the 

context of transportation. Prototype high-velocity separators separate in excess of 80% of small 

from large fish and could potentially be used in future mechanical screen bypass systems. 

During summer, juvenile anadromous salmonids migrating through the Columbia and 

Snake rivers encounter water temperatures that often exceed the 20EC (68EF) threshold 

mandated in the Washington State Clean Water Act. Temperatures exceeding 25EC (77EF), the 

upper incipient lethal limit for salmonids, have also been documented during the summer 

juvenile migration period. High summer water temperatures caused a catastrophic mortality 

within the juvenile collection facility at McNary Dam in summer 1994, which was thought to 

have resulted from operating turbine units at the south end of the powerhouse that draw in 

warmer water. Modifications to powerhouse operations and improved temperature monitoring 

have prevented a recurrence, because average mortality rates during the most recent five 

summers ranged from 0.6 to 1.0%. 

Passage timing for each route of dam passage varies with dam configuration, flow 

volume through the route, species, run, and life history type. Most studies were not designed to 

evaluate 24-hour passage behaviors for each species and life history type, making it difficult to 

precisely describe passage timing for each dam under varying conditions. In general, juvenile 

migrants that arrive during daytime tend to not pass through powerhouse routes (turbines and 

bypass) on arrival but will readily pass through surface-oriented outlets and daytime spill if 

xiii 
provided. For juvenile migrants that arrive at night, passage is more equally distributed amongst 

available routes. 

Results from studies of the Lower Granite Dam removable spillway weir and Wells Dam

deep-slot surface bypass systems demonstrate that surface bypass is a viable concept that can 

produce high rates of nonturbine fish passage with a relatively small percentage of project 

discharge. Surface bypass systems operated 24 hours per day reduce forebay residence times of 

juvenile salmon and exposure to predators relative to the existing program of 12-hour spill at 

night. Optimum surface bypass performance is attained when the entrance is at a location known 

to attract large numbers of fish. Marginal results from other locations indicate that site 

conditions may limit performance at some locations. Weir entrances have proven more 

successful at passing juvenile salmon and steelhead migrants than deep-slot entrances, especially 

when flow velocity through the weir increases gradually. Discharge through the entrance should 

be sufficient to create a strong velocity flow-field upstream, but high discharge alone will not 

overcome a marginal entrance location. Additional passage facilities such as a floating forebay 

curtain or turbine intake occlusion can help improve fish guidance through surface bypass 

systems. To achieve high passage efficiency and survival, successful surface bypass designs 

need to address guidance into the entrance, passage through the collector, discharge into the 

tailrace, and egress through the immediate tailrace. Recent data indicate that impact velocities 

up to 15.2 ms-1 (50 fts-1) are safe for juvenile salmonids for discharges greater than 1,000 cfs. 

Estimates of juvenile fish survival through turbines ranged from 0.81 to 0.92 for studies 

conducted from 1939 through 1968. Studies conducted under contemporary conditions using 

radiotelemetry and PIT-tag methodologies suggest mean survival through turbines generally 

remains in this range. Therefore, turbines continue to represent a significant source of mortality 

for juvenile salmon migrating through the FCRPS. Indirect mortality can be a significant 

component of total estimated turbine mortality, which future efforts to improve survival through 

turbines must address. 

While a statistically significant relationship between juvenile salmonid survival and peak 

turbine efficiency has not been demonstrated, operation of units within 1% of peak efficiency 

most often encompassed the operation that produced the maximum estimated survival. 

Therefore, continued operation within 1% of peak turbine efficiency is a reasonable operating 

guideline to follow until more detailed information on the relationship between unit operations 

and maximum fish survival for each family of turbines is developed. Development of more 

refined guidelines appears warranted because maximum survival can be as much as 3.2% higher 

than survival measured at peak efficiency, and this potential improvement in survival is large 

compared to that for other FCRPS configuration modifications. 

Recent radiotelemetry estimates of adult salmonid survival from Ice Harbor Dam to 

Lower Granite Dam on the Snake River have generally exceeded 90%, and PIT-tag estimates of 

survival from Bonneville Dam on the lower Columbia River to Lower Granite Dam have been 

over 80% for spring/summer Chinook salmon and 70–80% for steelhead. For adult salmonids 

that do not fall back at dams, estimates of survival range from 3% to 5% higher than for those 

that do fall back. Adult survival above the hydrosystem to hatcheries or spawning grounds is 

variable and estimates from Ice Harbor Dam using radiotelemetry range from 54% to 77%. The 

effects of passage through the hydrosystem on reproductive success are unknown. 

xiv 
Overall travel times and migration rates of radio- and PIT-tagged adult salmonids from 

Bonneville Dam to upriver Columbia or Snake River sites vary by species/run, time of year, and 

environmental conditions. Recent estimates of travel times and migration rates are similar to 

times and rates observed during the early development of the FCRPS, when there were fewer 

dams and the Snake River was unimpounded. Delays in dam passage for upstream migrating 

adult salmonids are associated with high levels of spill (≥40 kcfs), highly fluctuating spill, and 

high turbidity. In addition, elevated river water temperatures (>20EC) increase travel times 

between dams and through fishways at dams. Fishway entrance preferences are for deep/wide 

openings with significant attraction flow. The transition areas between collection channels and 

ladders are the fishway segments where adult Chinook salmon and steelhead are most likely to 

turn around and exit the fishway. Rates of fallback through a dam vary by species/run, 

environmental conditions, and dam. Higher rates are associated with high river flows, spill, and 

with adult fishway exit locations in close proximity to spillways. Fallback results in an

overestimation of adult counts in fish ladders. 

Estimates of repeat spawning rates for steelhead kelts that migrate downstream through 

the Snake and Columbia rivers are low (≈2%). While potentially negative effects of the 

hydrosystem on iteroparity rates have been recognized, the magnitude of the effects on survival 

and reproductive success are not clearly understood. Higher flows and spill significantly reduce 

travel and passage times for downstream-migrating kelts. 

Relative to salmonids, upstream passage at Columbia and Snake River dams is poor for 

adult Pacific lamprey (Lampetra tridentata) and white sturgeon (Acipenser transmontanus). 

American shad (Alosa sapidissima) have thrived in the FCRPS habitat, but their overall 

ecological impacts on anadromous salmonids have not been studied. 

xv 
xvi 
Acknowledgments 

An earlier draft was significantly modified to reflect comments and additional 

information provided by the Oregon Department of Fish and Wildlife, U.S. Fish and Wildlife 

Service, Idaho Department of Fish and Game, Fish Passage Center, Bonneville Power 

Administration, Columbia River Inter-Tribal Fish Commission, and Save Our Wild Salmon. We

greatly appreciate the time and effort contributed by personnel from these organizations to 

improving the content and usefulness of this technical memorandum. 

xvii 
xviii 
Acronyms and Abbreviations 

AHTS. Advanced Hydropower Technology Study — a program funded by the U.S. Department 

of Energy to develop advanced turbine technologies and designs. 

AWS. Auxiliary water supply — delivers supplemental attraction water to entrances used to 

attract adult salmon into adult fishways and ladders. 

BiOp. Biological Opinion issued by NOAA Fisheries Service under the Endangered Species 

Act. 

BPA. Bonneville Power Administration. 

Cfs. Cubic feet per second. 

COE. U.S. Army Corps of Engineers. 

ESA. Endangered Species Act. 

ESBS. Extended-length submersible bar screen. 

ESU. Evolutionarily significant unit — an independent population that can be listed under the 

Endangered Species Act. 

FCRPS. Federal Columbia River Power System. 

FGE. Fish guidance efficiency — the proportion of fish entering the turbine intake of a 

powerhouse that are guided into a juvenile fish bypass system. 

FPE. Fish passage efficiency — the proportion of fish passing a dam that pass the dam through 

nonturbine routes. 

ft msl. Elevation above mean sea level in feet. 

GBD. Gas bubble disease. 

HVF. High-velocity flume. 

ISAB. Independent Scientific Advisory Board — a regional body of scientists convened by 

NOAA Fisheries Service and the Northwest Power and Conservation Council to provide 

technical advice to researchers and decision makers. 

Kcfs. Thousand cubic feet per second. 

xix 
MGR. Minimum gap runner — a new turbine design that minimizes gaps between rotating and 

stationary turbine components. 

NMFS. National Marine Fisheries Service, also called NOAA (National Oceanic and

Atmospheric Administration) Fisheries Service. 

NPPC. Northwest Power Planning Council, now called the Northwest Power and Conservation 

Council. 

OPE. Orifice passage efficiency — the proportion of fish entering a turbine gatewell slot that 

exit the slot via an orifice and enter a juvenile fish bypass system within a specified amount 

of time, usually 22 or 24 hours. 

PATH. Plan for Analyzing and Testing Hypotheses — an analytical process implemented in 

response to the NOAA Fisheries Service 1995 BiOp and the Northwest Power and 

Conservation Council Columbia Basin Fish and Wildlife Program and undertaken by a panel 

of regional scientists to evaluate hypotheses pertaining to mortality associated with passage 

through the FCRPS. 

PIT tag. Passive integrated transponder tag. 

RKm. River kilometer. 

RPA. Reasonable and Prudent Alternative — a formal recommendation by NOAA Fisheries 

Service in a Biological Opinion to avoid jeopardy under the Endangered Species Act. 

RSW. Removable spillway weir. 

SAR. Smolt-to-adult return rate. 

SBC. Surface bypass collector. 

STS. Standard-length submersible traveling screen. 

TDG cap. Total dissolved gas cap — the total dissolved gas limit that must be maintained in the 

tailrace or forebay of a dam.

TDGS. Total dissolved gas supersaturation. 

TSP. Turbine survival program — a program funded by the U.S. Army Corps of Engineers to 

understand mechanisms of fish mortality within turbines and develop new turbine designs to 

improve juvenile fish passage survival through turbines. 

VBS. Vertical barrier screen — a stationary screen between the upstream and downstream

gatewell slots of a turbine intake that prevents fish that are guided into the upstream slot from

reentering the turbine intake through the downstream slot. 

xx 
Introduction 

This technical memorandum synthesizes available information on the behavior and 

survival of juvenile and adult Pacific salmon (Oncorhynchus spp.) during passage through eight 

dams operated by the U.S. Army Corps of Engineers (COE) on the lower Snake and Columbia 

rivers (Fig. 1). These eight dams and their reservoirs constitute the mainstem component of the 

Federal Columbia River Power System (FCRPS). This synthesis focuses on those aspects of 

passage considered relevant by NOAA’s National Marine Fisheries Service (NMFS or NOAA 

Fisheries Service) for judging FCRPS impacts upon specific salmon stocks listed under the 

Endangered Species Act (ESA), and in general, to all salmon stocks in the Columbia River that 

utilize habitats within or above the FCRPS. 

The amount of information developed through studies conducted over the past 60 years is 

large, and reporting all the available information is beyond the scope of this document. This 

memorandum focuses primarily on information gathered under contemporary conditions, or as 

the dams have been configured and operated since the 1995 FCRPS Biological Opinion (BiOp) 

was implemented. Results from studies conducted earlier than this time frame are discussed for 

context. The focus is on passage issues associated with the dams as they exist, not effects on 

salmon that might accrue from major changes such as dam removal. Nonetheless, it is 

recognized that many of the adverse impacts from FCRPS dams on salmonids would be reduced 

or eliminated with the removal of dams. 

The eight dams form back-to-back reservoirs over what was once a free-flowing riverine 

ecosystem. This stretch of river contained important habitat that salmon historically used for 

spawning, rearing, physiological development, preparation for seawater entry, growth, and 

shelter from predators. There is no doubt that this complex of eight dams and reservoirs has 

significantly changed this riverine ecosystem and affected the stocks of salmon that use and 

migrate through this habitat. The effects of the FCRPS on listed salmon evolutionarily 

significant units (ESUs) and potential benefits to these populations from actions undertaken in 

the hydropower system are being addressed elsewhere, such as in Williams et al. (2005), through 

life cycle modeling, and through technical deliberations of the Technical Recovery Teams. Here 

we present what is known about passage at the concrete under current conditions to provide a 

source of information that can be used by hydropower system and resource managers to judge 

actions and make decisions regarding dam operations and configurations in the future. 

As the National Research Council (NRC) pointed out, the long-term persistence and 

productivity of salmon populations are dependent in part on maintaining conditions that allow 

salmon to retain their genetic variability and local adaptations (NRC 1996). More recently, a 

viable salmonid population has been defined as an independent population of any Pacific 

salmonid that has a negligible risk of extinction due to threats from demographic variation, local 

environmental variation, and genetic diversity changes over a 100-year time frame. Four key 

parameters are considered when evaluating population viability status: abundance, population 
growth rate, population spatial structure, and diversity (McElhany et al. 2000). It is not possible 

at this time to assess potential impacts to the viability of salmon populations from specific dam

operations or configurations. Perhaps this question will be addressed in the future when 

appropriate data are available to make such an assessment. For now, we simply recognize that 

factors considered to be important to the viability of salmon populations over time have been 

identified and are an important consideration in recovery planning. 

Figure 1. Map of the federal Columbia River power system. 

2 
Juvenile Salmon Passage Through Spillways 

Background 

Downstream passage through spillways at major hydroelectric dams in the Columbia 

River Basin has been provided for many years to increase survival of juvenile salmonids. 

Historically, spill occurred operationally when project capacity or FCRPS generation needs were 

exceeded. However, as the hydroelectric system was developed, spill became less frequent. In 

1988, a 10-year spill program under a Fish Spill Memorandum of Agreement was implemented 

to provide spill at projects to meet regional goals for fish passage through nonturbine routes. 

Continued declines in salmonid populations prompted the eventual development of the more 

aggressive spill management plans required under the NMFS FCRPS BiOps. Since 

implementation of the 1993 and subsequent BiOps, spill has generally been provided in 

accordance with the prevailing spill program in most years, but there has been considerable 

variation in spill among years for a variety of environmental (river flow), system (excess 

generation spill, excess hydraulic capacity spill, turbine failures, maintenance-related

conditions), and fish research (test schedules) reasons. 

The generally recognized benefits of spill include reductions in mortality, migrational 

delays (through forebays, tailraces, and reservoirs), exposure to predation risk, high water 

temperatures, and diseases (Whitney et al. 1997, Muir et al. 2001, Giorgi et al. 2002). Previous 

studies have found that passage survival for juvenile salmonids at Columbia and Snake River 

dams is generally highest for spillways, followed by bypass systems, then turbines (Schoeneman 

et al. 1961, Whitney et al. 1997, Muir et al. 2001). 

The spillway passage route at all FCRPS dams includes a forebay, spill gates, ogee, 

stilling basin, and tailrace. The forebay typically consists of about 1 km of reservoir 

immediately above the spillway gates. Most spillway gates have a radial design with a 18.3-m

radius and 15.2-m width (COE 1996b). Two dams (Bonneville and McNary) have vertically 

operated lift gates of similar width. The number of spillbay gates per spillway varies from 8 to 

10 at lower Snake River dams to 18 to 23 at lower Columbia River dams. 

The ogee sections provide a transition for flow from below the gates to the stilling basin. 

Flow deflectors that help reduce dissolved gas production at many FCRPS dams are located on 

the ogee sections at elevations designed for each project. Below the ogee, the spilled flow enters 

a stilling basin designed to dissipate turbulent energy in a confined, armored zone, thereby 

minimizing the threat to the spillway’s structure. Beyond the stilling basin, the tailrace extends 

for about 1 km downstream. Spillway capacities are designed for the maximum probable flood 

and vary from 850 thousand cubic feet per second (kcfs) at lower Snake River dams to 2,290 

kcfs at lower Columbia River dams.

3 
Research Results and Discussion 

Spill Efficiency and Effectiveness 

Spill efficiency is the proportion of fish approaching a project that pass via the spillway. 

Spill effectiveness is spill efficiency divided by the proportion of total river flow passing over 

the spillway at the same time. Recent reviews of spill efficiency and effectiveness include Steig 

(1994), Giorgi (1996), Whitney et al. (1997), and Marmorek and Peters (1998). Estimates of 

spill efficiency vary by project. 

Most spill efficiency estimates are based on hydroacoustic methodologies, which make estimates 

for the general population, not specific species or stocks. Hydroacoustic estimates are sensitive 

to transducer position, fish size, and fish orientation. During the summer, hydroacoustic 

estimates are particularly susceptible to error caused by the presence of nonsalmonids, such as 

juvenile shad (Alosa sapidissima), and high summer effectiveness values reported should be 

viewed with this in mind. Radiotelemetry studies have been successfully conducted at some

dams, where yearling and subyearling Chinook salmon (Oncorhynchus tshawytscha) and 

steelhead (O. mykiss) were marked and released upstream, then their behavior and route of dam

passage were noted. With sufficient receiving antennas and sample size, estimates of spill 

efficiency and effectiveness can be derived. 

Steig (1994) reviewed studies at Snake and Columbia River dams conducted through 

1992 and noted that there is considerable variability in daily and weekly spill effectiveness. 

However, he concluded that most results fall around a 1:1 relationship between the proportion of 

water spilled and the proportion of fish passed in spill (i.e., 1.0 spill effectiveness). Giorgi 

(1996) reviewed estimates of spill effectiveness published through 1993 and noted that 

effectiveness is poorly estimated for most species due to a combination of sparse observations, 

imprecise estimates, and the reliance of most estimates on hydroacoustic monitoring, which is 

unable to distinguish among species. He cautioned that the assumption of a spill effectiveness of 

1.0 could not be justified in most cases and implied that a suite of estimates acquired with 

different methodologies should be considered when attempting to derive species-specific 

estimates at individual dams. Relying largely on Giorgi’s (1996) review, the PATH Hydro Work 

Group (PATH 1997) concluded that a range of spill effectiveness from 1.0 to 2.0 should be 

incorporated into sensitivity analyses of passage at Snake and Columbia River dams. 

Spill efficiency at Lower Granite, Little Goose, and Lower Monumental Dams can be 

estimated based on radiotelemetry observations for yearling Chinook salmon at Lower Granite 

Dam (Wilson et al. 1991), because of the similarity of the three projects. Combining the 

radiotelemetry observations with the assumptions that 0% of fish pass the spillway at 0% spill 

and 100% pass at 100% spill, the following relationship (Smith et al. 1993) can be applied:

Pf = 2.583Pw – 3.250Pw

2

 + 1.667Pw

3

where: Pf is the proportion of fish passing over the spillway (spill efficiency), 

 Pw is the proportion of total river flow passing over the spillway, and 

 spill effectiveness is defined as Pf ) Pw. 

4 
The shape of this relationship is uncertain outside of the range of observations (20–40% 

spill), even though Pf must logically go to 0 and 1.0 at the extremes (ISG 1996). 

Spill efficiency and spill effectiveness were examined in 1999 at Ice Harbor Dam using 

radiotelemetry (Eppard et al. 2000). The study concluded that spill efficiency was 82.6% and 

spill effectiveness was 1.8:1 during 2000 BiOp day conditions (approximately 45% spill). 

During nighttime BiOp conditions (approximately 100% spill), spill efficiency increased to 

96.1%; however, spill effectiveness was closer to a 1:1 relationship. This suggests that there 

may be a level of spill at Ice Harbor Dam above which there is no gain in spill effectiveness at 

passing fish. 

In 2003 radiotelemetry studies were conducted by Eppard et al. (2003) from 29 April to 6 

June at Ice Harbor Dam to evaluate relative spillway passage and project survival for hatchery 

yearling Chinook salmon under a 50% voluntary spill operation versus the 2000 BiOp spill 

operation. Overall spill efficiency during this study was 88.5% (95% CI, 83.7–93.2) and spill 

effectiveness was 1.5 (95% CI, 1.4–1.6). 

Hydroacoustic evaluations of these two spill conditions were also conducted at Ice 

Harbor Dam from 15 April to 15 July in 2003 (Moursund et al. 2003b). Spill efficiency for 

spring migrants was 75.9% and 51.7% for BiOp spill and 50% spill, respectively (significantly 

different), and spill effectiveness was 1.12 and 1.01, respectively. For summer migrants, spill 

efficiency was 70.8% and 64.4% for BiOp spill and 50% spill, respectively, and spill 

effectiveness was 10.7 and 1.27, respectively (significantly different). Moursund et al. (2003b) 

concluded that route-specific passage and project-wide performance metrics differed among 

operational treatments. 

Passage of radio- and PIT-tagged yearling Chinook salmon was evaluated at McNary 

Dam in 2002 by Axel et al. (in prep. a) during day and nighttime operations from 5 May to 5 

June. Nighttime spill remained relatively constant throughout this period (to 120% total 

dissolved gas [TDG] cap), but daytime spill increased considerably as of 21 May to 

accommodate increased river flows. Overall spill efficiency was 47% (95% CI, 27–67) and 

mean spill effectiveness (for Snake River migrants) was 1.12 (95% CI, 0.91–1.33). 

Axel et al. (in prep. b) evaluated passage behavior for radio-tagged Columbia and Snake 

River yearling Chinook salmon at McNary Dam in 2003. During the first three weeks of the 

study period, from late April to early June, spill was at BiOp levels (nighttime spill to 120% 

TDG cap), but involuntary daytime spill began on 26 May due to increased river flows. Spill 

efficiency was 47% for Snake River fish and 49% for Columbia River fish; mean spill 

effectiveness for Snake River migrants was 1.43 and for Columbia River migrants was 1.32. 

At John Day Dam in 1999 a constant nighttime spill of 60% was scheduled (but observed 

spill was actually 45%), and daytime spill was 0% and 30%, each scheduled for 3-day 

continuous periods within each replicate. During the spring at John Day Dam, Johnston and 

Nealson (1999) hydroacoustically measured nighttime spill efficiencies that were lower than 

reported previously, ranging from 1.4 to 1.7; however, there were potential problems with 

transducer placement. During the summer, 24-hour spill effectiveness ranged from 2.4 to 5.1, 

5 
while nighttime spill effectiveness ranged from 2.4 to 3.3. Both the spring and summer periods 

exhibited significantly higher 24-hour spill effectiveness during the 30% daytime spill treatment. 

Using radio-tagged steelhead, Hansel and Beeman (1999) found spill efficiency was not 

significantly different between the two treatments, at 46% and 54%, although efficiency was 

higher during the treatment that included daytime spill. For radio-tagged yearling Chinook 

salmon, spill efficiency was 53% and 65%; the periods when daytime spill was provided were 

significantly different and greater than the treatments without daytime spill. 

In the spring of 2000, Beeman et al. (2001) evaluated spill passage under two day/night 

treatments (0/60% and 30/60%) at John Day Dam using radio-tagged steelhead and yearling 

Chinook salmon. For Chinook salmon, spill efficiency was significantly higher for 24-hour spill 

(86% CI, 80–90) than for nighttime-only spill (75% CI, 68–81). For steelhead, spill efficiency 

ranged from 64% to 83% for 24-hour spill and 61% to 79% for nighttime-only spill (no CIs 

reported). Spill efficiency for Chinook salmon and steelhead passing through day spill was 92% 

and 68%, respectively. Spill efficiency for Chinook salmon passing through nighttime spill was 

78% for both spill treatments, and for steelhead it was 72% during the 12-hour treatment and 

78% during the 24-hour treatment. Also reported by Beeman et al. (2001), day spill at John Day 

Dam in 2000 was extremely effective for yearling Chinook salmon (2.9–3.2) and for steelhead. 

During nighttime spill (actual nighttime spill ranged from 50% to 54%), spill effectiveness 

ranged between 1.4 and 1.6 for Chinook salmon and 1.2 to 1.7 for steelhead. 

The same two spill treatments were evaluated for passage of radio-tagged subyearling

Chinook salmon at John Day Dam in the summer of 2000 (Beeman et al. 2002a). Spill 

efficiency was significantly higher for the 24-hour treatment (82%) than for the 12-hour 

treatment (54%), and spill effectiveness was greater for the 30% daytime spill (2.6–3.0) than for 

the 60% nighttime spill (1.1–1.6). 

In 2002 Moursund et al. (2003a) conducted hydroacoustic studies at John Day Dam to 

estimate fish passage efficiency for 24-hour (30%) and 12-hour (0% day–60% night) spill 

treatments. With 12-hour spill in the spring, spill efficiency was 78% ("6) and spill 

effectiveness was 2.90 ("0.30); with the 24-hour spill treatment, efficiency was 72% ("5) and 

effectiveness was 2.68 ("0.26). With 12-hour spill in the summer, spill efficiency was 58% 

("11) and spill effectiveness was 2.10 ("0.30); with the 24-hour spill treatment, efficiency was 

61% ("12) and effectiveness 2.30 ("0.32). None of the differences for either the spring or the 

summer were statistically different. 

Beeman et al. (2002a) also evaluated fish passage with 12- and 24-hour spill treatments at 

John Day Dam in 2002 and found no significant difference in spill efficiency between treatments 

for radio-tagged, hatchery yearling Chinook salmon. Nighttime-spill efficiency for yearling 

Chinook salmon was significantly higher during the 12-hour spill treatment than during the 24-

hour treatment, whereas nighttime spill efficiency for wild steelhead was similar during the two 

spill treatments. 

In contrast to all other dams, the powerhouse of The Dalles Dam is oriented nearly 

parallel to the natural course of the river, while the spillway is located on what was a shallow 

basalt bluff. The Independent Scientific Group (ISG 1996) suggests it is not surprising that this 

6 
project exhibits higher spill efficiency than many other projects, due to its unique configuration. 

Giorgi and Stevenson (1995) reviewed biological investigations that described smolt passage 

behavior at The Dalles Dam and discussed implications for future surface bypass research. They 

cited three investigations that indicated that spill effectiveness was near 2.0 when about 20% of 

the flow passed over the spillway. These investigations included Willis (1982), which describes 

a curvilinear relationship in which spill effectiveness is equal to or greater than 2.0 at spill below 

approximately 30% of total river flow, declining to about 1.4 at 60% spill, and further declining 

to 1.0 at 100% spill. 

A 1996 radiotelemetry study at The Dalles Dam by Holmberg et al. (1998) supports this 

general relationship. Spill effectiveness for yearling Chinook was 2.3 at 30% spill and 1.25 at 

64% spill in 1996. PATH (1997) reviewed the available information and suggested that a factor 

of 2.0 be applied at The Dalles Dam at spill levels up to 30%; when spill levels are above 30% 

spill, the relationship grades from 2.0 to 1.0. This relationship predicts a spill effectiveness of 

1.5 at 65% spill. 

Hansel et al. (2000) evaluated fish passage under two spill conditions (30% and 64%) at 

The Dalles Dam from 7 May to 29 May of 1999 using radio-tagged yearling Chinook salmon 

and steelhead. For 30% spill, they reported spill efficiencies of 51% (46–57) and 66% (60–71) 

for Chinook salmon and steelhead, respectively; for 64% spill, spill efficiencies of 79% (74–84) 

and 86% (82–90) for Chinook salmon and steelhead, respectively. Spill effectiveness at 30% 

spill was 1.7 for Chinook salmon and 2.2 for steelhead, and at 64% spill, 1.3 for Chinook salmon 

and 1.4 for steelhead. 

Beeman et al. (2000a, 2000b) evaluated spill efficiency and effectiveness during the 

spring and summer at The Dalles Dam under constant conditions of 40% spill. From 28 April to 

26 May, with river flows ranging from 217 to 305 kcfs, spill efficiency for radio-tagged yearling 

spring Chinook was 86% (95% CI, 83–89) during the day and 71% (95% CI, 67–76) at night; for 

steelhead, spill efficiency was 88% (95% CI, 84–91) during the day and 82% (95% CI, 77–85) at 

night. Spill effectiveness for both yearling spring Chinook and steelhead was 2.2 during the day, 

and 1.8 for Chinook and 2.1 for steelhead at night. 

For the period 28 June to 20 July, under 39–40% spill (with river flows ranging from 107 

to 281 kcfs) Beeman et al. (2000b) reported spill efficiency for radio-tagged fall Chinook was 

85% (95% CI, 78–91) during the day and 73% (95% CI, 63–82) at night at The Dalles Dam. 

Spill effectiveness was higher during the day than at night, 2.15 vs. 1.86, respectively. 

Moursund et al. (2001b) conducted a hydroacoustic evaluation of fish passage at The 

Dalles Dam in the spring and summer of 2000. For the period of 13 May to 6 June, under 

average spill of 32% (river flows averaged 234 kcfs), spill efficiency was 89% during the day 

and 83% at night. For the period of 6 June to 6 July, under average spill of 31% (river flows 

averaged 192 kcfs), spill efficiency was 75% during the day and 73% at night. 

Ploskey et al. (2000) conducted a hydroacoustic evaluation of fish passage at Bonneville 

Dam in 2000. Under average spill of 33% in the spring, spill efficiency was 44% and spill 

effectiveness was 1.36. Under average spill of 48% in the summer, spill efficiency was 49% and 

spill effectiveness was 1.03. 

7 
Schilt et al. (2002) conducted a hydroacoustic study to provide project-wide estimates of 

fish passage efficiency (FPE), spill efficiency, and spill effectiveness for river-run fish passing 

Bonneville Dam in 2002. In the spring the spillway discharged about 44% of total river flow; 

spill efficiency averaged 57% and spill effectiveness was 1.17. In the summer 48% of river flow 

was spilled; spill efficiency averaged 42% and spill effectiveness was 0.97. For spill levels less 

than 100, 100–125, and greater than 125 kcfs, spring spill efficiency was 41%, 54%, and 66%, 

respectively; summer spill efficiency was 22%, 39%, and 50%, respectively. 

For the spring of 2001 (unusually low river flow year) at Bonneville Dam, Evans et al. 

(2001b) reported spill efficiency for radio-tagged yearling Chinook salmon was 30% with 37% 

spill and 1% with 2% spill (adult attraction spill); spill effectiveness was 0.86 and 0.53 for the 

two spill levels, respectively. Overall for the spring season, spill efficiency was 16% with 22% 

spill and spill effectiveness was 0.7. During the summer, overall spill was 2.4% and spill 

efficiency for subyearling Chinook salmon was 2%. 

Evans et al. (2002) evaluated passage behavior of radio-tagged yearling Chinook salmon 

and steelhead at Bonneville Dam in 2002. During the early 2 May to 9 June study period, 49% 

of the river flow was spilled. For three spill treatments (75 kcfs during the day, up to 137.6 kcfs 

during the day, and up to 141.0 kcfs during the night), spill efficiency for yearling Chinook was 

42%, 63%, and 59%, respectively (57% overall); for steelhead, spill efficiency was 32%, 70%, 

and 49%, respectively (55% overall). Spill effectiveness for Chinook salmon during the three 

treatments was 1.3, 1.2, and 1.1, respectively (1.2 overall); for steelhead, spill effectiveness was 

1.0, 1.4, and 1.0, respectively (1.1 overall). 

Spill Survival 

Whitney et al. (1997) reviewed 13 estimates of spill mortality for salmonids (3 steelhead 

and 10 salmon) published through 1995 and concluded that 0–2% is the most likely mortality 

range for standard spillbays. They also pointed out that local conditions, such as back eddies or 

other situations that may favor the presence of predators, may lead to higher spill mortality. 

Some point estimates for mortality in spillbays with flow deflectors are higher than 

estimates for spillbays without deflectors. For example, the highest estimates of survival for 

yearling Chinook salmon and steelhead at Snake River dams were obtained from spillbays 

without flow deflectors, ranging from 0.984 to 1.000 (Muir et al. 1995b, 1996, 1998). Although 

lower survival estimates were obtained from spillbays with flow defectors (ranging from 0.927 to 

1.000) (Iwamoto et al. 1994, Muir et al. 1995b, Muir et al. 1998), differences in survival between 

the two types of spillbays compared pairwise were not significant for steelhead at Little Goose 

Dam or yearling Chinook salmon at Lower Monumental Dam. 

Eppard et al. (2003) also reported that at Ice Harbor Dam survival was lower over 

spillways with flow deflectors when flows were lower during the spring of 2002 and the 

summers of 2000 and 2002. These authors suggested that the lower survivals may have been due 

to hydraulic conditions in the stilling basin when flows were less than 90 kcfs. Survival of 

hatchery yearling Chinook salmon was 0.892 (95% CI, 0.841–0.943) in 2002. Survival of 

hatchery subyearling Chinook salmon was 0.894 (95% CI, 0.857–0.931) and 0.885 (95% CI, 

0.856–0.915) in 2002 and 2000, respectively. In addition, for yearling Chinook salmon at Lower 

8 
9 

Monumental Dam in 2003, relative survival was significantly higher (0.987 vs. 0.834) when 

submergence of flow deflectors increased as flows increased (Hockersmith et al. in prep.). 

Carlson and Duncan (2003b) suggested that turbulence over the spillway ogee increased with 

decreased flow over the spillway at Ice Harbor Dam, causing a higher probability of fish 

contacting spillway surface structures during lower discharge. Flow deflectors were originally 

designed to reduce gas supersaturation when involuntary spill was required during periods of 

higher flow and subsequent higher tailrace elevation. Hence, survival over spillways with flow 

deflectors may be reduced somewhat when spill occurs at lower flow levels. 

Survival of juvenile salmonids passing through spillways at lower Snake River dams has 

been evaluated at least once at all projects (Table 1). Among the most recent studies, Plumb 

et al. (2004), in evaluating the performance of a removable spillway weir at Lower Granite Dam, 

reported a spillway relative survival estimate of 0.931 ("0.060) for yearling Chinook salmon 

during 12-hour nighttime spill to the gas cap. Hockersmith et al. (in prep.) estimated relative 

survival for radio-tagged hatchery yearling Chinook salmon released in spillbays 4 and 7 during 

daytime operations at Lower Monumental Dam from 29 April to 6 June of 2003. Relative 

spillway survival was 0.896 and 0.895 for spillbays 4 and 7, respectively. Spillway survival for 

releases after 23 May (when average total river flow doubled, average powerhouse discharge 

increased threefold, and average tailwater elevation increased 1.2 m) was significantly higher 

than for earlier releases (0.987 vs. 0.834). 

In 2000, NOAA Fisheries Service estimated relative survival for river-run hatchery 

yearling and subyearling Chinook salmon passing through the spillway at Ice Harbor Dam (using 

PIT tags) was 0.978 and 0.885, respectively (Eppard et al. 2002a). The estimate for yearling 

Chinook salmon for this particular year was similar to previous spillway survival estimates at 

Little Goose (1.021) (Iwamoto et al. 1994) and Lower Monumental Dams (0.927 to 0.984) (Muir 

et al. 1995b). However, in 2002 at Ice Harbor Dam, analysis of PIT-tag data indicated spillway 

passage survival was 0.895 and 0.890 for hatchery yearling Chinook salmon under daytime and 

nighttime conditions, respectively; the overall survival estimate was 0.892. During the summer, 

spillway passage survival for hatchery subyearling Chinook was 0.876 and 0.915 for fish passing 

under daytime and nighttime conditions, respectively; the overall survival estimate was 0.894. In 

2003, Eppard et al. (2003) evaluated relative spillway passage survival under 50% voluntary spill 

operation versus BiOp voluntary spill operation (45 kcfs spill during the day and up to 100% 

spill at night). Spillway survival for hatchery yearling Chinook salmon was 0.928 and 0.948 for 

the 50% test spill and BiOp spill operations, respectively. Also in the summer of 2003, Absolon 

et al. (2003a) evaluated the survival of PIT-tagged subyearling Chinook salmon passing through 

the spillway at Ice Harbor Dam under conditions of BiOp “bulk spill” (BiOp spill using only two 

or three spillbays rather than all spillbays); mean survival from spillbays 2, 3, and 4 to McNary 

Dam was 0.96. 
Table 1. Location, species and run type, study year, fish-marking method, spillbay, test conditions, and survival estimate

at dams on the lower Snake and Columbia rivers. Dam abbreviations: LGR-Lower Granite; LGO-Little Goose; Monumental; IHR-Ice Harbor; MCN-McNary; JDD-John Day; TDA-The Dalles; BON-Bonneville. 

Dam Species/run type Year Method

Flow deflector Location Conditions Survival Refe

LGR Steelhead 1996 PIT tags no Bay 1 3.9 kcfs 1.01 SmitLGR Yearling Chinook salmon 2003 Radiotelemetry yes Bays 2–8 BiOp night 0.931 PlumLGO Steelhead 1997 PIT tags no Bay 1 4.9–10.0 kcfs 1.004 MuiLGO Steelhead 1997 PIT tags yes Bay 3 4.9–10.0 kcfs 0.972 MuiLGO Yearling Chinook salmon 1993 PIT tags yes Bay 3 3.8 kcfs 1.021 IwamLMO Coho salmon 1973 Freeze brands yes* Bay 2 4.5 kcfs 0.970 LonLMO Coho salmon 1973 Freeze brands yes Bay 4 4.5 kcfs 1.100 LonLMO Steelhead 1974 Freeze brands yes Bay 7 4.5 kcfs 0.978 LonLMO Steelhead 1974 Freeze brands no Bay 8 4.5 kcfs 0.755 LonLMO Subyearling Chinook salmon 1972 Freeze brands yes* Bay 2 13.1 kcfs 0.831 LonLMO Subyearling Chinook salmon 1972 Freeze brands yes* Bay 2 2.8 kcfs 0.840 LonLMO Yearling Chinook salmon 1994 PIT tags yes Bay 7 4.4–4.8 kcfs 0.927 MuiLMO Yearling Chinook salmon 1994 PIT tags no Bay 8 4.4–4.8 kcfs 0.984 MuiLMO Yearling Chinook salmon 2003 Radio, PIT tags yes Bays 4, 7 2.0–11.5 kcfs 0.900 HocIHR Yearling Chinook salmon 2000 PIT tags yes Bays 3, 5, 7 BiOp night 0.978 EppIHR Subyearling Chinook salmon 2000 PIT tags yes Bays 3, 5, 7 BiOp night 0.885 EppIHR Yearling Chinook salmon 2002 PIT tags yes All bays BiOp 0.892 EppIHR Subyearling Chinook salmon 2002 PIT tags yes All bays BiOp 0.894 EppIHR Yearling Chinook salmon 2003 Radiotelemetry yes All bays BiOp 0.948 EppIHR Yearling Chinook salmon 2003 Radiotelemetry yes All bays 50% spill 0.928 EppMCN Subyearling Chinook salmon 1955 Tattoo no Not specified Not specified 0.980 SchoMCN Subyearling Chinook salmon 1956 Tattoo no Not specified Not specified 1.00 SchoMCN Yearling Chinook salmon 2002 Radiotelemetry yes All bays BiOp 0.976 AxeMCN Yearling Chinook salmon 2003 Radiotelemetry yes All bays BiOp 0.928 AxeMCN Yearling Chinook salmon 2003 Balloon tag yes Bay 5 Various >0.98 HeisJDD Subyearling Chinook salmon 1979 Freeze brands no Bay 16 4.3 kcfs 0.98–1.2 RaymJDD Yearling Chinook salmon 2000 Radiotelemetry no All bays 0/60% spill 0.986 Cou

JDD Yearling Chinook salmon 2000 Radiotelemetry no All bays 30/60% spill 0.937 CouJDD Steelhead 2000 Radiotelemetry no All bays 0/60% spill 0.988 CouJDD Steelhead 2000 Radiotelemetry no All bays 30/60% spill 0.905 CouJDD Yearling Chinook salmon 2002 Radiotelemetry no All bays 0/54, 30/30 .993, 1.000 Cou

10
11

Table 1. Continued. Location, species and run type, study year, fish-marking method, spillbay, test conditions, and survi

spillway passage at dams on the lower Snake and Columbia rivers. Dam abbreviations: LGR-Lower Granite; L

LMO-Lower Monumental; IHR-Ice Harbor; MCN-McNary; JDD-John Day; TDA-The Dalles; BON-Bonnevill

Dam Species/run type Year Method 

Flow deflector Location Conditions Survival Refere

JDD Subyearling Chinook salmon 2002 Radiotelemetry no All bays 0/54, 30/30 .985, 1.003 CouniJDD Steelhead 2002 Radiotelemetry no All bays 0/54, 30/30 .958, .938 CouniJDD Yearling Chinook salmon 2003 Radiotelemetry no All bays 0/45, 0/60 .939, .934 CouniJDD Subyearling Chinook salmon 2003 Radiotelemetry no All bays 0/60, 30/30 .901, .955 CouniTDA Subyearling Chinook salmon 1997 PIT tags no All bays 64% spill 0.92 DawleTDA Coho salmon 1997 PIT tags no All bays 64% spill 0.87 DawleTDA Subyearling Chinook salmon 1998 PIT tags no All bays 64% spill 0.75 DawleTDA Subyearling Chinook salmon 1998 PIT tags no All bays 30% spill 0.89 DawleTDA Coho salmon 1998 PIT tags no All bays 64% spill 0.89 DawleTDA Coho salmon 1998 PIT tags no All bays 30% spill 0.97 DawleTDA Subyearling Chinook salmon 1999 PIT tags no All bays 64% spill 0.96 DawleTDA Subyearling Chinook salmon 1999 PIT tags no All bays 30% spill 1.00 DawleTDA Coho salmon 1999 PIT tags no All bays 64% spill 0.93 DawleTDA Coho salmon 1999 PIT tags no All bays 30% spill 0.96 DawleTDA Yearling Chinook salmon 2000 PIT tags no All bays BiOp 0.91 AbsolTDA Yearling Chinook salmon 2000 Radiotelemetry no All bays BiOp 0.92 CouniTDA Subyearling Chinook salmon 2000 PIT tags no All bays BiOp 0.897 AbsolTDA Subyearling Chinook salmon 2000 Radiotelemetry no All bays BiOp 0.826 CouniTDA Yearling Chinook salmon 2002 Balloon tags no Bays 4, 9, 

13 

40% spill 98.4, 98.9, 

95.6 

Norma

TDA Subyearling Chinook salmon 2002 Balloon tags no Bays 4, 9, 

11 

40% spill 93.6, 93.3, 

92.6 

Norma

TDA Chinook salmon 2002 Balloon tags no Bay 2 4.5,12 kcfs 0.949, 1.00 NormaTDA Chinook salmon 2002 Balloon tags no Bay 4 4.5,12 kcfs 0.965, 0.995 NormaTDA Chinook salmon 2003 Balloon tags no Bay 2 9–21 kcfs 0.931, 1.00 NormaTDA Chinook salmon 2003 Balloon tags no Bay 4 9–21 kcfs 0.966, 0.999 NormaBON Subyearling Chinook salmon 1974 Freeze brands no Bay 11 13 kcfs 0.958 JohnseBON Subyearling Chinook salmon 1974 Freeze brands yes Bay 14 13 kcfs 0.868 JohnseBON Subyearling Chinook salmon 1989 CWT/freeze brand yes Bay 5 6.8 kcfs 0.9604 LedgeBON Yearling Chinook salmon 2002 Radiotelemetry yes All bays 75 kcfs/cap 0.969 CouniBON Yearling Chinook salmon 2002 Radiotelemetry yes All bays 24-hour cap 0.98 Couni

*Flow deflector included dentates. 
Heisey et al. (2003) utilized the HI-Z balloon-tag-recapture technique to evaluate 

spillway survival in spillbay 5 at Ice Harbor Dam under various spill conditions and release 

depths in the spring and summer of 2003. Spring survival estimates for tagged yearling Chinook 

were greater than 0.98 for all conditions tested. One-hour summer survival estimates for tagged 

subyearling Chinook were 0.889 for bulk spill, shallow release; 0.923 for bulk spill, deep release; 

and 0.978 for dispersed spill, deep release. 

A number of methodologies have been used to estimate spillway survival at lower 

Columbia River dams, including identification of test fish by fin clips, freeze brands, coded-wire 

tags and freeze brands, balloon tags, PIT tags, and radio tags. Most recently at McNary Dam,

Axel et al. (in prep. a) using PIT- and radio-tag detections estimated 2002 spillway passage 

survival for hatchery yearling Chinook salmon was 0.976. In 2003 Axel et al. (in prep. b) 

estimated that spillway passage survival for radio-tagged hatchery yearling Chinook salmon at 

McNary Dam was 0.928. 

Estimates of spillway passage survival at John Day Dam include a study conducted in 

1979 by Raymond and Sims (1980), who found that spillway mortality relative to the tailrace 

was not different from 0. More recently, Counihan et al. (2001) evaluated spill survival of radiotagged yearling Chinook and steelhead under two daytime/nighttime spill conditions at John Day 

Dam in 2000. For the two spill conditions (0/60% and 30/60%), survival was 0.986 and 0.937 

for yearling Chinook, and 0.988 and 0.905 for steelhead, respectively. 

At The Dalles Dam, Dawley et al. (1998b) released PIT-tagged subyearling Chinook and 

coho salmon (O. kisutch) in 1997, and estimated spillway survival of 0.87 and 0.92, respectively, 

with 64% spill. Results from a 1998 study (Dawley et al. 1999) show that relative survival rates 

during 64% spill were 0.88 for coho salmon and 0.76 for subyearling Chinook salmon, while 

during 30% spill, survival was 0.96 and 0.92 for coho and subyearling Chinook salmon, 

respectively.

Analyses of data from 1999 show that relative survival rates during 64% spill were 0.94 

for coho salmon (O. kisutch) and 0.95 for subyearling Chinook salmon, while during 30% spill, 

survival was 0.96 and 1.00 for coho and subyearling Chinook salmon, respectively (Dawley et al. 

2000b). In 2000, Absolon et al. (2002) estimated relative survivals at a 40% spill volume of 0.95 

for yearling Chinook and coho salmon and 0.92 for subyearling Chinook salmon. 

After 2000 spill research at The Dalles Dam shifted to causative mechanisms for the 

observed high mortality. Direct survival studies and studies of scale hydraulic models indicated 

that fish retention time in the spill basin might be the cause of the increased injury and mortality. 

The hydraulic model investigations had shown that flow from bay 4 moved directly out of the 

stilling basin, while flow from more southern bays like bay 9 tended to recirculate for a period of 

time before moving out of the stilling basin. 

To evaluate the hypothesis that fish released in bay 9 would show higher mortality, 

Normandeau Associates Inc. (2003) evaluated direct survival by releasing HI-Z balloon-tagged 

yearling Chinook salmon directly into bays 4, 9, and 13 during the spring of 2002 (14–27 May), 

and subyearling Chinook salmon into bays 4, 9, and 11 during the summer (15–22 August). 

Control fish were released below bay 3. Spring survival for bays 4, 9, and 13 was 0.984, 0.989, 

12
and 0.959, respectively. Bay 13 survival was significantly lower than the other two. For the 

summer in bays 4, 9, and 11, survival was 0.936, 0.933, and 0.926, respectively. Eye injuries 

were most prevalent, followed by scrapes and operculum damage. Injuries were thought to be 

mainly from shear and physical contact with solid objects. 

In the fall of 2002 and the spring of 2003, Normandeau Associates et al. (2004) again 

used HI-Z balloon-tagged Chinook salmon to estimate direct passage survival through spillbays 

2 and 4 under low (fall) and high (spring) tailwater conditions at The Dalles Dam. In the low 

tailwater test, 48-hour survival rates for spillbay 2 ranged from 0.949 (deep release at 4.5 kcfs) to 

1.000 (releases at 12 kcfs). Survival estimates for spillbay 4 ranged from 0.965 to 0.995, at 4.5 

and 12 kcfs, respectively. Spring survival estimates for spillbay 2 ranged from 0.931 (deep/edge 

release at 9 and 21 kcfs) to 1.000 (deep/mid release at 18 kcfs). Spillbay 2 deep/mid-release 

survival estimates were high (0.975–1.000) at all four volumes. Survival rates for spillbay 2 

deep/off-center releases were greater than 0.99 for the 12 and 18 kcfs releases, but 0.931 for both 

the 9 and 21 kcfs spill volumes. Spillbay 4 survival estimates for four test conditions (all midreleases) in the spring ranged from 0.966 to 0.999. The researchers concluded that safe fish 

passage was maximized at spillbay volumes of 12 and 18 kcfs with a total spill of 108–112 kcfs. 

Fish released at greater depth and off-center in spillbay 2 appeared to have higher rates of injury; 

eye damage or head and body bruises were generally the most prevalent visible injury types in 

both the fall and the spring. 

At Bonneville Dam, Holmes (1952) estimated that subyearling Chinook salmon survival 

through the spillway was from 0.96 to 0.97, depending on how the data were analyzed. Johnsen 

and Dawley (1974) compared the survival of subyearling Chinook salmon passing through 

spillbays with and without flow deflectors, and found that relative survival was 0.868 and 0.958, 

respectively. Ledgerwood et al. (1990) found that survival of subyearling Chinook through 

spillbay 5 was not significantly different than for fish released downstream. Based on the 

balloon-tag methodology, the calculated survival probabilities for deflector and nondeflector 

spillways were both 1.0 at Bonneville Dam; however, fish passing through a spillbay without a 

spill deflector displayed a slightly higher injury rate (Normandeau Associates et al. 1996a). 

Counihan et al. (2003) evaluated spillway passage at Bonneville Dam in 2002 with radiotagged yearling Chinook salmon. The project was operated with powerhouse 2 priority, and spill 

was alternated between 75 kcfs day/gas cap at night and a test condition of gas cap for 24 hours; 

spillway survival was 0.969 and 0.980, respectively. 

Causal Mechanisms Affecting Spill Survival 

As pointed out in Table 1 and discussed in the Spill Survival subsection above, many 

point estimates of survival through spillways made in recent years under contemporary 

conditions using PIT- or radio-tag methodologies are less than 0.98, a value typically used in 

recent model studies of survival through the hydropower system (Marmorek and Peters 1998, 

NMFS 2000). In general, these lower survival estimates apply to Lower Monumental, Ice 

Harbor, Lower Granite, and The Dalles Dams, and suggest that conditions juvenile salmonids 

experience during spillway passage may differ among dams and within a dam spillway under 

various conditions. Here we summarize information developed to date on spillway passage 

survival at these dams. 

13
Ruggles and Murray (1983) reviewed fish responses to passage through spillways at 

Canadian dams and concluded that spillways were not a serious source of fish mortality. They 

pointed out that fish may be injured by rapid pressure change, rapid deceleration, shear forces, 

turbulence, abrasion, and the force of striking water in free fall. They noted that spillway 

deflectors have been used successfully to reduce air entrainment associated with deep plunging 

jets, and that injuries begin to occur when the impact velocity of a fish striking the water surface 

exceeds 16.2 ms

-1. Based on a need to design spillway flow deflectors for gas abatement, the 

COE initiated a literature review of physical injury through spillways through their Dissolved 

Gas Abatement Study. The study considered three injury types: immediate mechanical injury, 

short-term delayed injury, and longer-term delayed injury. It concluded that results from studies 

of impacts from gas abatement structures (such as flow deflectors) have been inconsistent, and 

effects from gas abatement structures could not be separated from effects associated with 

passage through the total spillway environment. Immediate direct mortality was the injury type

most studied and best understood, and long-term delayed mortality was the least studied and 

least understood. The report noted that gas abatement structures can produce hazardous 

conditions, such as shear zones (R2 Resources 1997). 

Several years of research conducted at The Dalles Dam was summarized by Ploskey et al. 

(2001). A spill discharge of 40% of total project discharge was adopted as the best operation and 

implemented in the spring of 2000. However, testing that year showed that survival was still low

in six of eight conditions tested, which included two spillway release locations (north and south), 

and daytime and nighttime releases during both the spring and summer periods. Average 

survival ranged from 0.842 for summer, daytime, north spillway to 1.026 for spring, daytime, 

north spillway (Absolon et al. 2002). Subsequently a committee of engineers and biologists used 

a 1:80 scale general hydraulic model at the COE Engineering Research and Development Center 

in Vicksburg, Mississippi, to evaluate physical conditions associated with these elevated 

spillway mortality rates. They concluded that large volumes of spill through the northern half of 

the spillway, designed to enhance tailrace egress, resulted in flow from southern spillbays 

moving northward within the stilling basin. This increased retention time in the basin, along 

with it being half the depth of the next shallowest basin of the FCRPS dams, suggested that 

mechanical injury from strike and shear in the turbulent stilling basin was causing the reduced 

survival. 

Results from more recent radiotelemetry (Beeman et al. 2003) and balloon-tag studies 

(Normandeau Associates Inc. 2003) corroborate these observations. Therefore, an extension of 

the pier wall between bays 6 and 7 downstream to the stilling basin end-sill was evaluated in the 

COE hydraulic model. In the model, the wall reduced retention time in the stilling basin when 

six northern-most spillbays were operated under uniform discharges of from 8 to 20 kcfs. The 

pier wall extension was installed at The Dalles Dam prior to 2004. Preliminary results based on 

balloon tagging indicate direct survival was greater than 0.98, and more than 96.8% of the test 

fish showed no signs of injury.1

 

1

 M. Langeslay, COE, Portland, OR. Pers. commun., May 2004. 

14
At Ice Harbor Dam, spillway survival of radio-tagged yearling Chinook salmon averaged 

0.978 in 2000, and 0.892 in 2002 (Eppard et al. 2002a, 2003). The 2000 Biological Opinion 

called for nighttime spill of up to 100 kcfs (the gas-cap limit) and daytime spill of 45 kcfs during 

the spring at Ice Harbor Dam, with uniform spill discharge across bays. To further investigate 

survival associated with higher (plunging flow) and lower (skimming flow) spill volumes, BiOp 

and 50% spills were tested during the spring of 2003 using a 2-day-block study design. Relative 

spillway passage survivals were similar, with 0.948 (95% CI, 0.915–0.981) for the BiOp spill 

and 0.928 (95% CI, 0.860–1.000) for the 50% spill. Also that year, estimated survival of 

balloon-tagged yearling Chinook salmon released at two locations (elevations) in spillbay 5 was 

greater than 0.98 during all conditions tested (Heisey et al. 2003). Although fish injuries ranged 

as high as 22% for releases made at the deepest elevation and were 8% or less for the shallower 

release location (Heisey et al. 2003), these release locations appeared to represent a relatively 

low percentage of the overall population passing the spillbay (Moursund et al. 2003b). Efforts to 

resolve this issue are continuing. In 2004 preliminary results from balloon-tag studies with 

yearling migrants released at locations more representative of the overall population indicate 

survival was still high (>0.98), but injury rates were reduced to 2%.2

Survival of subyearling Chinook salmon at Ice Harbor Dam during the summer averaged 

0.885 in 2000 and 0.894 in 2002 (Eppard et al. 2002a, 2003). In 2003 a bulk-spill pattern was 

developed where more than 10 kcfs was spilled through fewer bays, as compared to the pattern 

of uniform spill through all bays. Estimated survival of PIT-tagged subyearling Chinook salmon 

was 0.96 (95% CI, 0.90–1.10) (Absolon et al. 2003a). When compared to 2000 and 2002, these 

results suggest the bulk-spill pattern may reduce fish exposures to physical conditions that cause 

injuries. In contrast, 1-hour estimated survival under the bulk spill pattern based on balloontagged, river-run subyearling Chinook salmon was 0.89 and 0.92 for shallow and deep spillbay 

release locations, compared to an estimated 1-hour survival of 0.98 for fish released through a 

deep release under the uniform spill pattern (Heisey et al. 2003). However, the authors indicated 

there were problems related to high temperatures during this test, suggesting a need for caution 

when interpreting these data. 

At Lower Monumental Dam, Muir et al. (2001) found that survival of PIT-tagged 

yearling migrants was lower though spillbays with deflectors, compared to standard bays. The 

study did not address differences between spillbay locations, discharges, or tailwater elevations. 

Prior to 2003, additional deflectors were installed in the end bays of the spillway, and with these 

in place, from 25 to 40 kcfs could be spilled while maintaining gas criteria at downstream

locations under 24-hour spill. In 2003 Hockersmith et al. (in prep.) evaluated spillway survival 

based on radio-tagged yearling Chinook salmon where spill was distributed uniformly between 

spillbays. Relative survival through the spillway was 0.900 (95% CI, 0.843–0.961). Relative 

survival through spillbays 4 and 7 was similar, at 0.896 (95% CI, 0.779–1.031) and 0.895 (95% 

CI, 0.724–1.106), respectively. 

However, spill discharge early in the season was in the gas cap range noted above when 

total river discharge was less than 75 kcfs, but was outside this range later in the season when 

 

2

 M. Shutters, COE, Walla Walla, WA. Pers. commun., May 2004. 

15
total river discharge increased. Additional analysis of the data suggested there was a tailwater 

elevation effect, where survival was low when tailwater elevation was low (0.834 [95% CI, 

0.78–0.90], 440 ft msl, 76 kcfs river flow), and high when tailwater elevation was high (0.987 

[95% CI, 0.92–1.06], 444 ft msl, 150 kcfs river flow).3

In summary, fish survival through spillways at these dams is influenced by stilling basin 

depth and turbulence, hydraulic patterns in the basin, spillbay location within the spillway 

relative to the spill pattern being used, deflector elevation relative to tailwater elevation and thus 

total river flow, gate opening, and fish location when passing through the spillbay and under the 

control gate. Efforts to further explore the relationships between these factors are ongoing. 

Hydraulic model studies indicate that deflector submergence (tailwater elevation) directly 

influences the hydraulic behavior of the spill jet as it passes over the deflector. Under conditions 

of high discharge and low submergence of the deflector, the jet passes over the deflector and 

plunges abruptly into the stilling basin. This operation was tested during the summer of 2003 at 

Ice Harbor Dam and resulted in an estimated average survival of 0.96. Under conditions of 

moderate discharge and higher deflector submergence, the spillbay discharge jet is deflected 

upward, creating an undular, ramped, or submerged hydraulic jump. This operation was tested 

during the spring of 2003 and resulted in an estimated average survival of 0.99 at Lower 

Monumental Dam.

Under both conditions, the spillway discharge jet appears to dissipate relatively quickly 

and produces low impacts on juvenile salmonids passing through the discharge, even though jet 

velocities are high (>18.3 ms

-1). Conversely, under conditions of low discharge per spillbay, 

certain tailwater elevations, and uniform spill between bays, the discharge jet performs as 

designed and skims off the deflector and stays near the tailrace water surface for an extended 

distance downstream. This operation was tested during the spring of 2002 and resulted in an 

estimated average survival of 0.89 at Ice Harbor Dam, and 0.83 during the spring of 2003 at 

Lower Monumental Dam. 

Based on this information, a possible explanation of the low survival associated with 

skimming flow off the deflector is that a shear zone is created on the underside of the jet and fish 

in the lower portion of the nappe are exposed to this zone, which results in injuries. The zone of 

injury occurs until a point downstream where jet velocity drops to below 15.2 ms

-1, a threshold 

considered safe based on field and laboratory studies (Ruggles and Murray 1983, Pacific 

Northwest National Laboratory et al. 2001). If this explanation is the case, lowering the 

elevation of the deflector to produce hydraulic jump conditions a greater percentage of the time, 

or changing to higher discharges through fewer spillbays (“bulk” spill), may reduce injury and 

mortality rates. 

Clearly, additional field and hydraulic model studies will be needed to verify this 

explanation, understand the implications for gas abatement, and design solutions to these 

conditions. The observations of spillway survival at The Dalles, Ice Harbor, and Lower 

Monumental Dams suggest that survival of juvenile salmonids through spillways is influenced 

 

3

 E. Hockersmith, NWFSC, Pasco, WA. Pers. commun., January 2004. 

16
by many physical factors. Lessons learned from these studies should be applied to other 

spillways that have deflectors and training spill associated with the removable spillway weir 

(RSW) at Lower Granite Dam and others weirs that may be installed in the future. 

Forebay Passage and Predation 

Beamesderfer and Rieman (1991) found that forebay populations of northern 

pikeminnow (Ptychocheilus oregonensis) and smallmouth bass (Micropterus dolomieui) were 

present in substantial numbers in the forebay of John Day Dam. Poe et al. (1991) reported that 

the diet of northern pikeminnow in the forebay of John Day Dam was 66% salmonid smolts. In 

addition, detection histories for radio-tags (from hatchery yearling Chinook salmon) recovered 

from the Crescent Island Caspian Tern colony near the mouth of the Snake River indicated that, 

at a minimum, terns from this colony forage from Ice Harbor Dam forebay to McNary Dam

forebay (Axel et al. 2003). This suggests that delay of outmigrants in forebays could reduce 

survival due to increased predation, and project operations such as daytime spill that decrease 

forebay residence time could increase survival. 

Axel et al. (2003) evaluated passage behavior at Ice Harbor Dam in 2001 under low-flow 

(69.5 kcfs), no-spill conditions and reported a median forebay residence time for radio-tagged 

hatchery yearling Chinook salmon of 7.3 hours. Schools of yearling Chinook salmon were 

observed holding within the immediate forebay of Ice Harbor Dam on a daily basis throughout 

the study period (8–28 May). A previous radiotelemetry study conducted at Ice Harbor Dam in 

1999 by Eppard et al. (2000) reported that median forebay residence time for radio-tagged 

hatchery yearling Chinook salmon was 1.3 hours, with average river flows of 109.7 kcfs and spill 

averaging 45% (54 kcfs) during the day and 81% (94.2 kcfs) at night. 

Radiotelemetry data reported by Axel et al. (2003) regarding passage of radio-tagged 

hatchery yearling Chinook salmon at McNary Dam in 2001 also indicated prolonged residence 

time within the forebay under low-flow, no-spill conditions (7.6 hours). Detection histories 

indicated that once fish entered the forebay, they constantly moved back and forth across the 

spillway and powerhouse before passing. 

At Lower Granite Dam in 2003, median forebay passage times during removable 

spillway weir tests were 1.92, 1.72, and 2.28 hours for radio-tagged hatchery and wild steelhead, 

and hatchery Chinook salmon, respectively. For these same test groups, median passage times 

during spill-to-the-gas-cap tests were 7.37, 4.64, and 4.98 hours, respectively (Plumb et al. 

2003). 

Tailrace Passage and Predation 

The concept of developing spill patterns at FCRPS dams specifically for fish passage was 

first addressed systematically in the 1960s to facilitate adult salmon passage into the adult fish 

collection systems. Junge (1967) observed improved adult salmonid passage under intermediateto-large spill volumes if four or five gates at each end of the spillway were at low-volume

settings. At large dams, this resulted in a tapered spill pattern near each end and a flat spill 

pattern across the central portion of the spillway. At smaller dams, this produced a “crowned” 

pattern across the entire spillway tailrace, with the highest discharge in the middle bays. 

17
Junge evaluated adult salmon passage success by comparing ladder passage counts 

associated with various spill patterns. The spill patterns he developed that appeared best for 

adult passage conflict with what is thought today to be best for juvenile passage (high shoreline 

velocities), because Junge kept near-shore velocities low to facilitate adult migration and passage 

into fishway entrances located along shorelines. 

Smolt residence time in spillway tailraces is likely influenced by spill volume and 

pattern. High spill volume and water velocity push water, and presumably juvenile salmonids, 

out of the immediate tailrace, and help redistribute piscivorous predators (northern pikeminnow) 

away from the immediate spillway tailrace, reducing potential predation opportunities (Faler 

et al. 1988). Shively et al. (1996) found that ambient river flow velocities of at least 1 ms

-1 were 

necessary to keep northern pikeminnow from holding in areas near bypass outfalls, and that the 

degree by which water velocity eliminated northern pikeminnow holding increased as outfall 

distance from shore and water depth increased. 

Hansel et al. (1993) found that hydraulic cover such as eddies and backwaters at 

velocities below this threshold were preferred northern pikeminnow feeding habitats, particularly 

when near primary smolt outmigration paths. Spill patterns that facilitate rapid juvenile egress 

from the spillway stilling basin through the tailrace likely increase juvenile survival. Current 

spill patterns are developed to increase the survival of juvenile fish through tailraces, by 

emphasizing minimizing hydraulic cover and maintaining high water velocities near spillway 

shorelines. To not interfere with daytime adult passage, these juvenile spill patterns are often 

employed during nighttime hours only (COE 2004). 

As information is gained on the use and benefits associated with daytime spill to pass 

juveniles, greater consideration will likely be given to the use of juvenile spill patterns during the 

daytime. Spill patterns that attempt to satisfy both adult and juvenile passage criteria during the 

daytime have been developed for Lower Granite, Ice Harbor, McNary, John Day, The Dalles, 

and Bonneville Dams using scale hydraulic physical models at the COE Engineering Research 

and Development Center (ERDC) in Vicksburg, Mississippi. Patterns for each dam for daytime 

and nighttime periods are developed in coordination with salmon managers and incorporated into 

the Annual Fish Passage Plan (COE 2004). 

Eppard et al. (2000) evaluated tailrace residence times of radio-tagged yearling spring 

Chinook at Ice Harbor Dam during the spring of 1999. Spill levels during the study were as high 

as 45% of total river flow during the day and up to 100% at night. Median tailrace residence 

times were 2.7 , 5.7, and 7.3 minutes for fish that passed through the spillway, juvenile bypass 

system, and turbines, respectively, during daylight hours. At night during 100% spill, median 

tailrace residence time was 3 minutes for spillway-passed fish. During the spring of 2001 at Ice 

Harbor Dam (low flow, no spill), median tailrace residence times were 9.0 and 14.7 minutes for 

radio-tagged yearling spring Chinook that passed the through the bypass system and a turbine 

unit, respectively. 

At McNary Dam in the spring of 2002, radiotelemetry analysis of yearling Chinook 

salmon passage identified a statistically significant difference between median tailrace egress 

times for spilled fish (5.5 minutes) and bypassed fish (10.2 minutes) (Axel et al. in prep. a). 

While there was also a significant difference in tailrace egress times between fish bypassed 

18
during the day (9.4 minutes) and night (11.4 minutes), there was no day/night difference 

observed for fish passing through spill. Within the spillway, slightly longer tailrace egress times 

were observed for fish passing through the end bays. 

At McNary Dam in the spring of 2003, Axel et al. (in prep. b) also observed a significant 

difference between median tailrace egress times for radio-tagged yearling Chinook salmon 

passing through the spillway (5.8 minutes) and the bypass system (7.7 minutes). No diel 

differences in egress times were observed for spilled fish, but as in 2002, longer egress times 

were associated with fish passing through the end spillbays. 

At John Day Dam, Liedtke et al. (1999) observed good juvenile egress through the 

spillway tailrace with a new spill pattern; marked yearling and subyearling fish passed through 

the first 0.7 km of tailrace in 5–10 minutes. They also observed slower passage times and higher 

predation rates on fish that passed through end spillbays. 

In 2000 Duran et al. (2002) monitored tailrace egress at John Day Dam and concluded 

that spillway passage generally resulted in more rapid tailrace egress than bypass passage. 

Residence times for spilled fish were similar for radio-tagged fish released at daytime 30% spill 

and nighttime 60% spill. Residence times were longer for bypassed fish during the 60% 

nighttime than the 30% daytime spill. Spill-passed yearling Chinook exited the tailrace within 

about 10 minutes for all spill conditions and bypassed yearling Chinook took 6 minutes longer to 

exit tailrace (15 vs. 9 minutes) under the nighttime 60% spill treatment. Spill-passed steelhead 

tailrace exit times and relationships were similar, although the test was compromised by lower 

than requested spill percentages. South-spillway-passed fish took longer than north-spillway 

(12–14 vs. 7–8 minutes). For subyearling Chinook salmon, residence times for spill-passed fish 

were generally lower during 60% nighttime spill than for 30% daytime spill. North-spillway fish

had the lowest residence times. Subyearlings released at the bypass had significantly longer 

tailrace egress times during 60% nighttime spill than during 30% daytime spill (74 vs. 17

minutes).

At The Dalles Dam in 2000, Allen et al. (2001) reported that radio-tagged yearling and 

subyearling Chinook salmon that passed through the powerhouse in the spring and summer had 

double the tailrace residence times of north-spillway-passed or control fish. Sluiceway and 

south-spillway fish tailrace-residence times were similar to powerhouse times. 

Daily Spill Timing 

The early spill programs based daily spill timing on hourly monitoring of smolt 

abundance at projects where this information was available. If hourly information was not 

available, daily spill timing was determined prior to each season and was based on the best 

available diel passage information. This diel information was obtained from smolt monitoring or 

research activities conducted primarily at powerhouses, rather than spillways, and it was 

common to apply diel information collected at one dam to another. However, more recent 

studies have shown that diel passage timing through spillways is different than through

powerhouses. For example, while smolt monitoring information suggests 60–90% of the daily 

powerhouse passage at John Day and Lower Monumental Dams occurs from 1800 to 0600 hours 

(Ransom and McFadden 1987, Martinson et al. 1997), hydroacoustic studies indicate that both 

19
day and night passage of juvenile migrants through the spillway is high at both dams during 

involuntary daytime spill (BioSonics Inc. 1999a, 1999b). 

Observations at The Dalles and Bonneville Dams also indicate that spillway passage 

occurs at fairly constant rates, day and night, if daytime spill is provided (BioSonics Inc. 1997, 

Hensleigh et al. 1998). This suggests that juvenile migrations may be delayed if spill is managed 

based on powerhouse diel passage patterns. Studies conducted at John Day Dam in years with 

little daytime spill found that fish often milled in front of the dam when they arrived during the 

day and passed the dam at night, most often through the powerhouse (Giorgi et al. 1985, Sheer 

et al. 1997). Sheer et al. (1997) found that fish passing The Dalles Dam during 24-hour spill 

delayed very little, and most (85%) passed through the spillway. Hensleigh et al. (1999) and 

Liedtke et al. (1999) found a similar response at John Day Dam, where radio-tagged yearling and 

subyearling Chinook salmon and steelhead had relatively short forebay residence times and high 

(50–75%) spillway passage rates during years where high flows caused high levels of daytime 

spill. 

In radiotelemetry studies conducted in the spring and summer of 1999 at John Day Dam

during two day/night spill treatments (0/60% and 30/60%), Hansel et al. (1999a) reported 

forebay residence time was significantly shorter for yearling Chinook salmon passing during the 

daytime spill treatment (0.8 vs. 8.5 hours), and a similar result was observed for smaller 

(<200 mm) steelhead. With subyearling Chinook, there appeared to a greater delay for fish 

arriving during the day with spill than at night with spill (2.5 vs. 0.6 hours) during similar spill 

levels, suggesting a day vs. night effect that is independent of percent spill. Also, during the 12-

hour spill treatment, 55% of the fish arriving during the day delayed to pass at night. 

For similar daytime/nighttime spill treatments (0/60% and 30/60%) at John Day Dam in 

2000, Beeman et al. (2001) reported that median forebay residence time was longer for radiotagged yearling Chinook and steelhead when no daytime spill was provided, but this was only 

statistically significant for Chinook (4.6–12.3 hours without daytime spill, 1.4–3.2 hours with 

daytime spill). The researchers concluded that the use of 24-hour spill decreases forebay 

residence times of fish arriving during the day, increases spill efficiency of yearling Chinook, but 

does not significantly alter fish passage efficiency from values obtained during 12-hour spill. 

For radio-tagged subyearling Chinook, Beeman et al. (2002a) reported fish arriving at John Day 

Dam during the 24-hour treatment had shorter forebay residence times with less variability 

between day and night arrival periods than those arriving during the 12-hour treatment. Median 

residence time during the 24-hour spill for day arrival was 1 hour (range 0.7–1.6 hours) and for 

night arrival was 1.6 hours (1.2–2.6 hours). Median times for the 12-hour spill for day arrival 

was 4.8 hours (2.4–6.6 hours) and ranged from 0.8 to 4.2 hours for night-arriving fish. Project 

fish passage efficiency was significantly higher for the 24-hour spill treatment (91%) than for the 

12-hour treatment (79%). 

For two daytime/nighttime spill treatments (0/54% and 30/30%) at John Day Dam during 

the spring of 2002, Beeman et al. (2002b) reported that although median forebay residence times 

for radio-tagged salmonids tended to decrease with increased percent spill levels, median 

residence times for the 12- and 24-hour treatments overall were similar for yearling spring 

Chinook (1.7 vs. 1.6 hours) and wild juvenile steelhead (6.8 vs. 7.0 hours). 

20
Seasonal Spill Timing 

Historically, seasonal spill timing has been based on juvenile fish abundance. Early spill 

programs relied primarily on preseason planning dates and in-season estimates of cumulative fish 

passage as a trigger for the beginning and end of the spill season at each project. These 

programs were managed to provide spill for the middle 80% of the juvenile outmigration. These 

percentages were applied separately to the spring and summer migration periods and were based 

on actual fish sampling at each project via the smolt monitoring program (FPC 1995). 

The NOAA Fisheries Service 2000 BiOp proposed that the actual dates of spill be 

determined annually by the regional Technical Management Team and based on the best 

available monitoring and evaluation data concerning project passage, spill, and system survival 

research. The 2000 BiOp established Snake River spill planning dates as 3 April to 20 June and 

21 June to 31 August, and lower Columbia River dates as 10 April to 30 June and 1 July to 31 

August. 

Summer Spill 

The efficacy of continuing the NOAA Fisheries Service 2000 BiOp summer spill 

program during late July and August has recently been questioned for a variety of reasons and 

discussed in various regional forums. Because the majority of ESA-listed Snake River fall 

Chinook salmon are currently transported around FCRPS dams, some in the region view the 

benefits of late summer spill as being small and easily replaced by alternative biological offsets. 

The spill reduction would increase revenue from power generation and has led to a great deal of 

discussion regarding the cost effectiveness of the spill program. Others in the region believe that 

the adverse biological impacts of reducing spill have been greatly underestimated, the potential 

benefits of alternative biological offsets have been greatly overestimated, and the biological 

benefits of spill throughout the summer are clear and of utmost importance for both listed and 

nonlisted populations of Columbia River Basin salmonids. Therefore it is important that we 

address what is known regarding summer spill passage. 

In general, fewer spill studies have been conducted during the summer than during the 

spring migration period, as summer spill survival tests present several challenges to researchers. 

These include issues associated with handling fish during warm water temperatures, large sample 

sizes required to detect small differences in survival due to low detection probability, whether 

test fish represent the general population, and the availability of test fish for marking. Since 

1997, summer spill studies have been conducted using uniquely identified fish that were PIT, 

radio, or balloon tagged (Table 1). Here we summarize recent survival information for passage 

through specific spillways and survival through river reaches during periods of summer spill 

based on analyses presented in Williams et al. (2005). 

Dam-specific spillway passage survival has been estimated during the summer under 

contemporary spill conditions at Ice Harbor, John Day, and The Dalles Dams. Based on PIT 

tags, estimated mean survival through the Ice Harbor Dam spillway under the spill pattern called 

for in 2000 BiOp was 0.89 in 2000 and 2002 (Eppard et al. 2002a, 2002b). This compares to an 

estimated mean survival through the spillway of 0.96 in 2003 under a “bulk” spill pattern, where 

higher flow was concentrated in fewer spillbays (Absolon et al. 2003a). At John Day Dam in 

21
2003, Counihan et al. (in prep. b) used radiotelemetry and estimated subyearling Chinook salmon 

survival through the spillway was 0.901 (95% CI, 0.877–0.922) under 0% daytime and 60% 

nighttime spill, and 0.955 (95% CI, 0.938–0.970) under 30% spill throughout the 24-hour period. 

In 2002, Counihan et al. (in prep. a) reported that estimated survival through the spillway was 

0.985 (95% CI, 0.934–1.023) under 0% daytime and 54% nighttime spill, and 1.003 (95% CI, 

0.983–1.078) under 30% spill throughout the 24-hour period. At The Dalles Dam, estimated 

mean survival ranged from 0.75 to 1.00 based on radiotelemetry and PIT tags, and was roughly 

0.93 using balloon tags (Table 1). Some of these estimates are lower than expected. Potential 

explanations are discussed above in the Causal Mechanisms subsection on page 13. 

From 1999 through 2002, NOAA Fisheries Service collected, PIT tagged, and released 

river-run subyearling Chinook salmon (mostly wild fish from the Hanford Reach) at McNary 

Dam to estimate survival below McNary Dam. Study designs and release sites varied from year 

to year, but a series of releases into the tailrace of McNary Dam was included each year. We

calculated indices of exposure to the following river condition variables: river discharge 

(“flow”)(kcfs), amount of flow over spillways (kcfs), percentage of flow that passed over 

spillways, water temperature (°C), and water clarity (Secchi disk reading). We used various 

graphical methods, pair-wise product-moment correlations, and simple and multiple linear 

regression modeling to explore relations among indices of exposure to selected environmental 

factors and survival and travel time. 

Because of concomitant temporal trends in river conditions, exposure indices for release 

groups of PIT-tagged fish were generally highly correlated with each other and with release date. 

Correlation of such magnitude among predictor variables generally makes it difficult for multivariate statistical methods to distinguish the relative importance of the predictors’ influence on 

the response variable. Nonetheless, we explored bivariate patterns, and used multivariate 

methods to shed light on relations. Samples were of sufficient size that correlations with 

relatively little explanatory power (e.g., r

2

 = 0.16) were statistically significant (P < 0.05). One 

response to this is to lower the level required to declare a correlation significant. Our approach, 

however, was simply to focus on the amount of variability in the response variable that is 

“explained” by variability in the predictor (i.e., the r

2

 value). 

The data available from river-run subyearling Chinook salmon from 1999 through 2002 

were insufficient to make definitive statements regarding potentially complex dynamics among 

travel time, survival, and environmental (river) conditions between McNary Dam and John Day 

Dam. For example, strong correlations among river conditions (flow, spill, temperature) made it 

impossible to determine which variable had the strongest influence on response variables. 

Estimated survival was significantly correlated to flow, temperature, and percent spill at McNary 

Dam, but not strongly with percent spill at John Day Dam. The correlation with spill percentage 

at McNary Dam was probably due to common correlation with flow, as spill at McNary Dam

probably had little influence on survival because study fish were collected from the powerhouse 

at McNary Dam and released into the tailrace. Within most years there was a relatively narrow 

range in river condition values, with little overlap between years in some cases. Therefore, it 

was not possible to separate processes and relations that occur within migration seasons from

potential annual differences due to generalized “year effects.” 

22
The relationships identified among spill, flow, temperature, travel time, and survival of 

subyearling fall Chinook salmon between McNary and John Day dams are consistent with those 

found in the Snake River (Connor et al. 2003a, Connor et al. 2003b, Smith et al. 2003). We note, 

however, that weekly estimates of juvenile fish survival between McNary Dam and John Day 

Dam tailraces ranged from 0.52 to 0.66 in 2001 and were narrower in range and lower than in 

other years; weekly estimates of survival ranged from 0.45 to 0.96 in 2002, from 0.55 to 2.00 in 

2000, and from 0.75 to 1.03 in 1999. There was no spill at John Day Dam in 2001 during the 

period when the test fish used in this analysis passed the dam. In addition, river flow was lower 

and water temperatures were higher in 2001 than the other years. While we cannot identify the 

lack of spill in 2001 as the sole cause of the lower and narrower range in survival observed that 

year, based on the high estimates of survival through the John Day Dam spillway under 30% 

spill reported above, it is likely that lack of spill that year was at least partially responsible for 

the observed results. 

We also found that average survival was nearly constant for water temperatures below 

19.3°C, and nearly constant but considerably lower for water temperatures above 20.6°C. In 

laboratory studies, Marine and Cech (2004) found that exposure of Sacramento River fall 

Chinook salmon fry to water temperatures above 20°C resulted in decreased growth, increased 

osmoregulatory impairment, and increased vulnerability to predation. That temperature value 

was nearly the same temperature threshold we found to affect survival of subyearling fall 

Chinook salmon migrants in the river reach from McNary Dam to John Day Dam. Thus a 

temperature threshold may exist above which mortality in this reach is increased. 

Dissolved Gas/Gas Bubble Disease 

Dissolved Gas Standards 

Recommended total dissolved gas standards for surface waterways were developed by

the U.S. Environmental Protection Agency under the authority of the 1977 Clean Water Act 

amendment to the Federal Water Pollution Control Act of 1948. These standards were 

subsequently adopted by state environmental quality agencies; each Pacific Northwest state has 

slightly different statutes. 

In the case of the mainstem Snake and Columbia rivers, however, a common standard of 

110% total dissolved gas supersaturation (TDGS) was adopted. Each state has provisions for a 

short-term waiver of the standard. In 1994 NOAA Fisheries Service first applied for and was 

granted total dissolved gas waivers from Washington and Oregon. These waivers allowed total 

dissolved gas levels up to 115% in the forebay and 120% in the tailrace of each FCRPS dam. 

The 1995 FCRPS BiOp included these standard waivers in the spill program requirements. 

Since 1994, NOAA Fisheries Service has requested and received annual water quality waivers 

from each state. 

In 1997, the state of Washington replaced the dissolved gas standard annual waiver with 

a “fish passage exemption” in the Water Quality Standards for Surface Waters of the State of 

Washington (Chapter 173-201A-060 (4)(b)), which mandates allowable dissolved gas limits 

identical to those in the NOAA Fisheries Service BiOp, with the addition of a 1-hour maximum 

of 125%. This exemption specifically states that it is intended for “spillage for fish passage” and 

23
is “temporary.” In qualifying for this exemption, NOAA Fisheries Service has met annual 

reporting and monitoring requirements. 

Table 2 presents the estimated spill volumes which result in approximately 120% TDGS 

at tailwater gas monitors or 115% TDGS at forebay monitors. Spill volumes vary with forebay 

TDGS, powerhouse flow volumes, and spillway tailwater elevations. 

Dissolved Gas Supersaturation 

Soon after Bonneville Dam was completed in 1938, dead adult salmon were observed 

downstream by fishermen (Merrell et al. 1971). Unreconciled losses of adult salmon continued 

through the 1950s. Westgard (1964) first documented gas bubble disease (GBD) on the 

Columbia River in the McNary Dam spawning channel. Dissolved gas supersaturation was 

documented and unequivocally associated with spill at mainstem Columbia River dams in the 

late 1960s by Ebel (1969). By inference, Merrell et al. (1971) attributed earlier observed adult 

salmon losses to supersaturation. In 1967 run-of-the-river adult and juvenile salmon were 

observed with GBD and holding tests linked high dissolved gas and increases in temperature 

with mortality of juveniles (Ebel 1969). 

Adult salmon mortality was documented in conjunction with high levels of supersaturation (125–135%) from Wells to Chief Joseph Dams from 1965 through 1969 (Meekin and Allen 

1974). In 1968, when John Day Dam was completed but before turbines were in operation, dead 

adult Chinook and sockeye salmon (O. nerka) were found floating downstream and live fish with 

signs of GBD were captured (Beiningen and Ebel 1970). From 1968 to 1975, GBD in high-flow 

years contributed to high mortalities of juvenile salmonids migrating from the Snake River (Ebel 

et al. 1975). 

Many studies on GBD and its effects on salmonids were conducted in an attempt to 

define the threat in the mainstem Columbia and Snake rivers. The severity of GBD was 

dependent upon species, life stage, body size, level of total dissolved gas, duration of exposure, 

water temperature, general physical condition of the fish, and swimming depth (Ebel et al. 1975). 

A thorough review of the literature on dissolved gas supersaturation and of recorded cases of 

GBD was compiled by Weitkamp and Katz (1980) and updated by Fidler and Miller (1993). 

Despite numerous studies, there were still questions regarding the TDGS that migrating 

salmonids can safely tolerate and how to evaluate impacts. In 1994 NOAA Fisheries Service and 

BPA convened a panel of experts to review dissolved gas conditions on the river and assess 

impacts to salmonids from GBD resulting from voluntary spill (GBD Panel 1996). The panel 

concluded that not enough was known to accept the hypothesis that aquatic organisms in the 

Columbia and Snake rivers were not impacted by dissolved gas from voluntary spill. This 

conclusion initiated regional research and monitoring efforts to increase the knowledge base 

needed for spill management decisions. These efforts were administered by the Gas Bubble 

Disease Technical Work Group (now called the Water Quality Team) and was originally chaired 

jointly by NMFS and the Environmental Protection Agency. A comprehensive review of total 

dissolved gas (TDG) levels and risk assessment associated with spill and can be found in 

Appendix E of the NMFS 2000 FCRPS BiOp. 

24
Table 2. Estimated spill caps. 

Project 

120% TDG Limit 

(kcfs)a

Practical TDG Spill Limit 

(kcfs)b

Lower Granite 45–50 45–50 

Little Goose 60 40c

Lower Monumental 25–40 35 

Ice Harbor 95–105 95–105 

McNary 160–185 160–185d

John Day 85–160 150–160e

The Dalles 95–200 150 

Bonneville 145–150f 120–150g

a

 Spill cap based on observed gas levels at the tailwater monitor in 2002 and 2003. b

 Spill cap based on observed spill flows limited by the tailwater monitor or the next forebay monitor in 2002 and 

2003. c

 Because of entrainment of powerhouse flow into the spillway, the Little Goose spill levels are normally 

constrained by TDG levels in the forebay of Lower Monumental Dam. d

 The McNary spill limit is an estimate because a new spill pattern will be implemented in 2004. e

 The John Day limit depends on how the spill schedule is implemented as total river discharge increases. The spill 

pattern changes from concentrated to flat at about 108 kcfs spill. TDG levels actually decrease above this point then

increase with increasing spill. We are assuming the operators will avoid the 108 kcfs limitation. f The Bonneville spill limit is based on the new spillway tailrace monitor. g

 The TDG spill limit at Bonneville is dependent on the monitor site used for management. The tailwater site 

provides consistent TDG results; however, the Camas site is highly variable due to local environmental conditions 

and may, at times, further limit spill levels at Bonneville Dam.

GBD Monitoring and Research 

Beginning in 1994, the annual TDGS waivers from the states were granted with the 

stipulations that proper monitoring would be conducted and results of research and monitoring 

would demonstrate minimal effects from GBD at temporarily increased levels of 115% in 

reservoirs and 120% in tailraces at dams. A formal plan was developed by NMFS (1997) to 

monitor TDGS and signs of GBD in the aquatic biota. Critical uncertainties of the monitoring 

program were identified (Biological Monitoring Inspection Team 1995), gas bubble disease 

research priorities were developed (NMFS 1996), and thresholds of 15% prevalence and 5% 

severe GBD signs were set for continued voluntary spill. 

Through the 1990s, TDGS monitoring improved. By 1998 timely data distribution by 

COE included hourly dissolved gas levels and water temperatures from 41 monitoring sites in 

forebays and tailraces on the mainstem Columbia and Snake rivers (COE 1998b). Quality 

assurance and control measures, and improved gas measurement technology, increased data 

precision and provided confidence that fisheries management decisions regarding the threat from

TDGS were based on sound data. However, concerns still existed over whether the monitoring 

sites appropriately represent all conditions experienced by salmonids (ISAB 1999). 

Additionally, intensive monitoring throughout reservoirs and in tailraces has allowed a series of 

models to be developed that relate tailwater TDGS to spill, total project release, and forebay 

TDG for all mainstem dams from Grand Coulee and Lower Granite to Bonneville Dams, as well 

as from Dworshak Dam (COE 1998a). 

25
Through the 1990s, GBD monitoring of migrating juvenile and adult salmonids was also 

greatly improved. Numbers of fish, methods, locations, and times for sampling were adjusted to 

provide a representative network of samples. Quality assurance and control methods met 

regional requirements, providing confidence that fisheries management decisions were based on 

the best possible data given the present state of technology. 

Uncertainties associated with the GBD monitoring are 1) whether the relationship 

between smolt/adult mortality and gas bubbles in fins, gills, and lateral line is known; 2) whether 

clinical signs change during collection and examination; 3) whether signs in sampled fish 

represent the river site over the entire 24-hour period; 4) whether samples are taken at 

representative locations, including those of high risk from GBD; 5) whether sample size is 

statistically adequate for required confidence limits; 6) whether key signs of GBD and their 

relative significance are known; and 7) whether the 15% threshold level for GBD prevalence can 

be tolerated by the juvenile migrant population. Research conducted to address these 

uncertainties suggests: 

1. The relationship between prevalence and severity of GBD signs and mortality of juvenile 

salmonids is unresolved. Research has produced an equivocal relationship except at 

consistent 130% TDGS (Mesa et al. 2000a, Backman and Evans 2002). 

2. Varying pressures encountered by juvenile salmonids during passage through the 

collection and bypass systems at dams appears to not substantially alter existing GBD 

signs nor induce new signs (Absolon et al. 1999). Collection protocols require fish be 

collected at the dewatering screens in the bypass conduit, held in river water at ambient 

temperature, and processed within 15 minutes of sampling (Maule et al. 1997). 

Prevalence of GBD signs in fish collected from bypass systems was somewhat greater 

than that of fish collected by seines from reservoirs (Backman et al. 1998). The increased 

GBD signs may be caused by passage delays in front of dams prior to bypass system

entry. The ISAB (1999) called for more research to evaluate this difference. 

3. Fish samples collected for GBD assessment of smolt monitoring facilities at Snake and 

Columbia River dams adequately represented fish passing through the smolt bypass 

systems 24 hours per day (NMFS 1999a). 

4. Smolt monitoring sites appeared to provide samples that were generally representative of 

upstream and downstream locations. 

5. Evaluation of sample statistics by Maule et al. (1997) showed that numbers of sampled 

fish were generally sufficient to document a species-specific GBD prevalence when 

impacted migrants reached the NOAA Fisheries Service threshold of 15% prevalence or 

5% severe signs of GBD. An exception is at McNary Dam, where migrants from the 

Snake River cannot be differentiated from Columbia River migrants. Here a low 

prevalence in the total sample could be associated with a large prevalence within the 

Snake River subpopulation. However, at dissolved gas levels present during voluntary 

spill, this problem is insignificant. 

6. The GBD signs used as a GBD index for smolt monitoring are externally visible 

subcutaneous emphysema. Research to identify other indicators of GBD such as blood 

chemistry changes was not pursued. Acoustic assessment of internal gas emboli was 

determined too expensive and not suitable for use in field locations (Carlson 1995). 

26
Excision of gill lamella was investigated, but thought to be no better an index of GBD 

impacts than other less invasive methods. 

7. Signs of GBD can only be used as a subjective tool. The GBD Panel (1996) stated that 

when GBD signs are observed in any fish, there should be regional concern about 

survival. Also, the greater the prevalence and severity of GBD signs, the more concerned 

managers should be about fish survival. 

GBD Impacts

Results of GBD monitoring and research on juvenile salmonids in the 1990s show that 

incidents of high prevalence and severity of GBD signs and probable related mortality are 

associated with exceptionally high river flows or exceptional dam operation problems where 

powerhouse flows were limited. In 1990 a fire at John Day Dam caused 100% spill for an 

extended period, resulting in downstream TDGS levels greater than 130% and GBD prevalence 

up to 74% in steelhead and 38% in coho salmon. 

In 1993 turbine outages at Lower Granite Dam caused TDGS to increase, resulting in 

18% prevalence of GBD in migrants at Little Goose Dam. In 1995 and 1996, turbine outages at 

Ice Harbor Dam coupled with high river flows caused TDGS to exceed 130% during the spring 

freshet. PIT-tag interrogation data collected at Snake and Columbia River dams suggested losses 

of juvenile Chinook salmon coincident with the high levels of TDGS in the 67-km reach from

Lower Monumental Dam to the Snake River mouth in 1995 (Cramer 1996a) and 1996 (Cramer 

1996b, NMFS 1997). However, no losses of steelhead were measured in that same river reach 

for fish that prior to release at Little Goose Dam had been experimentally exposed to 

supersaturation sufficient to produce low-grade mortality (Monk et al. 1997a). Resident fish 

downstream from Ice Harbor Dam also showed high prevalence and severity of GBD signs in 

1995 and 1996 (Schrank et al. 1997, 1998). 

High river flows in 1996 and 1997 caused excessive spill at all mainstem dams, and 

TDGS exceeded 120% (the maximum allowed for voluntary spill) for extended periods 

throughout most of the lower Snake River and mid- and lower Columbia River. Results of GBD 

monitoring showed that prevalence of subcutaneous emphysema in juvenile salmonids was 

minor when TDGS was 120–125%, but was generally 10% or greater when TDGS was higher 

than 125% (NMFS 1998a). Under high river flow conditions, even when all possible system

management actions are taken, TDG often cannot be kept below 120%. 

In the early 1990s, river flow was low and often within powerhouse hydraulic capacities. 

When there was sufficient market for power, voluntary spill for fish passage and TDGS at 120% 

in tailraces and 115% in forebays could be regulated. Effects of GBD under these conditions 

appeared benign, signs were minimal or nonexistent, and there was no apparent GBD-related 

mortality (Maule et al. 1997, ISAB 1999). Even during periods of involuntary spill, GBD 

impacts appeared to be minor except when TDGS was over 120% (NMFS 1997, NMFS 1998a, 

NMFS 1999a, Backman and Evans 2002). 

Impacts of GBD as measured by visual external signs in river migrants are greater at 

higher temperatures, particularly when the ambient water temperature of supersaturated water 

increases (Ebel 1969, Ebel et al. 1971). Steelhead have a lower tolerance to TDGS than other 

27
species, based on Smolt Monitoring Program data (NMFS 1997, 1998a, 1999a) and laboratory 

tests (Fidler and Miller 1993). 

Depth Compensation

Gas solubility increases with increasing pressure. For each meter of depth there is a 10% 

reduction in the TDG saturation level relative to the surface saturation (Weitkamp and Katz 

1980). Dawley et al. (1975) and Weitkamp (1976) reported data suggesting depth compensation 

occurs with juvenile salmonids as gas solubility calculations would predict. Maule et al. (1997) 

observed that salmonids may migrate at protective depths and presented results suggesting that 

the depth of tagged fish would compensate for a surface TDG level of up to approximately 

124%. 

Employing depth-sensitive radio tags, Beeman et al. (1998, 1999) tracked juvenile 

salmonids between Ice Harbor and McNary Dams at depths of 1.8–2.5 m with a surface TDG 

level of 120%, and concluded the median depth of juveniles in the McNary pool was sufficient to 

protect fish from TDG levels of between 117% and 124%. The authors stated that a voluntary 

spill program with gas caps of 115% in forebays and 120% in tailraces can be expected to 

prevent gas bubble disease in juvenile Chinook and pose little threat to the more sensitive 

steelhead. 

Dissolved Gas Abatement 

Starting in 1994, a COE Dissolved Gas Abatement Study developed concepts for 

decreasing TDGS at the eight Snake and Columbia River dams. The COE is pursuing numerous 

potential structural modifications to spillways and stilling basins for dissolved gas abatement 

alternatives. While some of these alternatives may be implemented in the future, to date only 

flow deflectors have been implemented to reduce gas production at the FCRPS projects. 

None of the gas reduction alternatives evaluated met biological, operational, and 

economic criteria, nor did they decrease gas levels to the 110% water quality standard for the 

7-day, 10-year discharge events. Whitney et al. (1997) found that under appropriate operating 

conditions, flow deflectors generally lowered dissolved gas levels downstream from the projects 

by 10–20% for a given spill flow. Deflectors were installed at Ice Harbor and John Day Dams

and have provided gas abatement benefits in the upper portion of this range. Additional 

deflectors were installed in the end spillbays of Lower Monumental, McNary, and Bonneville 

Dams. These additional deflectors have further improved the dissolved gas levels below these 

dams. 

Spill Passage Conclusions 

1. Estimates of spill efficiency and effectiveness for juvenile salmonids vary by dam, the 

percentage of river flow spilled, spill pattern, time of day, species (or run), and configuration 

of the dam. Spill effectiveness is generally between 1.0 and 2.0, but can range considerably 

higher at certain dams. There is a strong indication from studies at Ice Harbor Dam on the 

Snake River and The Dalles Dam on the lower Columbia River that, above a certain level of

28
spill, there is no gain in spill effectiveness and very likely a reduction. However, spill 

efficiency will continue to increase with the percentage of total river flow spilled. 

2. Documented consequences of spill include increased fish passage efficiency (proportion of 

fish passing through nonturbine routes) and reduced migrational delays through forebays, 

tailraces, and reservoirs. Forebay residence time for downstream migrating juvenile 

salmonids is inversely related to spill. 

3. Spill has the potential for reducing exposure risk to predation, high water temperatures, and 

disease. The presence of piscine and avian predators in forebays and detection histories of 

tagged fish in forebays suggest that delay of juvenile salmonids in dam forebays may lead to 

reduced survival. 

4. Fish survival through spillways can be very high (near 1.00) and is often higher than turbine 

or bypass system survival when spill passage conditions are optimal. However, survival 

through spillways with deflectors or shallow stilling basins can be negatively influenced by 

stilling basin depth and turbulence, hydraulic patterns in the basin, spillbay location within 

the spillway relative to the spill pattern being used, deflector elevation relative to tailwater 

elevation and thus total river flow, gate opening, and fish location when passing through the 

spillbay and under the control gate. Survival through spillways with deflectors or shallow 

basins can be considerably less than 0.98, a value typically used in hydropower system

modeling exercises. Observations and lessons learned from spillway survival studies at The 

Dalles, Ice Harbor, and Lower Monumental Dams should be applied to other spillways with 

deflectors to ensure that adequate spillway passage conditions are provided at these locations 

as well. Examples include Little Goose Dam, where survival may be lower than previously 

measured under some conditions, and possibly training spill associated with the removable 

spillway weir at Lower Granite Dam and other weirs that may be installed in the future. 

5. Tailrace residence time for juvenile salmonids passing through spillways is influenced by 

spill volume and pattern. Generally, spillway passage results in more rapid tailrace egress 

than bypass system passage. Because piscine predators prefer hydraulic cover such as eddies 

and backwaters, spill patterns that facilitate rapid juvenile salmonid egress from the spillway 

stilling basin through the tailrace likely increase survival. 

6. In general, research results indicate that when daytime spill is provided, spillway passage of

juvenile salmonids occurs at fairly constant rates, day and night. Juvenile salmonid 

migrations may be delayed if spill is managed based on powerhouse diel passage patterns. 

7. Relatively few studies have been conducted on subyearling Chinook salmon survival through 

spillways during the summer migration period under contemporary conditions. Estimates of

mean spillway passage survival at Ice Harbor and The Dalles Dams since 1997 using 

traditional spill patterns ranged from 0.75 to 1.00. However, due to problems associated with 

stilling basin conditions at these dams, this wide range in spillway survival estimates may not 

represent other Snake and Columbia River dams. For example, estimated mean survival 

through the spillway for John Day Dam in 2002 and 2003 ranged from 0.96 to greater than 

1.00 when 30% of river flow was spilled throughout the 24-hour period. In addition, 

estimated mean survival through the Ice Harbor Dam spillway was 0.96 in 2003 under a 

“bulk” spill pattern, where higher flow was concentrated in fewer spillbays. 

29
8. Relatively few estimates of survival have been made through river reaches during periods of 

summer spill. The available information from the lower Columbia River as to whether 

summer spill improved reach survival was insufficient to make definitive statements 

regarding the relationships among travel time, survival, and environmental (river) conditions 

between McNary and John Day Dams. Strong correlations among flow, spill, and 

temperature made it difficult to predict the effect of any variable independently and 

impossible to determine which had the strongest influence on survival. This is also 

consistent with data on subyearling fall Chinook salmon in the Snake River. While we 

cannot identify the lack of spill in 2001 as the sole cause of the lower and narrower range in 

survival through the reach that year, it is likely that lack of spill was at least partially 

responsible for the observed results. We did find evidence that a temperature threshold of 

20.6o

C may exist in the reach from McNary Dam to John Day Dam, above which mortality 

increases. 

9. Results of GBD monitoring and research on juvenile salmonids indicate that incidents of 

high prevalence and severity of GBD signs and probable related mortality are associated with 

exceptionally high river flows or exceptional dam operation problems where powerhouse 

flows are limited. Under TDG cap conditions of 115% (forebay) and 120% (tailrace), signs 

of GBD are minimal or nonexistent, with no apparent GBD-related mortality. 

30
Juvenile Salmon Passage Through Mechanical 

Screen Bypass Systems 

Background 

In response to concerns for the mortality additional dams were expected to impart on 

juvenile anadromous salmonids during their seaward migrations in the late 1960s, NOAA 

Fisheries Service (then the Bureau of Commercial Fisheries) began to investigate means to 

mitigate the anticipated impacts. The development of systems to collect downstream migrants 

before they passed through turbines and the subsequent transport of the collected smolts around 

the remaining dams were two of the focal areas of original research during the earlier years, as 

they are currently. The original intent of the mechanical bypass systems then was to provide a 

method of collecting large numbers of downstream migrants at upstream dams for transportation 

to avoid the excessively high passage-related mortality of that period. 

The contemporary mechanical screen bypass systems were developed over the next 30 

years. The systems consist of a submersible fish screen within each turbine intake (for each 

turbine, there are three intakes and associated gatewells) that diverts migrants upward into a 

gatewell (Figure 2). Once in the gatewell, a vertical barrier screen (VBS) concentrates and 

further guides the fish into the upper gatewell area where they pass through a submerged orifice 

and into a collection channel that travels the length of the powerhouse. The channel conveys fish 

and orifice flow from all gatewells directly to the river, or to dewatering facilities that reduce 

flow to approximately 30 cfs. This reduced flow can then be routed directly to the river or to 

secondary dewatering/separation facilities for subsequent separation, sampling, holding (for 

delayed truck- or barge-loading), or direct loading onto barges. 

Smolt monitoring facilities are located within key bypass systems and allow researchers 

to estimate species composition, fish condition, run timing, and passage indices. PIT-tagged fish 

can also be detected at these facilities, with time and date of passage noted and fish diverted for 

further evaluation, if desired. Monitoring at these facilities is continuous within the season and 

requires that fish be routed through the fish separators and sampling facilities. 

At other dams, such as Ice Harbor Dam, monitoring is periodic and is done to determine 

if mechanical problems are causing poor fish condition. Based on 2001–2002 modifications at 

McNary Dam, secondary dewatering, separation, and sampling facilities can be bypassed 

completely by shunting collected fish to the dam’s tailrace through a full-flow bypass pipe with 

PIT-tag detection capabilities. Similar systems are scheduled for installation at Ice Harbor and 

Lower Monumental Dams prior to the 2005 migration season. 

31
Figure 2. Cross section of the mechanical screen bypass system at Lower Monumental Dam.

32
Two submersible fish-screen designs are now in use to guide fish away from turbine 

intakes and into juvenile bypass systems at Columbia River hydroelectric dams: a submersible 

traveling screen (STS) and an extended submersible bar screen (ESBS). The STSs utilize a 

monofilament mesh screen that rotates around large rollers at the top and bottom of the screen. 

The screen is rotated periodically to allow flow passing through the screen to flush the mesh 

surface clean of debris. STSs are currently installed at Lower Monumental, Ice Harbor, John 

Day, and Bonneville Dams. ESBSs are made of a fixed wedgewire screen material and have a 

brush sweep that is activated periodically to remove debris from the face of the screen. ESBSs 

are currently installed at Lower Granite, Little Goose, and McNary Dams. The Dalles Dam does 

not have a mechanical screen juvenile bypass system. 

Mechanical screen bypass system design criteria are described in NMFS (2004), COE 

bypass system design memorandums (COE 1995a, 1996a), the COE Fish Passage Plan (COE 

2004), and an intake design guidelines manual (ASCE 1995). NOAA Fisheries Service 

guidelines for locating and designing bypass outfalls are presented in NMFS (2004). Here we 

present essential fish passage information related to the development and operation of these 

systems. 

Fish Guidance Efficiency 

Definition and Methods 

Fish guidance efficiency (FGE) is the proportion of smolts passing through turbine 

intakes that are guided upwards into gatewells by the submersible fish screens. FGE is 

calculated as the number of guided fish divided by the total number of fish (guided plus 

unguided) passing through a turbine intake during a period of measurement (Brege et al. 1992). 

FGE estimates vary with sampling method, species, rearing history, stock, fish condition 

(disease and smoltification), project, time of day, day, season, turbine unit, fyke-net location, 

environmental conditions, and project operations. This spatial and temporal variability is likely 

related to complex interactions between biological and physical factors, such as the arrival of 

different stocks at a dam throughout the season. Williams et al. (1996) attempted to clarify some

of the mechanisms affecting FGE. However, FGE data based on fyke-net estimates was 

generally limited to studies designed to compare FGE of existing versus modified (prototype) 

intake and vertical barrier screens. These typically involved different hydraulic conditions and 

little could be concluded. Gessel et al. (1991) suggest that factors such as water temperature, 

turbidity, flow, photoperiod, physiological development, and predation by northern pikeminnow 

may affect FGE. 

There are four methods of estimating FGE: 1) intake fyke nets and gatewell dip baskets, 

2) hydroacoustics, 3) probability of detecting PIT tags, and 4) radiotelemetry. Early FGE studies 

utilized fyke-net frames beneath STSs to collect unguided fish in intakes (Krcma et al. 1986), 

and dip baskets (Swan et al. 1979) to collect guided fish in gatewells. Beginning in 1993, 

streamlined fyke-net frames and nets were designed to test extended screens and placed in 

gatewell slots located downstream from the guidance screen (Brege et al. 1994). The newly 

designed fyke nets and frame and placement downstream from the guidance screen minimized 

water resistance and possible FGE bias, which may have previously resulted from larger fish 

33
avoiding fyke collection. Gatewell dip-net recapture efficiency tests with yearling and

subyearling Chinook salmon produced recapture efficiencies of 95–100% (McComas et al. 1994, 

Brege et al. 1997a, Brege et al. 1997b, Brege et al. 1998). More recently, FGE estimates have 

been obtained from the probability of detecting PIT-tagged fish (Smith 1997, Krasnow 1998, 

Anderson et al. 1998), and radiotelemetry (Adams et al. 1998a) and hydroacoustic (Ploskey and 

Carlson 1999) methods. 

Differences exist among the FGE estimates derived from these methodologies, and each 

method has its advantages, disadvantages, and critical assumptions. For example, a strength of 

the fyke-net method is that it produces FGE estimates by individual species or life history type. 

On the other hand, the method generally measures FGE in only one gatewell of only one or two 

turbine units and for only relatively brief diel periods. The data tend to be highly variable and 

may not accurately represent even daily, much less seasonal, FGE over entire powerhouses. The 

method is probably better suited for comparing FGE during direct testing of various guidance 

technologies and configurations, with the possible exception of head-to-head comparisons 

between STSs and ESBSs. Recent tests of ESBSs at John Day Dam in 1999 suggested that 

gatewell dipping might not always recover all of the fish guided into a gatewell equipped with an 

ESBS (Brege et al. 2001). In this study, it was estimated that 20% of the guided fish were 

gatewell mortalities, and were not recovered by gatewell dipnetting because they had dropped 

out of the test gatewell prior to the commencement of dipnetting. 

On the other hand, Ploskey and Carlson (1999) previously found strong seasonal 

correlations between counts derived from hydroacoustic and fyke-net methods for both guided 

and unguided fish at John Day Dam. However, hydroacoustics significantly underestimated both 

guided and unguided fish passage relative to the fyke-and-dip-netting method. The two methods 

produced similar FGE estimates, but hydroacoustics undercounted both groups nearly equally 

relative to the netting method. In addition, hydroacoustics does not provide species-specific 

estimates of FGE, can count nonsalmonid species and debris, and passage routes are generally 

not sampled completely. Hydroacoustic can, however, provide continual fish count estimates 

over long time periods. 

The probability-of-detecting-PIT-tags method measures FGE of the entire powerhouse by 

species, but results are most reliable when groups of PIT-tagged fish pass during periods of near 

0% spill. When groups pass during periods of relatively high spill, the method produces results 

dependent on the questionable assumption that spill index and detection probability have the 

same linear relationship throughout the range of observed spill values (Smith 1997). 

Radiotelemetry has also been used to estimate FGE. Radio tags are implanted either 

gastrically or by surgery in fish that are generally larger than the population they represent. Both 

Adams et al. (1998a, 1998b) and Martinelli et al. (1998) concluded that surgical implantation 

was the preferred method for radio tags in most studies, though gastric implantation could be 

used in studies of short duration. Hockersmith et al. (2003) indicated that both tagging 

techniques would provide data comparable in quality to that derived from PIT tags in studies 

lasting 6 days or less. Radio-tagged fish are generally released well upstream of a particular dam

and are therefore normally distributed relative to other fish by the time they arrive at the dam.

34
Williams et al. (1996) compared the probability of PIT-tag detection with fyke-net 

estimates of FGE at McNary Dam and suggested a pressure field created by the fyke nets located 

under the STSs biased yearling Chinook FGE estimates upward. PATH (1998) developed a 

correction factor to adjust STS FGE estimates based on fyke-net position to improve the 

accuracy of the PATH retrospective model analysis for yearling Chinook salmon. The correction 

factor was the ratio of FGEs derived at McNary Dam with the nets in the downstream slot 

compared to the upstream slot (i.e., directly beneath the STS). Krasnow (1998) estimated the 

correction factor to be 0.82. The validity of applying the FGE correction factor for yearling 

Chinook salmon at McNary Dam to other intakes and species is unknown. 

For example, McNary Dam has a large, low-velocity intake compared to most other 

powerhouse intakes. Monk et al. (1999a) acknowledged the PATH FGE adjustment but chose 

not to apply it to the data for Bonneville Dam Second Powerhouse. In their view, the correction 

factor was not considered applicable to STS FGE at the Second Powerhouse because the intakes 

at McNary Dam and Bonneville Dam Second Powerhouse are hydraulically different, nor would 

the correction have affected the conclusions of their analysis. 

Estimations of FGE are inherently variable, regardless of the method used. This inherent 

variability makes it difficult to develop accurate and precise single-point estimates of FGE, and 

to use a typical scientific format that would express FGE as a mean with a standard deviation. 

Even with this variability, single-point estimates of FGE are often used for modeling or 

management purposes. 

Results 

Table 3 presents values commonly used by PATH (Anderson et al. 1998) and NOAA 

Fisheries Service (Krasnow 1998) in modeling analyses. They reviewed the available data and 

developed point estimates of FGE for each project and certain species and rearing history. 

NOAA Fisheries Service developed a spreadsheet model (SIMPAS) to evaluate benefits 

to fish survival associated with various FCRPS alternatives. FGE values used in SIMPAS differ 

slightly from Krasnow (1998). No single estimate of FGE by species and dam is universally 

accepted, and others in addition to those presented in Table 4 have been developed. To

characterize the variability in the data, Table 4 also presents ranges in the actual daily FGEs 

measured through various sampling methodologies. Point estimates of FGE that are used to 

model the hydropower system should generally fall within these ranges. 

Project operations can also affect FGE variability. For example, at Lower Granite Dam 

in 2002, Plumb et al. (2003) radio-tagged hatchery yearling Chinook salmon and hatchery and 

wild steelhead to estimate passage metrics when the removable spillway weir (RSW) was either 

on or off. Operation of the RSW affected FGE differently for the three groups of fish. For 

hatchery yearling Chinook salmon, total powerhouse FGE was 69% regardless of RSW 

operation. However, for hatchery steelhead, FGE dropped from 90% when the RSW was 

operating to 79% when it was not. The opposite trend occurred for wild steelhead, with the 77% 

FGE measured when the RSW was operating increasing to 83% when it was not. For the entire 

powerhouse, seasonal FGE was 68% for yearling Chinook salmon, 87% for hatchery steelhead, 

and 75% for wild steelhead. These values are similar to those reported for yearling Chinook 

35
salmon and steelhead at this dam in Table 3. However, during these studies, some components 

of the prototype surface bypass collector were still in place, which may have affected hydraulic 

conditions and study results. 

During 2003 Plumb et al. (2004) again radio tagged the same groups of fish at Lower 

Granite Dam to estimate passage metrics with the RSW either on or with the dam spilling to the 

gas cap and with all surface bypass components removed from the forebay. In this study, 

seasonal FGEs were 82%, 89%, and 96% for yearling Chinook salmon, hatchery steelhead, and 

wild steelhead, respectively, and are higher than those reported in Table 3, especially for wild 

steelhead. During the RSW treatment, FGEs were 80%, 88%, and 98% for yearling Chinook 

salmon, hatchery steelhead, and wild steelhead, respectively. During the gas cap spill treatment, 

FGEs were slightly higher for yearling Chinook salmon and hatchery steelhead, but slightly 

lower for wild steelhead, with measurements of 85%, 90%, and 95% FGE, respectively. 

Despite the variability discussed above, some general FGE trends have been observed. 

Side-by-side fyke-net estimates of FGE with STSs and ESBSs generally indicate that FGE is 

statistically significantly higher with ESBSs (McComas et al. 1993, Brege et al. 1994). FGE for 

yearling Chinook salmon at Lower Granite Dam generally increases over time throughout the 

migration season (Swan et al. 1990). A positive correlation was found between FGE and the 

level of smolt development exhibited by the migrant population (Giorgi et al. 1988). 

With subyearling Chinook salmon, an opposite trend appears to occur; FGE generally 

decreases as the season progresses (Brege et al. 1988a, Monk et al. 1999a). In the lower 

Columbia River, FGE of coho salmon is generally high and similar to steelhead, and sockeye 

FGE is generally lower than that of all other yearling migrants. FGE has been increased by a 

variety of adaptations to the basic STS, including extending the length of the screen from 6.1 to 

12.2 m (STS vs. ESBS), raising the operating gate to allow more flow up the gatewell, lowering 

STS screens to open the throat area, and using an inlet flow vane with ESBSs to direct more flow 

into gatewell slots. 

Recent PIT-tag data also indicate that smaller Snake River yearling Chinook salmon and 

steelhead smolts tend to be selected for guidance into bypass systems by submersible fish screens 

at higher rates than their larger cohorts, and that smaller individuals are also more prone to 

guidance multiple times as they migrate downstream through the FCRPS (Zabel et al. in prep., 

Williams et al. 2005). Zabel and Williams (2002) showed that smaller PIT-tagged fish returned 

as adults at lower rates than their larger cohorts for some years. Together, these results indicate 

that mechanical screen bypass systems have, to varying degrees, the potential to be selective for 

species, life history types, and other attributes such as size. Therefore, system managers should 

consider dam operations that utilize multiple routes to pass fish around dams to minimize any 

potential risk of reducing biodiversity. 

36
Table 3. Fish guidance efficiency (FGE) at Columbia and Snake River dams for 1999 configuration.

a

 

These values are commonly used by PATH and NOAA Fisheries Service in modeling 

exercises. 

Site (Screen type) Species PATH FGE (%)b NOAA FGE (%)c

Lower Granite Dam (ESBS) Yearling Chinook 78 78 

Subyearling Chinook - 53

Steelhead - 81

Little Goose Dam (ESBS) Yearling Chinook 82 82 

Subyearling Chinook - 45

Steelhead - 81

Lower Monumental Dam (STS) Yearling Chinook 61 61 

Subyearling Chinook - 49

Steelhead - 82

Ice Harbor Dam (STS) Yearling Chinook 71 71 

Subyearling Chinook - 46

Steelhead - 93

McNary Dam (ESBS) Yearling Chinook 95 95 

Subyearling Chinook - 62

Steelhead - 89

John Day Dam (STS) Yearling Chinook 67 64 

Subyearling Chinook - 34

Steelhead - 85

The Dalles Dam (None) d Yearling Chinook 46 46 

Subyearling Chinook - 46

Steelhead - 40

Bonneville Dam

 Powerhouse One (STS) Yearling Chinook 41 38 

Subyearling Chinook - 16

Steelhead - 41

 Powerhouse Two (STS) Yearling Chinook 43 44 

Subyearling Chinook - 18

Steelhead - 48

a

 These estimates likely have range, but that range changes with a number of factors and is not easily estimated. b

 Based on report to PATH from Anderson et al. (1998). c

 Based on NMFS sensitivity run #1 (assumes FGEESBS>FGESTS for wild yearling Chinook salmon). d

 FGE values for The Dalles Dam are based on passage through the ice and trash sluiceway. 

37
Table 4. Daily estimates of fish guidance efficiency (FGE) for existing conditions. Hydroacoustic 

estimates are not species- or rearing-history-specific. Estimates come from a variety of 

sources and include adjustments for fyke-net position to yearling Chinook salmon FGE with 

STSs. 

Species Fyke net

Hydroacoustics Radiotelemetry 

Project (Screen type) Min Max Min Max Min Max

Lower Granite (ESBS) Yearling Chinook 68a 93a 56b 62b

Steelhead 68a 93a 84b,c 86b,c

Subyearling Chinook 42b 53b

Little Goose (ESBS) Yearling Chinook 67d 87d 17e 100e 

Steelhead 88d 95d 61e 100e 

Subyearling Chinook

Lower Monumental (STS) Yearling Chinook 43f 83f

Steelhead 58f 95f

Subyearling Chinook 30g 40g

Ice Harbor (STS) Yearling Chinook 53h 85h 50i 100i

Steelhead 65h 95h

Subyearling Chinook

McNary (ESBS) Yearling Chinook 70j 96j 13 100

Steelhead 75j 97j 12 100

Subyearling Chinook 45j 86j

John Day (STS) Yearling Chinook 54k 78k 53l 84l

Steelhead 65k 87k 67l 86l

Subyearling Chinook 13k 55k

The Dalles NA 

Bonneville Dam — Yearling Chinook 21m 65m 15n 100n

 First Powerhouse Steelhead 27m 87m 15n 100n

Subyearling Chinook 2m 43m 0n 100n

Bonneville Dam — Yearling Chinook 22m 65m 0n 100n 29o 30o

 Second Powerhouse Steelhead 15m 70m 0n 100n 51o 52o

Subyearling Chinook 19m 62m 0n 100n

a

 Johnson et al. (1999). b

 Adams et al (1998c). c

 Includes range estimates for wild and hatchery steelhead. d

 Gessel et al. (1995). e

 Anonymous (1996). f Ledgerwood et al. (1987) and Gessel et al. (1993); range includes FGEs adjusted for fyke-net-frame position per 

Krasnow (1998). g

 Ledgerwood et al. (1987). h

 Brege et al. (1988b); range includes FGEs adjusted for fyke-net-frame position and raised operating gate per 

Krasnow (1998). i

 Brad Eppard, NWFSC, Pasco, WA. Pers. commun., February 2000. 

j

 McComas et al. (1995). k

 Krcma et al. (1986) and Brege et al. (1987); range includes yearling migrant FGEs adjusted for fyke-net-frame

position per Kransow (1998). l

 Hansel et al. (1999a). m Bonneville First: Monk et al. (1992a, 1993); Bonneville Second: Monk et al. (1994, 1995). n

 Ploskey et al. (1998). o

 Hansel et al. (1999b). 

38
Conclusions 

1. There is no single best way to estimate FGE. Each of the four methods used has advantages 

and disadvantages, and each is better suited to certain applications or conditions than others. 

2. Estimates of FGE are inherently variable, regardless of the method used. This variability 

makes it difficult to develop accurate and precise single-point estimates of FGE with high 

confidence. Even so, best estimates by dam have been developed. They are reported above 

in Tables 3 and 4 and are used in regional modeling exercises. 

3. Mechanical screen bypass systems have the potential to be selective for species, life history 

types, and other attributes such as size. Therefore, system managers should consider dam

operations that utilize multiple routes to pass fish around dams and thus minimize the risk of 

reducing biodiversity. 

Orifice Passage Efficiency 

Definition and Methods 

Once downstream migrants have been guided into gatewells by submersible fish screens, 

they exit into a bypass channel via an underwater orifice. It is important that fish not linger in 

gatewells for extended periods. Residence in this area can result in a variety of stresses, 

including those from delay and crowding (particularly in gatewells equipped with STSs), and 

excessive descaling and injury (particularly in gatewells equipped with ESBSs). 

Orifice passage efficiency (OPE) is the percentage of guided juvenile salmonids which 

exit a gatewell via the orifice in a given time period (usually 24 hours). The most recent 

estimates of OPE at Lower Granite, Little Goose, McNary, and John Day Dams and at 

Bonneville Dam First Powerhouse have been conducted by releasing either fin-clipped or PITtagged fish into the gatewell with the orifice open. With the fin-clipped fish, any remaining 

study fish are removed from the gatewell and counted after 24 hours. The percentage that left 

the gatewell in 24 hours is the OPE. With the PIT-tagged fish, OPE is calculated using the 

number exiting the gatewell as recorded by PIT-tag detectors in the bypass system. 

The regionally accepted minimum level for OPE with STSs installed is 70%. However, 

because of the increased flows and higher turbulence in gatewells associated with ESBSs, OPE 

levels approaching 90% are probably more appropriate for gatewells with these guidance 

devices. 

Results 

At Columbia and Snake River dams, where OPE has been estimated with either an ESBS 

or STS in place, estimates have generally been greater than 70% for yearling and subyearling 

Chinook salmon and steelhead, with the exception of yearling Chinook at McNary Dam (Brege 

et al. 1998) and steelhead at Lower Monumental Dam (Gessel et al. 1993), both of which had 

lower OPE estimates with ESBSs and with much greater variability (Table 5). 

39
Table 5. Most recent orifice passage efficiency (OPE) estimates for existing conditions (and prototype 

testing) at Snake and Columbia River dams. 

Project Species

OPEa

(%) Reference

Lower Granite Dam Yearling Chinook 95 Monk et al. 1997b 

 ESBS, 25-cm orifice Steelhead 97 

Little Goose Dam Yearling Chinook 97 Gessel et al. 1996 

 ESBS, 25-cm orifice -- -- -- 

Lower Monumental Dama Yearling Chinook 88 Gessel et al. 1993 

 STS, 30-cm orifice Steelhead 66 

McNary Dam Yearling Chinook 69 Brege et al. 1998 

 ESBS, 30-cm orifice Subyearling Chinook 79 

John Day Dam (ESBS w/ devices) Yearling Chinook 99 Brege et al. 2004 

 Existing STS, 35-cm orifice Subyearling Chinook 97 

Bonneville Dam First Powerhouse Yearling Chinook 80 Monk et al. 1999b 

 Existing STS, 30-cm orifice Subyearling Chinook 98 

Bonneville Dam First Powerhouse Yearling Chinook 90 Monk et al. 1999b 

 Prototype ESBS, 30-cm orifice Subyearling Chinook 97 

Bonneville Dam Second Powerhouseb Yearling Chinook 97 Monk et al. 2002 

 Modified screen system Yearling Chinook 94 

Bonneville Dam Second Powerhouseb Subyearling Chinook 100 Monk et al. 2002 

 Modified screen system Subyearling Chinook 99 

 Existing STS, 30-cm orifice 

a

 All estimates (except Lower Monumental) developed using releases of fin-clipped fish; Lower Granite estimates 

are results of PIT-tagged and fin-clipped releases averaged together. b Estimates made using PIT-tagged fish. 

Discussion 

Although FGE and OPE are higher in gatewells equipped with ESBSs than those 

equipped with STSs, hydraulic conditions are also considerably more turbulent in the ESBSequipped gatewells due to higher water velocities caused by more water being forced up the 

gatewells by the longer ESBSs. Additionally, the degree of gatewell turbulence at different dams

varies with turbine discharge and gatewell volume. In recent studies at John Day Dam (which 

has relatively severe gatewell conditions), Brege et al. (2001) found that FGE and OPE were 

both high and within acceptable ranges in gatewells equipped with ESBSs. However, in a 

different test where fish were exposed to gatewell conditions over a longer period, gatewellreleased marked fish were recovered at a high rate (99%), but also experienced 8% descaling and 

2% mortality. 

These results suggest that at some sites, improvements in FGE and OPE in gatewells 

equipped with ESBSs relative to STSs might be offset by increases in descaling and injury for 

the small percentage of fish that do not exit the ESBS gatewells rapidly. The fate of these fish is 

uncertain and may require additional attention, especially at McNary Dam where a 50% increase 

in turbine capacity is currently being evaluated. The primary issue with ESBSs is not delay but 

the condition and survivability of the small percentage of fish that do not exit the turbulent 

ESBS-equipped gatewells quickly. Therefore, there may have been a trade off between the two 

40
guidance systems. STSs have lower FGE and OPE than ESBSs, but might be better for those 

fish that are guided in terms of overall condition. 

 However, to correct the above problems at John Day Dam, the COE modified the 

porosity of VBSs and, as a contingency, added a gatewell flow-damping feature (which later 

evaluations in 2002 showed were not required). These adjustments were successful, as descaling 

and mortality rates returned to acceptable levels, averaging 4.0% and 0.1%, respectively, for 

yearling Chinook salmon and 1.0% and 0.1% for subyearling Chinook salmon, respectively 

(Brege et al. 2004). 

There is no indication that fish diverted with ESBSs at Lower Granite Dam, where high 

numbers of fish are collected, are in poorer condition or suffer higher mortalities than expected. 

Descaling and injury of fish sampled by the smolt monitoring program and overall facility 

mortality are low. In addition, adult returns of PIT-tagged fish are consistently higher for fish 

collected and bypassed at Lower Granite Dam than from other dams, including those equipped 

with STSs (Williams et al. 2005). 

Two locations where increased gatewell flow may impact fish condition are McNary 

Dam and the Bonneville Dam Second Powerhouse. To address the potential for increased 

accumulations of fine debris and injury problems not anticipated with clean VBSs, methods to 

removed debris from the VBSs are being designed and tested at these locations. 

Conclusions 

1. At dams where OPE has been estimated with either an ESBS or STS in place, OPE estimates 

have generally exceeded the regionally acceptable values for yearling and subyearling 

Chinook salmon and steelhead. Two exceptions were OPE for yearling Chinook salmon at 

McNary Dam and for steelhead at Lower Monumental Dam. In both cases, OPE was lower 

than acceptable with an ESBS as the guidance device. 

2. While OPE is generally higher for ESBSs than STSs, some earlier studies indicated that 

overall condition of juvenile fish might be poorer with the former. More recent studies on 

modified ESBSs suggested otherwise, as does some adult-return PIT-tag data. Thus ESBSs 

appear to guide higher percentages of smolts without concomitant increases in descaling and 

injury rates when appropriately designed VBSs are coupled with ESBSs at specific dams. 

Separators and Separation Efficiency 

Background and Methods 

Stress indices have repeatedly been shown to be elevated in yearling Chinook salmon 

smolts when in the presence of steelhead smolts (Park et al. 1983, Congleton et al. 2000). 

Moreover, steelhead smolts, particularly those raised in hatcheries, are much larger and more 

physical than yearling Chinook salmon smolts. Separation of these species at collection facilities 

is likely necessary to maximize survival of yearling Chinook salmon smolts. 

Separation of juvenile migrant salmonids by size is now an integral part of the juvenile 

bypass operations at FCRPS dams. Separators at Columbia River juvenile fish bypass facilities 

41
sort fish using an array of appropriately spaced bars oriented parallel to flow passing through a 

tank. This allows smaller fish to sound and pass between the bars, while excluding larger fish. 

The strategy for using bars for separation falls into two categories: single- and two-stage, 

depending on the objective of the process. Single-stage separators are in use at Ice Harbor, John 

Day, and Bonneville Dams, where the primary purpose of separation is to monitor the condition 

of juvenile salmon and steelhead passing through the project bypass system. These units 

remove adult salmonids, large incidental species, and debris using “wetted” separation in which 

smaller fish (including salmonid smolts) fall between separation bars along with incidental flow, 

while large animals and most debris are carried across a short section of exposed bars into 

another flume for eventual return to the river. Smolts can then be sampled without concern for 

injury induced by larger adult and incidental fish and debris in the sample holding area. At John 

Day Dam, the adult fish separation bars are wetted by small low-pressure jets along their upper 

surface to facilitate fish movement along the bars. Following the separation and sampling 

process, facilities using single-stage separators bypass all fish to the river downstream from the 

dam. The Dalles Dam has no juvenile fish bypass system or separator. 

Two-stage separators are used at McNary, Lower Monumental, and Little Goose Dams 

and are intended to separate large steelhead from smaller Chinook salmon smolts, as well as 

remove adult salmonids, large incidentals, and debris prior to collection for transport. Size 

separation also allows selective bypass or transport of one or both smolt size classes. The 

separators at these sites rely on “wet” separation by keeping the fish completely submerged 

throughout the process. Wet separators depend on sounding behavior as an avoidance response 

to shallow conditions above the separation bars to achieve separation (Gessel et al. 1985). All 

fish are introduced to the upstream compartment along with transport flow from the bypass 

channel (Katz et al 1999). The smaller fish are filtered between bars in the upstream

compartment, while larger smolts are removed in the second section. Large incidentals, adults, 

and debris pass through the unit to the third compartment for return to the river. 

Results 

Because fish are continually submerged, wet separation was thought to be less stressful to 

fish. However, separation efficiency of operational wet separators has been usually less than 

70% for small fish and varies considerably among years This is due to small fish passing over 

the bulkhead from the small to large fish section of wet separators where they are subsequently 

held or transported with larger fish. In addition, some fish hold under the bars for extended 

periods rather than exit expeditiously from separator units. With existing wet separator designs, 

especially those at Lower Monumental and McNary Dams, fish that delay in the wet separators 

become fatigued as a result of swimming in resistance to the hydraulic conditions within the unit.

The separator at Lower Granite Dam is unique in that a single-stage wet separation 

process is employed to segregate adult salmonids, large incidental species, and debris from

smaller fish. Presently, smolts are not separated by size. This system is scheduled for 

modification when the entire Lower Granite Dam juvenile fish bypass facility is upgraded. 

A research program was initiated in 1996 to evaluate potential improvements to existing 

wet separators, and to develop new separator concepts (Katz et al. 1999, McComas et al. 2001). 

42
These studies evaluated the effects of many physical separator designs, including separation bar 

length, slope, color, and spacing as well as effects of transport flow, water velocities through the 

separator, depth over separation bars, and exit orifice orientation and configuration (McComas 

et al. 1998, 2000, 2001). Other work considered parameters independent of physical design 

which may affect separation potential, such as ambient light and fish density. 

Design criteria from those research efforts resulting in positive effects on separation 

efficiency and separator exit efficiency were incorporated into a separator insert used for 

comparison to the operational separator at McNary Dam (McComas et al. in prep. b). In addition 

to enhancements to increase separation efficiency, the insert included a perforated-plate false 

bottom to reduce the volume available for residence under the separation bars, and a more 

efficient submerged exit-orifice configuration. 

Evaluation of the insert against the McNary Dam operational separator over a series of 2-

day replicates resulted in measurably higher separation efficiency values using the insert than for 

the McNary Dam operational separator condition, though the difference was only significant for 

large fish groups and the total salmonid catch. Using lights above the separator enhanced 

separation efficiency for all small salmonid groups, although the relationship was significant 

only for the total small salmonid catch and for the total salmonid catch. Because use of the insert 

did not result in a biologically significant increase in descaling or adverse stress levels compared 

to the McNary Dam operational condition, it was installed for use in the Lower Monumental 

Dam operational separator at the beginning of the 2002 juvenile migration. 

In addition to studies aimed at improving the function and design of existing operational 

separators, a second approach explored alternatives to the existing separator design. The most 

promising alternative concept to emerge from interagency brainstorming sessions was the highvelocity flume (HVF) design. Under this strategy, smolts enter a section of open flume directly 

after transport from the bypass channel. While traveling at velocities not normally present in 

current operational separator designs (1–2 ms

-1), smaller smolts could sound between 

appropriately spaced separation bars within the flume, effecting separation from larger smolts 

unable to fit between the bars. Both groups would continue to different holding areas without 

the interruption caused by abrupt velocity reduction of incoming transport flow, and without 

migration timing delays, stress, and fatigue induced by combating flows within the separator. 

Results using an evaluation HVF separator at McNary Dam during the 1997 and 1998 

juvenile salmonid migration periods indicated that over 80% separation could be achieved for the 

total catch of all species combined (McComas et al. 2000, 2001). Based on these observations, a 

full-scale prototype HVF separator was constructed at Ice Harbor Dam.

Results from cumulative evaluations of design and operational criteria using the 

prototype HVF separator from 1999 through 2001 suggest that separation efficiency values of 

over 80% are possible for the total salmonid smolt catch, with moderate (5.3%) overall 

descaling, and virtually no timing delays in the separator unit (McComas et al. in prep. b). 

However, most research to date has been conducted when fish abundance was relatively low. 

Studies are now proposed to test HVF separators under conditions of high fish abundances at Ice 

Harbor Dam. In addition, other studies have proposed a method for removing adults, large 

incidental fish, and debris prior to entering the smolt section of the HVF separator (McComas 

43
et al. in prep. a). Although no HVF separators are in operational use at juvenile bypass facilities 

in FCRPS dams, work is ongoing to complete the development of this concept as a possible 

option to, or replacement for, the current separator design. 

Conclusions 

1. Small yearling Chinook salmon smolts appear to be stressed when in the presence of larger 

steelhead smolts. Separation may be necessary to reduce extended holding of small fish with 

larger fish, especially in the context of transportation. 

2. Past research has demonstrated that separation efficiencies in excess of 80% can be attained 

with a high-velocity flume separator concept. Research continues to further refine this 

method of separation for potential use in mechanical screen bypass systems. 

Water Temperature Effects 

Background 

During the summer, juvenile salmonids migrating through the Columbia and Snake rivers 

encounter water temperatures that often exceed the 20EC (68EF) threshold mandated in the 

Washington State Clean Water Act (WAC 1992). Of greater concern, Columbia and Snake 

River temperatures exceeding the 25EC (77EF) upper incipient lethal water temperature for 

salmonids (Brett 1952) have been documented during the summer juvenile migration period. In 

recent years, considerable effort has been expended to investigate the effects of and to identify 

potential measures to decrease high summer river temperatures. 

The NMFS 2000 Biological Opinion (NMFS 2000) includes three Reasonable and 

Prudent Alternative (RPA) actions pertinent to the temperature issue and juvenile salmonids. 

The first, RPA Action 141, calls for the action agencies to investigate the relationships among 

elevated water temperature, fish disease, and fish mortality during critical migration periods. 

The second, RPA Action 142, directs COE to identify and implement measures to address 

juvenile fish mortality associated with high summer water temperatures at McNary Dam. The 

third, RPA Action 143, calls for the action agencies, in consultation with other regional entities, 

to develop and coordinate a plan to model the effects of alternative Snake River operations (flow 

augmentation). 

Results 

Several studies have addressed RPA Action 141 so far (effects of temperature on fish 

disease and mortality). In the summer of 2001, researchers sampled subyearling Chinook 

salmon at McNary Dam on the Columbia River to assess the prevalence of the bacterial 

pathogens Flexibacter columnaris and Renibacterium salmoninarum (Rs; the causative agent of 

bacterial kidney disease) and to measure relative levels of heat-shock proteins in liver tissue 

(Tiffan et al. 2003a). These authors reported: 

1) of 181 fish sampled, 2 tested positive for F. columnaris; 

2) of 61 fish necropsied, 8 had kidneys characteristic of infection by Rs; 

44
3) of 120 fish from which kidney samples were collected and analyzed by an enzyme-linked 

immunosorbent assay (ELISA), none were positive for Rs; and 

4) results from heat-shock protein sampling indicated that exposure to water temperatures 

above 22EC (71.6EF) evoked a strong peak in this physiological parameter. 

In the summer of 2002, Haskell et al. (2003) conducted a second year of study at McNary 

Dam, again assessing the prevalence of bacterial pathogens and levels of heat-shock protein. 

These authors reported: 

1) no F. columnaris was observed in 599 fish randomly sampled, although 3 fish found 

during smolt monitoring operations tested positive for the pathogen; 

2) low Rs levels were detected in 4 of 96 fish sampled; and 

3) a mid-August peak in heat-shock protein levels coincided with a water temperature 

exceeding 23°C (73°F). 

Tiffan et al. (2003a) and Haskell et al. (2003) characterized the incidences of bacterial 

pathogens as low and recommended laboratory studies be conducted to quantify short- and longterm effects of elevated heat-shock protein levels. In laboratory tests conducted by Mesa et al. 

(2002), effects of a single, acute thermal stress on survival, predator avoidance, and physiology 

of juvenile fall Chinook salmon obtained from the Hanford Reach of the Columbia River were 

studied. They found that the thermally-stressed fish showed little direct mortality and no 

increases in vulnerability to predation. Marked physiological responses of fish to the 

temperature stress were noted, including transient increases in plasma cortisol, glucose, and 

lactate, as well as dramatic and persistent increases in levels of liver heat-shock protein 70. 

In response to a catastrophic loss of subyearling Chinook salmon at the McNary Dam 

juvenile fish facility in the summer of 1994, the NOAA Fisheries Service 1995 BiOp required 

the action agencies to take measures to reduce the potential of a recurrence of the incident 

(NMFS 1995). In mid-July 1994, the water temperature at the McNary Dam juvenile fish 

facility reached 20EC (68EF). A few days later, the facility experienced a catastrophic loss of 

subyearling Chinook salmon over a period of several days, coincident with water temperatures 

exceeding 21EC (70EF). In prior years, large thermal gradients between operating and 

nonoperating turbine units were observed and thought to be associated with instances of fish 

mortality. In the days leading up to the 1994 incident, although thermal profile data showed 

elevated water temperatures, no large gradients were present (Hurson et al. 1996). In a later 

analysis, Coutant (1999) suggested that the cause of acute summer mortalities at McNary Dam

was cumulative exposure to high temperature water received over a period of several days. 

Occurrences of acute mortality on the scale of the 1994 loss over several days have not recurred 

at McNary Dam. For example, total juvenile fish facility mortality during the summer at 

McNary Dam was 2.2% during both 1997 and 1998, with a one-day peak mortality rate of 

approximately 8.0% in early July 1998 (Hurson et al. 1999). For the most recent 5-year period 

(1999 through 2003), summer fish facility mortality has averaged 1.0%, 0.6%, 0.7%, 0.7%, and 

1.0% for the respective years, with no notable daily mortality peaks (data available at 

www.fpc.org). 

45
Continuing concern over the McNary Dam temperature issue resulted in inclusion of 

RPA Action 142 in the NOAA Fisheries Service 2000 BiOp (NMFS 2000). RPA Action 142 

directs the COE to identify and implement measures to address juvenile fish mortality associated 

with high summer temperatures at McNary Dam. Enhanced temperature monitoring has been in 

place at McNary Dam since the 1990s to monitor temperatures in the forebay, across the 

powerhouse, in gatewells, at locations within the collection channel, and at the fish monitoring 

facility. Temperature gradients within the system are known to occur when turbine units at the 

south end of the powerhouse are put into operation following an off-line period. Surface waters 

are warmer in the south forebay than in the forebay upstream from turbine units at the north end 

of the powerhouse. Turbine units at the south end of the forebay draw this warmer surface water 

into the bypass system via the gatewells, creating temperature differentials in the collection 

channel and increasing temperature in downstream portions of the bypass system. 

To the extent possible, COE limits occurrence of temperature increases through 

operational means, that is, by restricting operation of the south end turbines (Units 1, 2, and 3), 

and by loading the powerhouse from the north end. Among the recommendations in RPA Action 

142 is investigating the feasibility of developing a hydrothermal computational fluid dynamics 

model of the McNary forebay. Model development is currently underway through a contract 

with the University of Iowa.4

 The completed model will allow different operations (especially 

the 50% McNary turbine capacity increase) and potential structural mitigation measures to be 

evaluated. 

In 2001 two large industrial mixers were installed in the forebay upstream from turbine 

units 1 and 2. Evaluation of the cooling effect of mixers on water column temperatures was 

conducted by Tiffan et al. (2003a). Results indicated that the mixers were ineffective, decreasing 

temperature inside the mixer plume and in gatewells by only 0.5EC (0.9EF) and 0.1EC (0.2EF), 

respectively. In 2002 Haskell et al. (2003) operated turbine unit 1 experimentally to determine 

effects of turbine operation on water temperatures. Results of this study (Haskell et al. 2003) 

showed that turbine unit 1 gatewell temperatures increased by up to 3EC (5EF) upon starting the 

unit, followed by a gradual temperature decline lasting two or more hours. Haskell et al. (2003) 

released subyearling Chinook salmon tagged with temperature-sensing radio transmitters into the 

bypass system. Tagged fish were exposed to thermal gradients of up to 3EC (5EF) when turbine 

unit 1 was started, tagged fish resided in gatewells for a mean time of 7.8 hours, and fish tended 

to hold in water less than 5 m deep within the gatewells. Haskell et al. (2003) concluded that 

fish condition, prior stress, and acclimation temperature could affect capacity to respond to 

temperature increases such as those observed in the gatewells of turbine unit 1 and recommended 

additional work to explore the effects on survival and performance. 

Because juvenile migrant salmonids experience high summer water temperatures during 

migration through the lower Snake River, the effect of elevated water temperatures on ESAlisted Snake River fall Chinook salmon is a primary regional concern. This issue has been 

addressed through a number of biological field studies examining effects of flow augmentation 

and through RPA Action 143 of the NOAA Fisheries Service 2000 Biological Opinion (NMFS 

 

4

 P. Ocker, COE, Portland, OR. Pers. commun., November 2003. 

46
2000). Williams et al. (2005) provides additional information on the effects of water temperature 

on the survival of juvenile migrants in the FCRPS. 

Dauble et al. (2003) estimated that Snake River adult fall Chinook salmon are currently 

restricted to about 20% of the Snake River habitat that was available prior to dam construction. 

Brownlee Dam, a high water storage dam in Hells Canyon, has altered annual temperature cycles 

below it in the lower Snake River. Connor et al. (2002) noted that the presently available 

spawning and rearing habitat is cooler and has lower growth potential than areas favored by the 

species prior to hydroelectric development. They suggested that slower development of fall 

Chinook salmon to migrational size leads to delayed seaward migration, resulting in an unnatural 

exposure of smolts to elevated summer water temperatures in the Snake River. 

A number of studies have investigated the biological consequences and temperature 

effects of flow augmentation. Flow augmentation using relatively cool water from Dworshak 

Dam is intended to increase migration speed and survival of smolts through the Lower Granite 

Dam reservoir. Connor et al. (1998) PIT-tagged subyearling Chinook salmon in the Snake River 

from 1992 to 1995 and monitored PIT-tag detections at Lower Granite Dam. They concluded 

that detection rate at Lower Granite Dam was positively related to mean summer flow and 

negatively related to maximum summer water temperature, supporting the concept that summer 

flow augmentation is a beneficial interim recovery measure for improving the survival of 

subyearling fall Chinook salmon in the Snake River. 

From 1992 to 2001, Connor et al. (2003a) used mark-recapture data to study migrational 

behavior of wild subyearling Chinook salmon in the Snake River. Results suggested that flow 

augmentation increases the rate of seaward movement during one of four migrational phases and 

that augmentation likely prevents reversion to parr. Connor et al. (2003b) evaluated the results 

from wild subyearling Chinook salmon tagged and released in the Snake River from 1998 to 

2000 and concluded that survival of these fish generally increased with increasing flow and 

decreased with increasing temperature. 

Tiffan et al. (2003b) tagged naturally produced subyearling Chinook salmon with 

temperature-sensing radio tags and tracked the fish in the Little Goose Dam reservoir on the 

Snake River during the summers of 1998 and 1999. They determined that temperatures selected 

by juvenile migrants were not significantly different from mean water temperatures and no areas

of thermal refugia existed in the reservoir. Further, they suggested that augmentation with coldwater releases from Dworshak Dam may increase survival by lowering water temperatures by up 

to 4EC (7EF). Smith et al. (2003) studied survival and travel time of Lyons Ferry Hatchery 

subyearling Chinook salmon from 1995 to 2000. These researchers determined that survival 

from release locations in the free-flowing reach of the Snake River to the tailrace of Lower 

Granite Dam was significantly correlated with flow, water clarity, and temperature. Survival 

decreased as flow decreased, as water clarity increased, and as temperature increased.

Smith et al. (2003) were unable to determine which of the three factors were most 

important, because the factors were highly correlated among themselves. They concluded that 

augmenting Snake River summer flow with relatively cool water will increase discharge while 

decreasing temperature of the river and will likely increase the speed of seaward migration, 

thereby benefiting the recovery of ESA-listed Snake River fall Chinook salmon. 

47
RPA Action 143 directs the action agencies, in consultation with other regional entities, 

to develop and coordinate a plan to model the effects of alternative Snake River operations (flow 

augmentation). Implementation of RPA Action 143 has occurred through the NOAA Fisheries 

Service Regional Forum’s Water Quality Team. The Water Quality Team technical workgroup 

addressing RPA Action 143 is a cooperative effort, open to all regional entities. The RPA 143 

Workgroup has met since March 2002 and has recently produced a comprehensive report 

detailing their progress toward meeting RPA Action 143 (NMFS 2003). Included in the report, 

and of particular interest to those considering the biological implications of Snake River flow 

augmentation, is an evaluation of biological needs including various life cycle stages of ESAlisted anadromous salmonids in the Snake River from Hells Canyon Dam downstream to the 

Columbia River confluence and in the Clearwater River from Dworshak Dam downstream to the 

Snake River confluence.

Other studies have also addressed RPA Action 143. Connor et al. (2000) described a 

method to forecast survival and cumulative percent passage for subyearling Chinook salmon at a 

dam, thereby allowing managers to effectively time water releases. Cook et al. (2003) monitored 

thermal conditions at the confluence of the Clearwater and Snake rivers and in the Lower Granite 

Dam reservoir and applied the results to a three-dimensional hydrodynamic and water quality 

model. They also applied a two-dimensional, laterally-averaged hydrodynamic water quality 

model to the three Snake River reservoirs downstream from Lower Granite Dam.

Conclusions 

1. During the summer, juvenile anadromous salmonids migrating through the Columbia and 

Snake rivers encounter water temperatures that often exceed the 20EC (68EF) threshold 

mandated in the Washington State Clean Water Act. Temperatures exceeding the 25EC 

(77EF) upper incipient lethal water temperature for salmonids have also been documented 

during the summer juvenile migration period. 

2. High summer water temperatures caused a catastrophic mortality within the juvenile 

collection facility at McNary Dam in the summer of 1994. The mortality was thought to 

have been the result of operating turbine units at the south end of the powerhouse, which are 

supplied with warmer water. Since then, modifications to powerhouse operations and 

improved temperature monitoring have presumably prevented a recurrence, with average 

mortality rates during the most recent five summers ranging between 0.6% and 1.0%.

3. The two common fish pathogens, F. columnaris and R. salmoninarum, appear to have been 

of little concern relative to survival of subyearling Chinook salmon within the bypass system

at McNary Dam during two recent summers. 

4. To date, a single laboratory study found that thermally-stressed subyearling Chinook salmon 

showed little direct mortality and no increase in vulnerability to predation, even though test 

fish responded physiologically to the single, acute thermal stress. 

5. In the Snake River, colder than normal water temperatures in spring delay development of 

subyearling Chinook salmon, resulting in later-than-normal summer migrations that coincide 

with increasing water temperatures. However, once migrating, these fish appear to respond 

positively in terms of their migration and survival rates due to flow augmentation from 

Dworshak Dam.

48
Effects of Bypass Systems on Smolt Condition and Survival 

Background 

Over the last 30 years, many improvements have been incorporated into existing juvenile 

bypass systems at Columbia and Snake River dams (Williams and Matthews 1995), and new 

systems have also been constructed. This work intensified during the 1990s, with existing 

bypass systems at Little Goose (1990), McNary (1994), and John Day Dams (1998) replaced 

with upgraded facilities, and completely new facilities built at Lower Monumental (1993) and 

Ice Harbor Dams (1996), and at the Bonneville Dam Second Powerhouse (1999). The Lower 

Granite Dam facility, which was completed as a state-of-the-art facility in 1975, received major 

structural improvements during the early 1980s, but was never completely upgraded or rebuilt. 

Each of these new facilities was tested to locate and correct any areas within the structures, 

flumes, and pipes that might cause descaling, injury, or mortality to juvenile fish passing through 

them. In addition, the region’s state and federal fisheries agencies monitor daily samples of the 

fish for descaling, injury, and mortality to ensure that these facilities continue to be operated in a 

manner safe for fish. Starting in 1993, all hatchery spring/summer Chinook salmon from the 

Snake River were adipose-fin clipped, which allowed hatchery fish to be distinguished from wild 

fish. Fish condition at the Snake River dams and at McNary Dam on the Columbia River is 

detailed in Hurson et al. (1996) and Hurson et al. (1999). More recent fish condition information 

is collected as part of the annual juvenile fish transportation program at each of the Snake River 

dams and at McNary Dam on the Columbia River.5

 This information is summarized below. 

Results 

Generally, passage through the fish bypass systems on the Snake River resulted in high 

levels of fish condition, based on observations of low rates of descaling or direct mortality (Table 

6). Descaling ranges from a low of 1.5% for wild yearling Chinook salmon at Lower Granite 

Dam to a high of 10.1% for the more easily descaled sockeye salmon at Little Goose Dam. It is 

noteworthy, however, that overall descaling is similar for the older Lower Granite Dam facility 

and the new Lower Monumental Dam facility and is lower at both of these facilities than at the 

recently upgraded Little Goose Dam facility. Mortality during passage through the Snake River 

facilities follows a similar pattern as descaling, ranging from a low of less than 0.1% for hatchery 

and wild steelhead at Lower Granite Dam to a high of 6.1% for subyearling Chinook salmon at 

Little Goose Dam. As with descaling, overall mortality tends to be higher at Little Goose Dam

than at Lower Granite or Lower Monumental Dams. 

The juvenile bypass system at Ice Harbor Dam was completed and evaluated in 1996. 

Hatchery steelhead and yearling Chinook salmon released at two locations in the collection 

channel were recaptured and examined. Descaling and mortality for both species were less than 

0.2% (Table 6) (Gessel et al. 1997). At this facility, the mechanical screen bypass system

operates continuously throughout the migration season, while the sample facility operates 

intermittently to determine whether descaling and injury rates have increased or are excessive. 

 

5

 D. Hurson, COE, Walla Walla, WA. Pers. commun., November 2003. 

49
50

While this provides a cursory examination of descaling and mortality annually, the small 

numbers of fish examined minimize the usefulness of any data collected. 

Recent descaling and mortality rates for fish passing through bypass systems on the lower 

Columbia River are presented in Table 7. As at the Snake River dams, negative effects of 

passage through these systems appear to be minor in most cases. At McNary Dam, the first dam 

that fish encounter after exiting the Snake River, overall descaling rates appear to be slightly 

higher than for the Snake River dams. Conversely, direct mortality appears to be similar 

between the two areas. 

At John Day Dam, measures of bypass system survival were comprised of mortality 

observations at the sampling facility (Martinson et al. 2003) and data gathered during a bypass 

system evaluation in 1998 and 1999 (Absolon et al. 2000a, 2000b). As part of this evaluation in 

1998, hatchery Chinook salmon yearlings and steelhead were released at various points within 

the system. In 1999, three groups of Chinook salmon fry were also released into the upstream 

end of the elevated flume and recaptured in the sample tank. Based on measurements taken at 

the sampling facility, descaling at John Day Dam was similar to that measured at McNary Dam 

and at the Snake River dams. Based upon data gathered during the bypass system evaluation, 

direct mortality was also similar to that measured at the other dams. For the Chinook salmon fry 

releases in 1999, no descaling or injuries were observed on any of the recaptured fish. 

At Bonneville Dam, descaling in both the First and Second Powerhouses was again 

within the ranges observed at all of the upstream dams. Direct mortality at sampling facilities in 

the First and Second Powerhouses was noted by Martinson et al. (2003). Again, these values 

were similar to values observed at upstream dams. 

Although these estimates give some measure of the upper limit of descaling and direct, 

immediate bypass system passage mortality, they cannot reflect mortality through the entire 

systems because sampling locations are typically some distance upstream from the outfalls. 

Also, some mortality observed within a system may have resulted from prior injuries, and, 

conversely, some live fish observed in samples may die of passage effects later. 
51

Table 6. Average descaling and mortality rates for juvenile salmonids passing through fish bypass systems at Snake Rive

2002. 

Descaling Mortalit

Chinook Steelhead Other Chinook 

Facility 

Yearling 

hatchery 

Yearling 

wild Subyearling Hatchery Wild Sockeye Coho

Yearling 

hatchery 

Yearling 

wild Subyearling Ha

Lower Granite 2.7 1.5 2.8 4.1 1.7 5.7 3.0 0.3 0.3 1.2 <Little Goose Dam 6.2 4.9 3.2 6.7 3.5 10.1 6.4 0.6 0.7 6.1 Lower Monumental Dam 

3.1 2.5 2.3 4.5 2.7 3.7 3.9 0.2 0.2 2.3 

Ice Harbor Dam * * * * * * * * * * * Descaling and mortality for all species and rearing types measured were less than 0.2% for the 1996 juvenile bypass system evaluatioTable 7. Average descaling and mortality rates for juvenile salmonids passing through fish bypass systems at lower Colu

1998 to 2000. 

Descaling Mortality

Chinook Steelhead Other Chinook Steelh

Facility Yearling Subyearling Hatchery Wild Sockeye Coho Yearling Subyearling Hatchery 

McNary Dam 6.4 2.9 5.7 3.7 9.6 3.9 0.2 1.0 0.4 

John Day Dam 3.9 1.1 6.1 1.9 8.2 2.5 0.5 0.2 0.3 Bonneville Dam First Powerhouse 

5.0 1.7 9.6 4.2 20.3 3.6 0.2 0.4 0.1 

Bonneville Dam Second Powerhouse* 

2.5 0.5 5.5 1.9 6.6 1.0 0.8 0.5 0.4 

* The figures for Bonneville Dam Second Powerhouse are for the years 2000–2002, because the juvenile bypass system modifications b

2000. 
Discussion 

Today’s juvenile bypass systems are vastly improved compared to those that existed 

during the 1970s and early 1980s. The descaling and injury and system mortality rates for all of 

the above facilities are substantially lower than those reported by Williams and Matthews (1995), 

which were reported primarily for fish collected at the then state-of-the-art facility at Lower 

Granite Dam. This is in spite of the fact that the criteria used to measure descaling are far more 

comprehensive today than during the earlier years. In addition, recent post-handling and 

-marking mortality rates are greatly reduced compared to those reported by Williams and 

Matthews (1995). Marsh et al. (1996a, 1996b, 1997, 2000, 2001, 2003, 2004) and Harmon et al. 

(2000) reported annual 24-hour post-handling and -marking mortality rates ranging between 

0.2% and 1.6%, with most values below 1.0%. These values differ little from those reported as 

bypass system mortality rates for fish that were bypassed, but not handled and marked. 

Passage survival studies conducted by NOAA Fisheries Service at Bonneville Dam from

1987 through 1990 and in 1992 involved releases of differentially marked subyearling Chinook 

salmon into various passage routes at Bonneville Dam, including the Second Powerhouse bypass 

system (1987 to 1990) and the First Powerhouse bypass system (1992). These studies used fish 

identified with freeze brands and marked with coded-wire tags. Relative short-term survival for 

treatment groups was based on data from recapture of juvenile test fish downstream at Jones 

Beach (RKm 74) during seaward migration. Relative long-term survival for test fish using the 

various passage routes was to have been based on recoveries of coded-wire tags from fisheries, 

hatcheries, and spawning ground surveys. 

Results from the above survival studies were reported in Dawley et al. (1988, 1989), 

Ledgerwood et al. (1990, 1991, 1994), and Gilbreath et al. (1993). Results from Second 

Powerhouse tests conducted from 1987 to 1990 indicated that relative survival of bypassreleased groups averaged 8.3% less than reference groups released in the tailrace immediately 

downstream from the dam, 7.6% less than turbine releases, and 17.4% less than groups released 

approximately 3.2 km downstream from the dam (Dawley et al. 1998a). Using a similar 

methodology, subyearling Chinook salmon were released at the First Powerhouse in 1992 

(Ledgerwood et al. 1994). Results indicated that relative survival of bypass-released groups was 

11.8% less than turbine-released groups and 28.3% less than groups released approximately 3.2 

km downstream from the dam.

Due to the unexpected low survival of bypass-released groups, a separate evaluation was 

conducted at the Second Powerhouse bypass system from 1990 to 1992 (Dawley et al. 1998a). 

In these studies, referred to as direct assessments of passage survival, a trap-net attached to the 

submerged outfall was used to recapture study fish immediately upon exit from the bypass 

system. Results indicated that the bypass system was not responsible for the considerable 

survival differences noted in the previous paragraph, although descaling, injury, and stress 

among release groups did increase as the distance traveled through the bypass system increased. 

It was hypothesized that increased stress and fatigue of fish traveling through the bypass system, 

in combination with a poor outfall location, resulted in high predation rates on juvenile 

salmonids exiting the bypass system. Hence outfall location and configuration are important 

considerations for maximizing survival of bypassed juvenile salmonids. 

52
Based on the results of these studies, the COE modified the bypass system at the Second 

Powerhouse in 1999. The collection channel was modified to improve hydraulic conditions, a 

new dewatering facility that uses floor and wall screens was installed, a new 2.7-km-long 

conveyance pipe was installed, and outfalls were constructed at Hamilton Island, well 

downstream of Bonneville Dam. Gilbreath and Prentice (1999) evaluated the new bypass 

system. They found that none of the hatchery-reared steelhead that traversed the system died or 

were descaled or injured. For run-of-the-river steelhead, none showed adverse signs of passage 

effects other than a 0.3% mortality rate. Median passage time from the upstream end of the 

collection channel to Hamilton Island was about 3 hours for hatchery-reared steelhead and about 

1 hour for run-of-the-river steelhead. The median passage time through the length of the 

collection channel was about 1 hour for hatchery-reared steelhead, compared to only a few 

minutes for run-of-the-river steelhead. For hatchery-reared yearling Chinook salmon, none of 

the fish released were injured or descaled. However, mortality was 0.1%, which the researchers 

attributed to handling and tagging. For run-of-the-river yearling Chinook salmon, descaling was 

2.0% and mortality was 0.5% after passage through the new system. 

Median passage time from the upstream end of the collection channel to Hamilton Island 

was about 46 minutes for both hatchery-reared and run-of-the-river Chinook salmon. The 

median passage time through the length of the collection channel was about 4 and 7 minutes for 

hatchery-reared and run-of-the-river Chinook salmon, respectively. During the summer smolt 

migration period, run-of-the-river subyearling Chinook salmon were released to pass through the 

system. Descaling and injury was nil and mortality was 0.1%. Median passage time was 5 

minutes through the length of the collection channel and 42 minutes from the upstream end of 

the collection channel to Hamilton Island. 

Finally, recent PIT-tag data indicate that passage through multiple or individual bypass

systems can reduce smolt-to-adult return rates (SARs) of hatchery-reared, but not wild, smolts 

compared to those that were never detected in any bypass system during downstream passage. 

The exception is for smolts that were only detected at Lower Granite Dam (Sandford and Smith 

2002). These fish consistently returned as adults at equal or higher rates than those not detected 

in any of the bypass systems during downstream passage. There is also evidence that juvenile 

bypass systems tend to select fish that are smaller than those represented in the general 

population (Zabel et al. in prep.), and that smaller fish survive at lower rates than larger fish 

(Zabel and Williams 2002). This confounds the causal relationship between decreased SARs and 

detection history at dams. A more detailed review of this subject is presented Williams et al. 

(2005). 

Conclusions 

1. Contemporary mechanical screen bypass systems are vastly improved compared to the 

original systems that operated during the 1970s and early 1980s. Recent very low descaling, 

injury, and system-mortality rates are testimonials to the improved conditions for juvenile 

migrants using these passage routes. 

2. Outfall location and configuration are important considerations for maximizing survival of 

juvenile salmonids that are bypassed back to the river below dams. 

53
3. SAR rates for PIT-tagged in-river migrants are related to bypass history. Smolts detected at 

dams other than Lower Granite Dam often return as adults at lower rates than fish not 

detected at any dams. For hatchery (but not wild) smolts, detection frequency is inversely 

related to SAR. A confounding factor is that smaller smolts with lower SARs tend to be 

more easily guided into bypass systems than larger smolts with higher SARs. 

Effects of Bypass Systems on Blood Chemistry 

Background 

Stress agents, or stressors, are external and internal stimuli that induce quantifiable 

biochemical responses in higher animals, including fish (Hane et al. 1966, Grant and Mehrle 

1973). These physiological responses prepare animals for the “fight or flight response,” which is 

the first stage of the general adaptation syndrome otherwise known as stress (Selye 1956). 

Stress, then, is an adaptive response by an animal to any of an innumerable number of 

environmental stressors. The response enhances the probability of survival; therefore, it is not 

necessarily negative, unless the stress is chronic in nature or the animal has been physically 

injured or forced to exhaustion. The primary stress response involves changes in blood plasma

concentrations of cortisol and adrenaline, which influence levels of secondary indicators 

including lactate, glucose, liver glycogen, leucocyte count, free fatty acids, and the balance of 

various electrolytes (Mazeaud et al. 1977). 

Results 

Several stressors related to passage through fish bypass facilities at hydroelectric dams

have been shown to alter concentrations of stress indices in juvenile salmonids under 

experimental conditions. For example, elevated plasma cortisol and glucose levels have been 

associated with crowding and handling (Wedemeyer 1976, Congleton et al. 1984), descaling 

(Gadomski et al. 1994), acclimation temperature (Barton and Schreck 1987a), and confinement 

(Strange et al. 1978). 

Under field conditions, Maule et al. (1988) among others demonstrated that juvenile 

Chinook salmon plasma cortisol and glucose concentrations could increase cumulatively as the 

fish passed successive points in a bypass system, corroborating similar results from laboratory 

work with sequential handling (Barton et al. 1986, Congleton et al. 1999). However, Congleton 

et al. (1999) found that, generally, peak plasma cortisol levels in fish following handling 

experiments did not appear to be affected by repeated stressful experiences. They point out it is 

possible that fish in the multiple stress groups experienced some form of chronic, low-level 

stress, which was masked by the elevated pre-stress plasma cortisol levels in these groups. 

Lactate, also used as a stress index, has been shown to increase in response to the stress 

of exertion (Mesa and Schreck 1989, Gadomski et al. 1994). As with glucose and cortisol, blood 

plasma concentrations of lactate can rise dramatically with exposure to this stressor, returning to 

preexposure levels after several hours following suspension of, or acclimation to, the stimulus. 

Blood plasma indicators have been used routinely as stress indices during the fish bypass 

system evaluation process for upgraded facilities constructed at FCRPS projects since 1990. At 

54
55

each dam, blood samples were collected from Chinook salmon and steelhead juveniles at 

successive points within the bypass system. Because blood plasma indicator levels in fish 

captured from a gatewell are similar to levels found in hatchery-reared (native) smolts 

(Congleton et al. 1984), results were compared to samples taken from smolts collected 

concurrently from gatewells. This process was repeated several times through the smolt 

migration to include populations originating from different places and changes in other 

environmental factors. In all cases, blood plasma concentrations of cortisol, glucose, and lactate 

were measured as indices of stress resulting from passage through the facility. Mean levels of 

indicators (averaged across the entire smolt migration) are included in Table 8 by facility for 

each species evaluated. 

Results from these studies varied by site. For yearling Chinook salmon, there were 

significant increases in plasma cortisol and glucose concentrations at Little Goose (Monk et al. 

1992b), Lower Monumental (Marsh et al. 1995), and John Day Dams (Absolon et al. 2000a) as 

fish passed from the gatewells through the sample tanks, while no significant differences were 

reported for any indicator levels from Ice Harbor (Gessel et al. 1997) and McNary (Marsh et al. 

1996a) Dams. Lactate levels increased significantly only at Little Goose Dam. Subyearling 

Chinook salmon were evaluated only at McNary Dam and showed a change only for mean 

lactate values, which declined significantly before returning to gatewell levels in raceways. 

Steelhead plasma cortisol concentrations increased significantly through all bypass 

systems except at McNary Dam, and significantly elevated levels of plasma glucose were 

recorded at Little Goose, Lower Monumental, and McNary Dams. Mean lactate concentrations 

in steelhead showed significant increases only at Little Goose and Lower Monumental Dams. 

Discussion 

In general, blood plasma stress indices are within the normal range of expected values, as 

fish pass through current bypass systems. In cases where levels were measured following the 

bypass process (e.g., from raceways), stress indicator concentrations returned to near gatewell 

levels within a few hours of raceway residence. 

Results to date suggest that the physiological effects of passage through fish bypass 

facilities on juvenile Chinook salmon and steelhead are nominal, as measured by blood plasma 

stress indices, and follow a typical sequence for fish subjected to an acute stressor followed by 

acclimation to or removal of the stress (Wedemeyer 1976, Mazeaud et al. 1977). Several 

researchers have reported that effects of stress as measured by hepatic plasma indicators 

diminish within 24 hours following removal of the stressor in a protected environment 

(Gadomski et al. 1994, Schreck and Knoebl 1997, Sharpe et al. 1998). For example, transported 

fish showed an initial increase in plasma cortisol levels immediately after loading onto a barge, 

but the effect abated to preloading levels after 3 hours (Maule et al. 1988). 
Table 8. Recent juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) mean blood plasma str

concentrations by juvenile fish facility sample location for COE-operated hydroelectric projects on the Columbirivers. 

Blood plasm

concen

Hydroelectric project 

Evaluation year Authority Species Sample location

Cortisol 

(ng/ml) 

Glu

(mg

John Day 1998 Absolon et al. (2000a) Yearling Chinook salmon Gatewell 103.2 63

Pre-separator 151.6 76Pre-sample tank 160.1 73

Steelhead Gatewell 98.8 88

Pre-separator 192.7 81

McNary 1994 Marsh et al. 1996a Yearling Chinook salmon Pre-sample tank 179.0 107

Gatewell 92.5 93Post-dewaterer 97.5 97

Separator 102.4 91

Raceway (0-h) 89.5 87Raceway (2-h) 98.4 93Raceway (4-h) 84.7 86Raceway (6-h) 79.8 88

Raceway (10-h) 92.3 103

Steelhead Gatewell 83.2 153

Post-dewaterer 84.4 108

Separator 90.5 109

Raceway (0-h) 101.2 132Raceway (2-h) 108.0 137Raceway (4-h) 86.9 132Raceway (6-h) 75.8 131

Raceway (10-h) 100.9 161

56
Table 8. Continued. Recent juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) mean bloo

indicator concentrations by juvenile fish facility sample location for COE-operated hydroelectric projects on the Snake rivers. 

Blood plasm

concen

Hydroelectric project 

Evaluation year Authority Species Sample location

Cortisol 

(ng/ml) 

Glu

(mg

McNary, cont. 1994 Marsh et. al 1996a Subyearling Chinook salmon Gatewell 57.2 92

Post-dewaterer 82.6 85

Separator 75.9 88Raceway (0-h) 84.4 73Raceway (2-h) 70.8 82

Marsh et al. 1996a Subyearling Chinook salmon Raceway (4-h) 75.6 80

Raceway (6-h) 68.1 87Raceway (10-h) 88.3 89

Ice Harbor 1995 Gessel et al. 1997 Yearling Chinook salmon Gatewell 140.3 105

Pre-separator 140.7 95Pre-sample tank 135.9 97

Steelhead Gatewell 101.0 131

Pre-separator 165.5 117Pre-sample tank 188.8 112

Lower 1993 Marsh et al. 1995 Yearling Chinook salmon Gatewell 115.7 62Monumental Post-dewaterer 144.6 68

Pre-separator 144.7 81

Raceway (0-h) 152.3 103Raceway (2-h) 127.2 110Raceway (4-h) 121.4 111Raceway (6-h) 110.3 108

Raceway (10-h) 166.6 104

Steelhead Gatewell 84.4 119

Post-dewaterer 154.2 116

Pre-separator 184.0

6

127

147

Raceway (0-h) 138. 

57
58

Table 8. Continued. Recent juvenile Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) mean bloo

indicator concentrations by juvenile fish facility sample location for COE-operated hydroelectric projects on the Snake rivers. 

Blood plasm

concen

Hydroelectric project 

Evaluation year Authority Species Sample location

Cortisol 

(ng/ml) 

Glu

(mg

Raceway (2-h) 173.1 14Raceway (4-h) 69.3 14

Lower Monumental, cont. Raceway (6-h) 103.2 13

Raceway (10-h) 174.1 14

Little Goose 1990 Monk et al. 1992b Yearling Chinook salmon Gatewell 75.7 9

Post-dewaterer 77.3 8Pre-separator 112 8

Raceway (0-h) 140.7 10Raceway (2-h) 160.5 16Raceway (4-h) 129.4 15Raceway (6-h) 85.5 15

Raceway (10-h) 81.7 16Pre-barge 79.4 10

Steelhead Gatewell 42.2 13

Post-dewaterer 114.5 12

Pre-separator 142.3 11Raceway (0-h) 125.1 10Raceway (2-h) 91.2 15Raceway (4-h) 61 14Raceway (6-h) 65.3 12Raceway (10-h) 103.5 14Pre-barge 125.4 14
The relationships between physiological indicators of bypass-induced stress and 

subsequent survival during seaward migration are not well-documented or understood. There is 

evidence that short-term survival may be directly impaired as a result of stress in poor-quality 

Chinook salmon smolts. Under controlled conditions, Barton et al. (1986) demonstrated that 

multiple acute disturbances, similar to those encountered during the bypass process, resulted in 

cumulative physiological responses. Plasma indicator levels of healthy animals returned to 

control levels within 30 hours with no mortality after three stress events, while the same

treatment for unhealthy fish resulted in 50% mortality within 3–6 hours after the third event. 

Indirectly, bypass stress may also somewhat impair survival during subsequent migration. 

Barton and Schreck (1987b) found a positive relationship between metabolic rate and the plasma

cortisol level in stressed fish. They concluded that even relatively minor events can reduce 

available energy stores in fish by as much as one quarter, leaving the animal with substantially 

fewer reserves to cope with future environmental challenges such as temperature adaptation, 

disease, and demands on swimming stamina. On the other hand, Mesa et al. (2000b) found that 

imposition of an acute stressor on fish heavily infected with Rs did not lead to higher infection 

levels or increased mortality relative to infected but unstressed fish. 

As with all animals, weaker, more vulnerable fish are likely targets for predators. For 

example, Congleton et al. (1984) demonstrated a significant positive correlation between 

impaired predator avoidance and crowding stress resulting in plasma cortisol levels of 75–

150 ng/ml. In another study, Gadomski et al. (1994) could not find a significant relationship 

between juvenile salmonid descaling and predation by northern squawfish (Ptychocheilus 

oregonensis), but noted there was evidence of increased predation of descaled fish compared to 

controls at higher descaling rates. 

Conclusions 

1. Passage through components of mechanical screen bypass systems induces physiological 

changes related to the general adaptation syndrome, or stress, in juvenile anadromous

salmonids. 

2. However, the physiological effects of passage through fish bypass facilities are nominal for 

juvenile Chinook salmon and steelhead, as measured by blood plasma stress indices, and 

follow a typical sequence for fish subjected to an acute stressor followed by acclimation to or 

removal of the stressor. Unless exhaustion or physical injury are involved, the majority of

bypassed fish are probably minimally affected by at least a single, and possibly several, 

stressful experiences. 

3. There is both pro and con evidence that multiple stress experiences can cumulatively 

diminish survivability, particularly in animals of inferior quality. Some multiple stress 

events may inhibit survival while others may not. Post-stress survival likely depends upon 

which of an innumerable array of stressors are involved in combination with each fish’s 

particular sensitivity to that stressor in terms of its overall condition, health, and environment 

at the time. 

59
Diel Passage and Timing 

Background 

Beginning in the late 1960s, sampling from turbine intake gatewells and at smolt 

monitoring facilities within powerhouses of Columbia and Snake River dams consistently 

showed that the majority of smolts passed through the dams’ powerhouses at night (Long 1968, 

Sims and Ossiander 1981, Brege et al. 1996), and that the timing of this pattern could be affected 

by holding behavior in gatewells and collection channels. Since then, hydroacoustic 

investigations and radiotelemetry data (for example, Johnson and Dauble 1995, BioSonics Inc. 

1996b, BioSonics Inc. 1998, Ploskey et al. 1998, Vendetti et al. 2000) from most dams

confirmed this as a consistent diel passage pattern and provided more details regarding specific 

hours of peak passage into turbine intakes. 

The more recent studies also showed that peak passage hours differed among passage 

routes. For example, passage through sluiceways at Bonneville and The Dalles Dams generally 

peaked in early morning. Diel passage through spillways is more variable, paralleling 

powerhouse patterns at some dams, but showing high daytime passage and morning peaks at 

others. In the following subsections, we review diel passage by project. 

Lower Granite Dam 

Since 1994 both radiotelemetry and hydroacoustic studies have been conducted in the 

forebay of Lower Granite Dam to assess fish behavior and efficiency relative to a surface bypass 

collector (SBC) in front of turbine units 4, 5, and 6. Johnson et al. (1998b) found that fish 

passage into the SBC and spillway was higher during daytime hours than nighttime hours. 

However, passage at the powerhouse pier nose and inside the turbine intake was the reverse 

(Figure 3). In 1999 spill occurred from 1800 to 0600, and highest peak spillway passage 

occurred at 2300. However, the diel distributions of fish passage were similar for the SBC and 

turbines, with peak passage occurring between 0600 and 1700 (Anglea 1999) (Figure 4). Anglea 

et al. (1999) found that nighttime spill efficiency was 57% and downstream passage was slightly 

lower at night, while abundance of smolt-sized fish was higher during the day. 

Adams and Rondorf (1999) tracked radio-tagged spring Chinook salmon smolts as they 

approached the SBC. In 1998 from 66% to 78% of all radio-tagged fish entered the fish bypass 

(via the turbine intakes) between 1900 and 0600. With the exception of juvenile hatchery spring 

Chinook salmon, most fish also entered the SBC during early evening and nighttime hours. 

60
0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

12345 6789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

Passage (proportion)

SBC

Turbine

Spill

Figure 3. Diel distributions for surface bypass collector (SBC), pier nose, spillway, and within-turbine 

sample locations at Lower Granite Dam, 1998, based on fixed-location hydroacoustics 

(adapted from Johnson et al. 1998b). 

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

Passage (proportion)

SBC

Pier nose

Spillway

In-turbine

Figure 4. Diel distribution of fish passage for surface bypass collector, powerhouse, and spillway with 

fixed-location hydroacoustics at Lower Granite Dam, 1999 (adapted from Anglea 1999). 

61
Seventy-six percent of wild steelhead and wild fall Chinook salmon and 80% of hatchery 

steelhead entered the SBC between 1900 and 0600. A summary of all radiotelemetry and 

hydroacoustic studies conducted to evaluate the effectiveness of the SBC, including all 

information related to diel passage timing and movement, can be found in Anglea et al. (2002). 

Adams et al. (1997) and Adams and Rondorf (1999) compared forebay residence times of 

hatchery Chinook salmon, hatchery steelhead, and wild steelhead for the period of 1994–1998. 

Flow and spill varied throughout the period, were generally highest in 1997, and the peak spill in 

May was approximately 115–120 kcfs. Comparing forebay residence times for high flow years 

(1996–1998) to the lower flow years (1994 and 1995) suggests that in general, high flows and 

increased spill volume and hours reduced forebay residence time. For example, hatchery 

steelhead median residence times were 14.2 hours and 26.7 hours in 1995 and 1994, respectively, 

compared to 0.7–4.0 hours from 1996 to 1998. Hatchery yearling Chinook median residence 

times were 6.8 and 5.0 hours in 1995 and 1994, respectively, compared to 1.0–1.9 hours from

1996 to 1998. Wild steelhead median residence times showed little difference between years and 

ranged from 0.7 to 3.5 hours between 1995 and 1998. 

In an attempt to improve surface bypass at the dam in 2001, the COE installed a 

removable spillway weir (RSW) in spillway 1. Using radiotelemetry in spring 2002, Plumb et al. 

(2003) reported that route of passage at the dam varied depending upon the time of day (Table 

9). Higher proportions of salmon smolts passed through the RSW during the daytime than 

during the nighttime. 

Table 9. Routes of passage in spring 2002, determined via radiotelemetry (adapted Plumb et al. 2004). 

Daytime (0500–1959) Nighttime

Stock RSW Spill

Turbine 

routes RSW Spill

Turbine 

routes 

Yearling Chinook 60% 20% 20% 49% 26% 25%

Hatchery steelhead 73% 6% 21% 38% 22% 40%

Wild steelhead 69% 14% 17% 42% 25% 33%

In 2003 Plumb et al. (2004) confirmed the above trends. As they approached the dam,

smolts were distributed shallower during daytime and deeper during nighttime. Hence they are 

more apt to pass the dam through the deeper passage routes afforded by turbines and spillways at 

night than during day. Performance of the RSW was optimal during the day, regardless of 

species or rearing type. 

Little Goose Dam 

In three years of research by Vendetti et al. (2000), passage of radio-tagged juvenile 

subyearling Chinook salmon showed a slightly different diel pattern for passage through the 

powerhouse at Little Goose Dam (Figure 5). Peaks in the number of detections were observed 

between 0500 and 0600 hours and again in the evening between 1800 and 2000 hours, with the 

evening peak being the larger and longer of the two. 

62
Number of detections by hour

Mean hourly flow (m X 1,000)

Time (hours) 

1996

100

150

200

250

300

350

400

450

1.0

1.3

1.6

600 

800 

1,000 1,200 1,400 1,600 1,800 2,000

1995

Flow

Detections

1.2

1.4

1.6

12:00

16:00

20:00

04:00

08:00

00:00

500

1000

1500

2000

2500

3000

3500

1.0

1.5

2.0

1997

3

Figure 5. Total number of detections over the entire study period vs. mean hourly discharge at Little 

Goose Dam, 11 July–24 August 1994, 17 July–24 August 1995, 17 July–26 August 1996, and

15 July–23 August 1997 (adapted from Vendetti et al. 2000). 

63
Lower Monumental Dam 

To assess the downstream migrational patterns of smolts passing the dam and to estimate 

the effectiveness of spill in passing migrants, hydroacoustic investigations were conducted at 

Lower Monumental Dam from 1986 through 1989 (Wright et al. 1986, Ransom and McFaden 

1987, McFaden 1988, Ransom and Sullivan 1989). As at other dams, turbine passage rates were 

highest during nighttime, with a peak at 2300 hours. During the four years of study, nighttime

project passage (powerhouse and spill) ranged from 70% to 84%. 

Spillway operation varied over the four years of tests. In 1986 spill varied from just 

nighttime spill (1800–0600 hours) to a constant 24-hour spill. Examination of diel passage 

during these different flow regimes suggested that both fish behavior and dam operations 

influenced the timing of spillway passage. During a period when spill was relatively constant

throughout the day, a pronounced peak in passage occurred at 2200 hours. However, this 

evening peak was enhanced during periods when spill was off during daylight hours and started 

at 1800. In studies from 1987 to 1989, the spillway was only operated from 1800 to 0600 hours, 

and highest passage rates occurred at 2300 hours and declined steadily from 2300 to 0600 hours. 

Ice Harbor Dam 

Three years of hydroacoustic investigations at Ice Harbor Dam revealed diel passage 

patterns similar to other Columbia and Snake River dams (Johnson et al. 1983, Ransom and 

Ouellette 1988). Most migrants passed the dam at 2300 hours. Sluiceway diel passage rates 

were highest from 0600 to 1300 hours. Turbine passage rates were highest from 2100 to 0600 

hours. 

During all three years of study, hourly passage rates through the spillway were more 

variable than through the turbine units or sluiceway. Generally, spillway passage rates were low 

in the early morning and then increased steadily to a peak at 1200 hours. The rate then declined 

rapidly, reaching a low point at 1700 hours, followed by a secondary peak at 2100 hours (only 

slightly lower than the peak at 1200 hours). 

In 1999 a radiotelemetry study of yearling Chinook salmon passage was conducted at Ice 

Harbor Dam to determine tailrace egress and routes of passage under varying levels of spill and 

powerhouse flow (Eppard et al. 2000). At 1800 hours each day, powerhouse flow was reduced 

while spill was increased to the maximum level, based on dissolved gas levels, until 0600 hours 

the following day. The test fish were released in the tailrace of Lower Monumental Dam each 

morning (0800 hours) via the bypass outfall. Receivers were positioned 1 km above Ice Harbor 

Dam and across the powerhouse and spillway, in the juvenile bypass channel, and on each 

submersible traveling screen. Each individual fish could be timed from the study entrance line 

through the passage route. Of the 580 fish detected 1 km above the dam, diel passage was fairly 

evenly distributed. A total of 302 and 278 fish passed the project during the day and night, 

respectively, although release location may have affected these results. Spilled fish had a lower 

forebay residence time, and fish first detected after dark had a lower forebay residence time than 

those first detected during daylight hours. 

64
McNary Dam 

No hydroacoustic or radiotelemetry studies have been conducted that provide a robust 

source of diel passage information at McNary Dam. Studies of OPE using an orifice trap 

provide information on hourly passage from the gatewell(s) sampled. Using an orifice trap, 

McComas et al. (1997) found that passage from gatewells into the juvenile bypass channel 

appeared heaviest within a few hours of dawn and dusk, with the dusk peak having generally 

larger numbers of fish. Orifice trap data does not provide information on when fish first arrived 

in the forebay or entered the gatewell. 

John Day Dam 

Diel passage patterns have been reported as part of the smolt monitoring program at John 

Day Dam since the 1970s. This sampling, done mainly in gatewell slots 3A and 3B, has shown 

that all species of juvenile salmonids tended to pass through the John Day Dam powerhouse 

during hours of darkness (Brege et al. 1996). In more recent years, further observations have 

confirmed that passage occurs predominantly from 1800 to 0600 hours (Martinson et al. 1998) 

(Table 10). 

Table 10. Percent night passage (1800–0600 hours) for each season at John Day Dam, 1985 to 1997 

(Martinson et al. 1998). 

Year 

Yearling 

Chinook 

Subyearling

Chinook 

Wild 

steelhead 

Hatchery 

steelhead Coho Sockeye

1985 83.2 83.7 N/A N/A 91 86.8

1986 75.5 80.1 N/A N/A 95.9 81.9

1987 84.5 85.4 93.6 85.6 95 94.9

1988 80 80.7 80.8 70.3 83.9 87.1

1989 86.4 86.2 73.6 79.4 93 79

1990 79.7 84.4 76.3 94.8 95.6 85

1991 89.9 77 91 92.3 96.2 83.6

1992 82.8 78.7 95.3 91.5 96 94.9

1993 83.3 87.8 83.4 80.7 95.1 86.5

1994 80.9 68.1 91.6 81.4 92.2 94.5

1995 80.7 79.7 87.9 75.8 91.5 79.5

1996 68.6 70 81.6 74.7 80.2 65.6

1997 62.6 73.1 67 70.6 73.7 59.6

Average 79.8 79.6 83.8 81.5 90.7 83

Min. 62.6 68.1 67 70.3 73.7 59.6

Max. 89.9 87.8 95.3 94.8 96.2 94.9

Giorgi and Stevenson (1995) reviewed all gatewell sampling, hydroacoustic, and 

radiotelemetry data from 1980 to 1989. They concluded that even though none of the reports 

reviewed offered robust estimates of diel passage, there was consistent agreement among all 

evaluation tools and across many years. The data indicated that smolts at John Day Dam exhibit 

a strong tendency to sound and pass through deep passage entrances during nighttime hours. 

65
Both the hydroacoustic and radiotelemetry data corroborated the gatewell sampling and also 

showed that timing of spillway passage through the deep spill intakes (14.3–17.7 m below 

normal operating pool) paralleled powerhouse passage. 

One factor that could influence passage timing measured at the dam is the release 

location of the radio-tagged fish during the earlier and even some more recent studies. In 1996, 

Holmberg et al. (1998) saw diel passage patterns similar to those summarized by Giorgi and 

Stevenson (1995). However, the test fish were released only a short distance upstream (8 km) 

from the dam. In 1997 Hensleigh et al. (1999) observed a similar pattern for fish also released a 

short distance upstream from the dam. In contrast to previous years, they saw no apparent diel 

pattern for fish released at McNary Dam. They also reported that, with high spill volumes in 

1997, 50–75% of radio-tagged fish passed through the spillway, as compared to roughly 40% in 

previous years. 

In 1997 BioSonics Inc. (1999a) reported higher numbers of fish passing the dam at night 

based on hydroacoustic studies. They estimated that 54–89% of the fish passed through the 

spillway. Also, during the spring when spill level was similar day and night, they observed that 

fish passed the spillway at a rather uniform rate. In 1999 Hansel and Beeman (1999) saw 

differences in species response to dam operations. Steelhead generally passed at night regardless 

of spill discharge. Further analysis of the data suggests that steelhead smaller than 200 mm

(primarily wild) did readily pass the spillway throughout the diel period.6

 Yearling Chinook 

salmon appeared to readily pass the project if daytime spill was provided. Information on John 

Day Dam spill effectiveness and efficiency is discussed in the Spill Efficiency and Effectiveness 

section. 

Time to pass the project is another important consideration. In 1995 Sheer et al. (1997) 

estimated that forebay residence time of spring Chinook was 10 hours during conditions of low 

spill (<14 kcfs). For 1996 Holmberg et al. (1998) reported delays of less than 1 hour during 

moderate spill conditions (47–125 kcfs). In 1997 Hensleigh et al. (1999) observed forebay 

residence times of 0.3 hour when spill discharge exceeded 300 kcfs. 

To summarize, data collected during the 1980s suggested there was a strong and 

consistent tendency for fish to pass the project at night. However, it is difficult to compare 

results from recent and earlier studies, due to differences in sample sizes, release points for 

tagged fish, and radio-tagged fish release timing. Based on radiotelemetry and hydroacoustic 

study results since 1997, it appears that yearling migrants, especially yearling Chinook, will 

readily pass the spillway during the day if spill volumes are 30% or greater. Daytime spill 

volumes other than 30% have not been tested using recent release protocols for radio-tagged fish. 

More recently, Hansel and Beeman (1999) found that juvenile steelhead generally passed 

the dam at night regardless of the spill discharge when they arrived. Yearling Chinook salmon 

passed similarly when arriving during a daytime spill discharge rate of 68%, but during 30% 

daytime spill, they passed at that time. Fish arriving at night generally passed at night. 

 

6

 G. Fredricks, NOAA Fisheries Service, Portland, OR. Pers. commun., March 2000. 

66
According to Anglea et al. (2001), radiotelemetry studies conducted in 1999 and 2000 

and hydroacoustic studies conducted after 1999 were the only studies that collected sufficiently 

reliable passage and survival data for studies conducted at John Day Dam. They also reported 

that it appeared a majority of juveniles passed the dam at night (especially for steelhead, where 

often over 90% passed at night, regardless of treatment). In addition, they reported that the 

addition of daytime spill increased the proportion of juvenile salmonids passing the spillway 

during the day (especially yearling Chinook salmon in 2000). 

The Dalles Dam 

Diel passage through the powerhouse at The Dalles Dam is similar to that seen at other 

Columbia and Snake River dams. Both gatewell dipping (Long 1968) and hydroacoustics 

(Magne et al. 1983, Steig and Johnson 1986, Johnson et al. 1987, BioSonics Inc. 1996a) 

documented that the majority of all salmonids entered gatewells between 1900 and 0700 hours. 

In studies conducted by BioSonics Inc. (1996a), both spring and summer migrants exhibited 

peak passage during hours of darkness. 

As with Bonneville Dam, diel passage through the sluiceway does not seem to follow the 

nighttime passage pattern. Early studies by Nichols (1979) and Nichols and Ransom (1980, 

1981) reported that sluiceway passage peaked during daylight hours, typically around midday. 

More recent hydroacoustic studies have also found that average daylight passage through the 

sluiceway was the predominant pattern at The Dalles in the spring. During the summer period, 

passage rates through the sluiceway also showed low nighttime passage, but the peak was during 

the afternoon rather than morning. After reviewing all available literature, Giorgi and Stevenson 

(1995) concluded that, regardless of the sampling approach employed, during the spring juvenile 

salmonids pass via the sluiceway at The Dalles Dam through most of the 24-hour period, with 

peak passage occurring during daylight hours. 

There are few measures of diel passage at The Dalles Dam spillway, because in most of

the studies conducted there spill was only provided at night and did not span a 24-hour period. 

However, in studies conducted in 1996 by BioSonics Inc. (1996a), spill levels were maintained 

for 24-hour periods and slightly higher morning passage rates via spill were found during the 

spring. This morning peak was even more pronounced during the summer migration. In 1999 

Ploskey et al. (1999a) found that diel distribution of fish passage during the spring through the 

spillway was similar between the 30% and 64% spill treatments. The proportion of spillway fish 

passage was relatively uniform, except for a substantial peak between 2000 and 2100 hours. 

During the summer, fish passage through the spillway was again fairly uniform, except for peaks 

at 0600–0700 hours and 2000–2200 hours during 30% spill, and from 1800 to 1900 hours during 

64% spill. 

More recently, Hansel and Beeman (1999) examined FPEs and spill and sluiceway 

passage efficiencies of yearling Chinook salmon and steelhead at The Dalles Dam. With the 

exception that the FPE of juvenile steelhead was significantly greater during the day than night 

during the 30% spill discharge (95% vs. 86%, respectively), neither FPE nor spill passage 

efficiency differed significantly between day and night for either species. However, differences 

between diel passage conditions did affect spillway passage location. During the adult spill 

pattern, radio-tagged fish passed mostly through the southern end of the spillway, whereas 

67
during the juvenile spill pattern, they passed predominantly through the northern end. Median 

residence time in the near-dam forebay area was generally less than 0.4 hour for both species, but 

juvenile steelhead during the daytime 30% spill discharge had a median residence time of 

2.3 hours. 

Using radio tags, Beeman et al. (2000a) found that residence times were significantly 

longer for steelhead arriving during the day than at night, and that steelhead greater than 200 mm 

in length had significantly shorter residence times than smaller steelhead. Presumably, this 

reflects a behavioral difference between wild and hatchery steelhead, because the former tend to 

be smaller. In a similar study on subyearling Chinook salmon, Beeman et al. (2000b) reported 

total project diel passage was 59% and 41% during day and night, respectively. 

With hydroacoustics, Moursund et al. (2001a, 2001b) used relative passage rates between 

the major routes of turbine, sluice, and spillway to examine temporal differences in passage. 

Turbine passage peaked at night in the spring, but that peak shifted to afternoon by summer. 

Spillway passage showed a peak in the morning hours, and this trend remained through the 

summer. There was a pronounced trend for fish to preferentially pass through the sluice during 

the day rather than at night. This trend was clear in both the spring and the summer. 

Bonneville Dam 

Passage patterns at Bonneville Dam are more complex than at other dams because there 

are three separate structures: two powerhouses and an unattached spillway. There are also two 

passage routes at each powerhouse: the turbines and ice/trash sluiceway. For this reason, the 

available information is provided separately for each powerhouse and the spillway. 

First Powerhouse 

Gessel et al. (1986) dipped gatewells over 24 hours during six periods throughout the 

spring migration and observed peaks in passage shortly after dawn and dusk, with the evening 

peak being typically much higher. From 1992 to 1995 the Smolt Monitoring Program sampled 

juvenile migrants moving through the downstream migrant channel on a 24-hour basis 

(Martinson et al. 1999). During this time, passage for all species increased at dusk, at about 

2000 hours, and peaked at 2200 hours (Table 11). Since 1995 the Smolt Monitoring Program

has sampled for 8 hours per day only (from 1600 to 2400 hours) and has reported the typical 

dusk peak in turbine passage observed at Snake River and lower Columbia River dams

(Martinson et al. 1996, 1997, 1998). 

In 1998 Martinson et al. (1999) observed that the 8-hour passage patterns of tule and 

bright stocks of subyearling Chinook salmon followed the same general pattern as spring 

migrants with only slight differences. Increases in tule passage began earlier, at about 1800 

hours, had a smaller peak at 2200 hours, then averaged a lower percent of total passage for the 

remaining hours. Upriver bright passage resembled the spring migrants, with a larger, more 

abrupt increase at 2200 hours and a gradual decline in passage thereafter. 

68
Table 11. Percent night passage (1800–0600 hours) for 1992–1995 at Bonneville Dam First Powerhouse 

(Martinson et al. 1999). 

Year 

Yearling 

Chinook 

Subyearling 

Chinook 

Wild 

steelhead 

Hatchery 

steelhead Coho Sockeye 

1995 52.8 65.8 68.3 62.9 70.7 56.2

1994 49.6 52.4 52.2 53.1 66.7 74.6

1993 43.2 56.2 67.1 62.4 68.1 63.6

1992 52 44 62.3 61.3 60 69

Median 50.8 54.3 64.7 61.9 67.4 66.3

Min. 43.2 44 52.2 53.1 60 56.2

Max. 52.8 65.8 68.3 62.9 70.7 74.6

Hydroacoustic data from Ploskey et al. (1998) also showed that mean hourly smolt 

passage into turbines at the First Powerhouse was higher during night hours than during the day 

in 1996 and 1997. The data were more variable in the spring than in the summer. The authors 

also observed that diel passage through the sluiceway at the First Powerhouse varied from

turbine passage. During both the spring and summer smolt migrations, the majority of passage 

through the sluiceway occurred during the early morning hours with a peak at approximately 

0300 hours. Passage was reduced during daytime hours, with a secondary peak shortly after 

sunset. 

Second Powerhouse 

The patterns of diel variation in fish passage rates during the spring migration, seen at the 

First Powerhouse, do not seem to be as strong or consistent at the Second Powerhouse. Studies 

by BioSonics Inc. (1998) showed little diel variation at unit 11A (both guided and nonguided), 

with no peak at dusk. Spring studies by Ploskey et al. (1998) showed a nighttime peak for a 

couple of treatment days, but the majority of days had highly variable data, with no consistent 

pattern. 

During summer migration studies by BioSonics Inc. (1998), subyearling Chinook salmon 

exhibited a minor diel variation in unit 11A with highest fish passage between 2100 and 2200 

hours. However, Ploskey et al. (1998) observed a pronounced nighttime peak from 2200 to 2300 

hours during summer smolt passage through the turbines at the Second Powerhouse. 

Similar to the First Powerhouse sluiceway, spring-time fish passage through the sluice 

chute at Second Powerhouse occurred during early daytime, between 0600 and 1300 hours 

(BioSonics Inc. 1998). A second, smaller increase in passage rates was observed between 1800 

and 2100 hours. During the summer migration, the peak passage rates through the sluice chute 

occurred between 0600 and 1800 hours. The hours of lowest passage through the sluice chute 

were from 2100 to 2200 hours, coinciding with the hours of highest fish passage of guided fish 

into unit 11A. 

69
Spillway 

A limited amount of hydroacoustic data is available pertaining to diel timing of juvenile 

salmonids through the spillway. BioSonics Inc. (1998) found that fish passage rates were higher 

during nighttime at the spillway in the spring. These results were not adjusted for changes in 

spill levels; however, because of high river flows, spillway discharges showed little variation 

during the spring. During the summer, fish passage rates were also highest at night at the 

spillway; however, this was in part due to spillway operations, when spillbay 5 (with transducer) 

was typically closed between 0500 and 2200 hours. 

In 1999 Plumb et al. (2001) found that regardless of release site, more fish were first 

detected at the spillway (37%) than at Bonneville Dam First (31%) or Second Powerhouse 

(29%). Median forebay residence times for hatchery steelhead were 5.6, 3.9, and 0.3 hours for 

the two powerhouses and spillway, respectively. Corresponding times for yearling Chinook 

salmon were 1.0, 0.2, and less than 0.1 hour. Additionally, fish from all releases passed the dam

throughout the diel period, with slightly higher numbers during nighttime passage (2000–0659 

hours). 

During the low-flow year of 2001, Evans et al. (2001a) found that passage rates were 

highest for yearling Chinook salmon during the day at the spillway and at the First Powerhouse, 

but nighttime passage was the highest at the Second Powerhouse. Overall diel passage was 

generally flat. A higher proportion of the total daily passage occurred during the day (0500–

2059 hours), but hourly passage was highest at night, with peak hourly passage at sunset (2100 

hours). Also, the median residence times in the forebays of Bonneville Dam ranged from

8 minutes to 9.7 hours, depending on forebay area. In a similar study on subyearling Chinook 

salmon, Evans et al. (2001b) found that median forebay residence time was shortest for the 

Second Powerhouse (0.7 hour), compared to 2.4 hours at the First Powerhouse and 1.5 hours at 

the spillway. Diel passage was very similar to that reported above for yearling Chinook salmon. 

Diel Passage Conclusions 

1. Passage timing for each route varies with dam configuration, flow volume through the route 

of passage, species, and life history type. Most studies were not designed to evaluate 24-hour 

passage behaviors for each species and life history type, making it difficult to precisely 

describe passage timing for each dam under varying conditions. 

2. In general, juvenile migrants that arrive during daytime tend to not pass through powerhouse 

routes (turbines and bypass) on arrival, but will readily pass through surface-oriented outlets 

and daytime spill if provided. 

3. In general, for juvenile migrants that arrive at night, passage is more equally distributed 

amongst available routes. 

70
Juvenile Salmon Passage Through Sluiceways 

and Surface Bypass Systems 

Many of the older FCRPS dams were constructed with surface-oriented ice and trash 

sluiceways to pass ice and debris from the forebay to the tailrace. These include (year 

completed) Bonneville Dam First Powerhouse (1938), McNary Dam (1954), The Dalles Dam

(1957), and Ice Harbor Dam (1962). The sluiceways included adjustable weir gates above 

turbine intakes that passed water from the forebay into a longitudinal channel around the dam to 

the tailrace. In addition, the Bonneville Dam Second Powerhouse was constructed in 1983 with 

a surface outlet located in one corner of the powerhouse to pass ice and debris to the tailrace. 

Juvenile yearling salmon generally migrate in the upper portion of the water column and 

approach FCRPS dams near the surface (Dauble et al. 1999, Johnson et al. 2000). However, 

most turbine intakes and all spill gate sill elevations are greater than 12.2 m below forebay water 

surfaces. Thus juvenile migrants have to follow flow lines downward into turbine and spill 

intakes, a behavior that is counter to their normal surface orientation. Also, juveniles can 

accumulate in the immediate forebay during the day at sites where spill is limited or nonexistent, 

and pass the dam at night. Delay of juvenile migrants in forebays may increase losses to 

predation (Raymond and Sims 1980). Recognizing these behaviors, researchers began to 

evaluate the sluiceways for their potential to pass juvenile salmonids. 

The Ice Harbor Dam sluiceway was evaluated in 1995 prior to installation of a juvenile 

bypass system in the sluiceway channel. Combinations of wide, surface-overflow entrances 

(6.1 m wide, 1.8 m deep, and 2.3 ms

-1 entrance velocity) and deeper, slotted entrances (up to 

1.8 m wide, 12.2 m deep, and up to 1.2 ms

-1 entrance velocity) were studied. The wide, surfaceoverflow entrances had the highest passage rates based on hydroacoustics (BioSonics Inc. 

1996b) and radiotelemetry, where a total of 57% of the radio-tagged Chinook salmon used sluice 

gate 2B, which was a surface-skimming flow (Swan et al. 1996). 

The Dalles Dam sluiceway is located along the entire length of the 22-turbine-unit 

powerhouse, which is oriented parallel to the river channel centerline. Surface flow passes 

through weir-type chain gates into a channel that has a hydraulic capacity of 5,000 cfs. 

Typically, three gates above turbine unit 1 at the west end of the powerhouse are opened to pass 

3,200 cfs through the sluiceway. During periods of no spill, sluiceway passage was estimated at 

40–55% (Giorgi and Stevenson 1995). However, sluiceway passage is reduced during periods of 

spill. For example, in 2000, estimated sluiceway passage was 6% and 7% during the spring and 

summer, respectively, during 40% spill (Moursund et al. 2001b). 

The Bonneville Dam First Powerhouse is perpendicular to the river channel centerline. 

Estimates of sluiceway efficiency have ranged from 83% for steelhead (Willis and Uremovich 

1981) to 13% for subyearling Chinook salmon (Krcma et al. 1982), and vary highly by species, 

71
flow, time of day, estimation method, forebay elevation, the number of turbines operating, and 

which sluiceway weirs are open. Although the capacity of the northern-most sluice gates was 

reduced in the early 1980s by installation of a juvenile fish bypass system, total sluiceway 

capacity was not reduced, and remains at approximately 1,000 cfs. In 2002 estimated sluiceway 

passage was 33% (95% CI, 32–34%) for spring and 29% (95% CI, 28–30%) for summer with 

gates 7A and 10C operating, under conditions of 24-hour spill, and the Second Powerhouse 

being operated as the priority powerhouse. Sluiceway passage during both the spring and 

summer was higher during the day than at night (Ploskey et al. 2003). 

The Bonneville Dam Second Powerhouse sluice chute is located immediately south of 

turbine unit 11, the southernmost of eight turbines at the powerhouse. The entrance is 4.6 m

wide by 6.1 m deep and discharges approximately 3,000 cfs. Strong lateral flows toward the 

north and south ends of the powerhouse concentrate juvenile fish at these locations (Monk et al. 

1999a). In 1998 with the six southernmost turbine intake extensions removed (over turbine units 

11–14), 52% of the radio-tagged steelhead and 36% of the radio-tagged yearling Chinook salmon 

approaching the powerhouse passed through the sluice chute, compared to 21% of the tagged 

steelhead and 14% of the tagged yearling Chinook salmon detected in the juvenile bypass 

system. When the chute was closed, 50% of the steelhead and 30% of the yearling Chinook 

salmon were detected in the bypass system (Hensleigh et al. 1998). The combined efficiency of 

the sluice chute and juvenile bypass system relative to passage through the chute and turbine 

units 11–13 was 90% during both spring and summer periods, based on hydroacoustics. When 

the chute was closed, guidance into the juvenile fish bypass system was 55% and 30% of the 

total fish passing turbine units 11–13 during the spring and summer, respectively. For the sluice 

chute alone, chute efficiency (proportion of fish passing the chute relative to passage through 

turbine units 11–13 and the chute) was 83% and 81% in the spring and summer, respectively. 

The effectiveness of the sluice chute was high; about five times more fish passed the chute than 

would be expected based on the proportion of water passing the chute (Ploskey et al. 1999b). 

Sluiceway survival is a function of direct mortality that occurs during passage through 

the sluiceway and plunge into the immediate tailrace, and delayed mortality that occurs 

downstream. Estimated survival through sluiceways varies with life history type, time of day, 

and spill conditions. For example, estimated survival through The Dalles Dam sluiceway in 

1998 was 0.960 (95% CI, 0.874–1.054) for coho salmon under 30% spill during the daytime, 

while estimated survival of subyearling Chinook salmon was 0.889 (95% CI, 0.806–0.980) 

during 30% daytime spill. In 2000 estimated survival of yearling Chinook salmon was 0.945 

(95% CI, 0.895–0.998) during daytime and 0.940 (95% CI, 0.889–0.995) at night during 40% 

spill. Estimated survival of subyearling Chinook salmon was 0.955 (95% CI, 0.849–1.074) 

during daytime and 0.972 (95% CI, 0.864–1.094) at night during 40% spill (Absolon et al. 2002). 

These sluiceway evaluations suggest that downstream migrants use nonturbine, surfaceoriented passage routes, especially during daytime and periods of nonspill. Relatively high 

passage rates resulted in the sluiceways being highly effective, given their low hydraulic capacity 

relative to total river discharge. However, their low hydraulic capacity also limited further 

increases in the percentage of fish that could be passed through these routes. 

Wells Dam on the upper Columbia River (completed 1967) is configured differently than 

all other run-of-the-river dams on the Snake and Columbia rivers. Powerhouse turbine intake 

72
ceilings are 21.3 m deep, with 11 spillway gates located over and between the 10 turbines. Each 

of the five turbine pairs has one surface bypass entrance 4.9 m wide by 22.3 m deep. The surface 

bypass entrance velocity is approximately 0.6 ms

-1, and each surface bypass slot passes up to 

2,200 cfs. Bypassed fish and flow pass through the surface entrances into an afterbay between 

the entrance and the spill gate, and are discharged to tailwater through the spillway gates, which 

control bypass entrance flow and velocity. Surface bypass flow is 5–7% of the hydraulic 

capacity of each pair of turbines (Johnson 1996). Evaluations of a surface bypass system at 

Wells Dam demonstrated that surface-oriented fish will pass in large numbers through the 

surface route with a relatively small flow. Surface bypass entrance efficiency (ratio of fish 

passing into the bypass relative to fish passing both the bypass entrance and turbine pair) is 

approximately 90% (Johnson et al. 1992), and ranges from 84.3% to 95.0% for spring migrants 

and 76.5% to 97.0% for summer migrants (Skalski et al. 1996). 

Prototype Surface Bypass Systems 

Based on the successful performance of the Wells Dam surface bypass system, prototype 

surface bypass systems have been tested at a number of locations in the Pacific Northwest to 

determine whether fish behavior principles observed at Wells Dam could be applied to dams

with appreciably different and more typical powerhouse and spillway configurations, but with 

similar fish passage performance results. Evaluations of prototype surface bypass systems 

commenced in 1995 at Wanapum and Rocky Reach Dams, at Lower Granite Dam in 1996, at the 

Bonneville Dam Second Powerhouse in 1997, and the Bonneville Dam First Powerhouse in 

1998. Prior to the 1998 evaluation, the Lower Granite surface bypass collector prototype was 

deepened to simulate the Wells Dam intake, and a behavioral guidance device was installed. A 

J-block occlusion device was installed at the west turbine units of The Dalles Dam in 2001, and a 

removable spillway weir was installed at Lower Granite Dam in 2002. 

In 1995 a 16.8-m-deep prototype surface collector was installed upstream of turbine units 

7–10 at Wanapum Dam. A single slot entrance, 4.9 m wide by 15.2 m deep, with a hydraulic 

capacity of 1,400 cfs was located directly above turbine unit 8. The structure occluded the upper 

portion of the turbine intakes. Due to poor performance of the slot entrance, reduced turbine 

entrainment associated with the turbine intake occlusions, and increased entrainment through 

turbines without intake occlusions, the prototype collector was extended in 1996 to also occlude 

turbine units 4–6. Additional collector entrances were not added. Mean collector efficiency 

(percent of fish entering the prototype collector relative to the total passing into the collector plus

the number passing into reference units) was 30% in 1995 relative to turbine unit 8, and averaged 

12% relative to turbine units 7–10. 

Horizontal distribution across the powerhouse was skewed toward turbine units not 

covered by the occlusion device when compared to distribution in years before the prototype 

collector was installed (Ransom et al. 1996). Furthermore, spillway efficiency during surface 

bypass prototype testing was higher (43%) than in previous years (30%) (Kumagai et al. 1996). 

This suggests the prototype surface collector may have reduced entrainment at the occluded 

units, and fish passed over the spillway, increasing spillway effectiveness. The prototype 

collector was again extended in 1997 to occlude turbine units 1–3, and mean collector efficiency 

relative to turbine unit 8 averaged 15%, and was 0.9% relative to the entire 10-unit powerhouse 

during the spring (Kumagai et al. 1997). 

73
Rocky Reach Dam has an 11-unit powerhouse oriented parallel to the river channel 

centerline, similar to The Dalles Dam. Juvenile salmon accumulate at the downstream half of 

the powerhouse between the powerhouse and a nonoverflow wall connecting the powerhouse to 

the west shoreline (Dauble et al. 1999). Evaluations of various prototype surface collection 

configurations occurred from 1995 through 2002. In 2003 a permanent surface bypass facility 

was constructed consisting of a single entrance, 12.2 m wide by 15.2 m deep, located 

immediately downstream from turbine unit 1 with a hydraulic capacity of 6,000 cfs. Most of this 

flow is screened and pumped back to the forebay. Fish from the surface collector and screened 

juvenile fish bypass system in turbine units 1 and 2 are routed across the downstream face of the 

powerhouse and spillway to monitoring facilities and an outfall located on the east shore. 

Estimated fish passage efficiency (proportion of fish passing through nonturbine routes including 

the surface bypass, intake screen bypass, and spillway) was 66%, 68%, 31%, and 42% for 

yearling Chinook salmon, steelhead, sockeye salmon, and subyearling Chinook salmon, 

respectively (Mosey 2003). 

Lower Granite Dam has a six-unit powerhouse located immediately to the south of eight 

spillbays. In 1996, a prototype surface bypass collector was installed immediately upstream

from turbine units 4–6, consisting of a channel 6.1 m wide by 18.3 m deep, the bottom of which 

was at the same elevation as the turbine intake ceiling. Attraction flow passed through various 

entrance configurations and was conveyed through a longitudinal channel to an adjacent spillbay. 

Maximum discharge through the collector was 3,500 cfs, and flow control was provided by the 

spillbay gate. 

From 1996 through 2000, 11 different entrance configurations were tested at the Lower 

Granite Dam surface bypass collector to evaluate the effects of entrance characteristics, such as 

discharge, shape, orientation, area, velocity, and change in rate of accelerations (Anglea et al. 

2002). In 1998 based on hydraulic model studies, a 4.6-m extension was installed on turbine 

units 4–6 to emulate hydraulic conditions observed at Wells Dam. The upper 17% of the turbine 

intakes was occluded, which decreased the magnitude of downward velocities at mid-depths in 

the forebay upstream of the collector (Johnson et al. in prep.). Also in 1998, a 330-m-long 

floating curtain was installed to affect the horizontal distribution of juvenile salmon in the 

forebay and divert them from turbine units 1–3 to units 4–6 and the collector. The height of the 

curtain varied from 17 to 24 m. Hydroacoustic evaluations were used to evaluate passage for the 

general population and compare collector to noncollector passage routes. Radiotelemetry was 

used to evaluate the responses of yearling Chinook salmon, hatchery and wild steelhead, and 

subyearling Chinook salmon to the various configurations tested. 

In 1997, 12% of fish passing the dam did so through the collector, based on 

hydroacoustics (Johnson et al. 1998a). Similarly, from 6% to 10% of radio-tagged hatchery and 

wild steelhead and hatchery spring Chinook passed through the collector as compared to the total 

passing the dam, and from 43% to 65% of the radio-tagged fish that passed within 6 m of the 

entrances passed through the collector (Adams et al. 1998c). In 1998 the objective was to 

determine whether horizontal and vertical distribution could be modified by installation of the 

forebay curtain and collector modifications to increase the number of fish near the entrances and 

passing through the collector. More fish were observed in front of the collector and fewer fish 

were entrained into turbine units 4–6. Passage efficiency was from 14% to 34% depending on 

species, and from 14% to 51% of fish within 10 m of the entrances passed through the collector. 

74
A total of 92%, 61%, and 67% of radio-tagged hatchery steelhead, hatchery spring Chinook 

salmon, and wild steelhead, respectively, that approached turbine units 1–3 stayed to the north of 

the forebay curtain (Adams and Rondorf 2001). Additional research was conducted in 1999 and 

2000 with similar results. 

Research conducted from 1966 to 2000 showed that surface flow bypass is a valid 

concept for Lower Granite Dam (Johnson et al. in prep.). Both spillway efficiency and passage 

of fish through nonturbine routes increased. For example, performance of the surface bypass 

collector increased spillway fish passage efficiency from 30% to 57% in 1998 (Johnson et al. 

1998b). It also improved passage through nonturbine routes at the powerhouse. In 1998 an 

estimated 83% of the downstream migrants were guided into the bypass system relative to total 

powerhouse passage, and this increased to 90% when passage through the surface bypass 

collector was included. Also, concentrating inflow in a single surface entrance produced the 

optimum configuration. Finally, surface bypass efficiency alone was not high enough for it to be 

a stand-alone bypass system, but when used in combination with spill and the juvenile bypass 

system, passage past the dam through nonturbine routes exceeded 90% with more than half of 

these fish using the collector (Johnson et al. in prep.). 

The Bonneville Dam First Powerhouse consists of 10 turbines and is separated from the 

spillway by Bradford Island. A prototype surface collector in front of turbine units 3–6 was 

installed in 1998 and evaluated to determine whether occluding the upper intake and passing 

high flows through a deep-slot entrance would successfully attract fish. The bottom of the 

collector was from 11.0 to 12.8 m below the water surface and flow through the entrances was 

routed into the turbine intake. Both 1.5- and 6.1-m-wide entrances were tested, which passed 

approximately 1,000 and 3,000 cfs, respectively. 

Ploskey et al. (1999b) estimated that collector entrance efficiency was approximately 

90% for both spring and summer migrants for both entrance configurations, based on 

hydroacoustic targets that passed into and under the prototype collector. However, while 45% of 

the radio-tagged steelhead and 40% of the radio-tagged yearling Chinook salmon detected 

upstream from the First Powerhouse passed near the prototype collector, 67% of these fish did 

not enter the collector and instead moved south along the face of the structure, and initially held 

upstream from turbine units 1 and 2 (Hensleigh et al. 1998). 

In 2000 the prototype collector was expanded southward through turbine unit 1 to attempt 

to collect fish moving laterally (as observed in 1998). Estimated collection efficiency (the 

number entering the collector relative to the total number passing turbine units 1–6) was 83%, 

78%, and 81% for radio-tagged steelhead, yearling spring Chinook salmon, and subyearling 

spring Chinook salmon, respectively. Also, 71% and 59% of the radio-tagged steelhead and 

yearling Chinook salmon approached more than one entrance before passing (Evans et al. 

2001a). Multibeam hydroacoustic evaluations conducted to evaluate behavior in front of the 

collector found extensive milling of fish within 5 m of the entrances (Johnson et al. 2001). 

The Dalles Dam forebay has strong lateral flows toward the west end of the powerhouse 

and spillway that concentrate fish into these areas. Weir entrances located at the very western 

end of the powerhouse effectively pass fish into the sluiceway. The combination of flow patterns 

and high sluiceway fish passage efficiency, in combination with the knowledge from Wanapum

75
and Lower Granite Dams where passage of juvenile salmon into turbines was reduced by 

occluding the upper portion of the turbine intakes with prototype surface bypass structures, 

suggested that intake occlusion at The Dalles might reduce turbine entrainment rates. In 2002 

the upper 13.7 m of the turbine intakes of two fish attraction units and four main turbine units at 

the west end of the powerhouse were outfitted with “J” shaped occlusions. The long stem of the 

“J” blocked the intake and the nearly horizontal leg extended 6.1 m upstream, then upwards 3.0 

m at a 90° angle. In 2002 there was no difference in turbine entrainment with the occlusions 

installed or removed during the spring, and turbine entrainment was actually higher during 

periods with the “J” device installed during the summer (Johnson et al. 2003). 

Turbine priorities were different in 2002 compared to previous years, which resulted in 

every other turbine unit being operated. In 2002 spill passage efficiency dropped to 45% and 

38% during the spring and summer, respectively, which was lower than in previous years and 

may have been affected by the turbine operations that year. For example, in 2000, spill 

efficiency was 86% and 74% during the spring and summer, respectively (Moursund et al. 

2001b). The turbine operations in 2002 may have influenced turbine entrainment and the 

performance of the occlusions.

A new concept in surface bypass collectors, an RSW, was installed in the southernmost 

spillbay at Lower Granite Dam in 2001. The RSW is nearly 15.2 m wide and has a 5.9-m-long 

ramp extending upstream. The approach ramp was incorporated into the design based on Haro et 

al. (1998), where juvenile Atlantic salmon passed more readily over weirs with gradual velocity 

increases upstream of the crest as compared to sharp-crested weirs. The weir crest is from 3.4 to 

4.6 m below normal water surface elevations and transitions to the existing ogee of the spillbay. 

Discharge through the weir is 7,000 cfs at minimum operating pool. The weir is designed to 

rotate 90° in the upstream direction to a lowered position to not reduce spill capacity during high 

river discharge. 

In November 2001, a preliminary study using balloon-tagged hatchery yearling Chinook 

salmon found no significant differences in survival or injury rates between fish released into the 

RSW and into an adjacent spillbay that was partially-opened and outfitted with a deflector. 

Estimated 1- and 48-hour survival probabilities for fish passing through the RSW were 0.992 

(90% CI, 0.983–1.000) and 0.981 (90% CI, 0.966–0.995), respectively. Estimated 1- and 48-

hour survival probabilities for fish passing through the adjacent spillbay were 1.00 and 1.00, 

respectively, and the survival probabilities between the two routes were not significantly 

different (P > 0.05) (Normandeau Associates et al. 2002). 

The RSW was evaluated in 2002. During the tests, spill of 7,000 cfs was provided 

through the weir, spill through adjacent bays was provided to establish satisfactory tailrace 

egress conditions, and total spill was 15,000 cfs and 23,000 cfs. During spill-to-the-gas-cap 

tests, spill of approximately 45,000 cfs was provided though seven spillbays 12 hours at night. 

Surface bypass collector components remained in turbine units 4–6, the floating curtain was 

attached to the southern end of the surface collector, and surface collector components north of 

the powerhouse that had routed collector flow to the spillway were removed. 

In 2002 the bypass efficiency of the RSW (the ratio of fish passing the weir to fish 

passing through all routes) was 56%, 62%, and 61% for radio-tagged hatchery yearling Chinook 

76
salmon, hatchery steelhead, and wild steelhead, respectively. The bypass efficiency of the RSW

and the adjacent training spill was 78%, 73%, and 78% for radio-tagged hatchery yearling 

Chinook salmon, hatchery steelhead, and wild steelhead, respectively. In comparison, spill 

passage efficiency during the spill-to-gas-cap tests was 62%, 66%, and 54% for radio-tagged 

hatchery yearling Chinook salmon, hatchery steelhead, and wild steelhead, respectively. While 

training spill adjacent to the RSW was greater than 50,000 cfs for part of the evaluation period 

because of high river flow, the combined bypass efficiency of the RSW and training spill during 

these periods was similar to the combined bypass efficiency observed when adjacent training 

spill was 8,000 and 16,000 cfs (Plumb et al. 2003). 

In 2003 the surface bypass collector was removed, which eliminated the intake 

occlusions in turbine units 4–6, and the floating curtain in the forebay was stored well upstream

from the dam and had no influence on fish behavior and distributions. Flow through the RSW

was again 7,000 cfs and training spill through adjacent spillbays totaled 12,000 cfs. Due to a 

more stable hydrograph, flow through spillbays adjacent to the RSW was more consistent than in 

2002. The bypass efficiency of the RSW was 58%, 69%, and 67% for radio-tagged hatchery 

yearling Chinook salmon, hatchery steelhead, and wild steelhead, respectively. The bypass 

efficiency of the RSW and the adjacent training spill was 66%, 74%, and 71% for radio-tagged 

hatchery yearling Chinook salmon, hatchery steelhead, and wild steelhead, respectively. In 

comparison, spill passage efficiency during the spill-to-gas-cap tests was 52%, 59%, and 54% for 

radio-tagged hatchery yearling Chinook salmon, hatchery steelhead, and wild steelhead, 

respectively.

Passage through the RSW was greatest during daylight hours. Median forebay passage 

times during RSW tests were 1.92, 1.72, and 2.28 hours for radio-tagged hatchery and wild 

steelhead, and hatchery Chinook salmon, respectively. For these same test groups, median 

passage times during spill-to-the-gas-cap tests were 7.37, 4.64, and 4.98 hours, respectively. The 

estimated relative survival of radio-tagged hatchery yearling Chinook salmon passing through 

the RSW was 0.980 (95% CI, 0.957–1.03), compared to estimated survival through the spillway 

under spill-to-the-gas-cap spill of 0.931 (95% CI, 0.871–0.991). There were no significant 

differences between the treatments (Plumb et al. 2004). 

In 2002 and 2003, acoustically tagged hatchery yearling Chinook salmon and hatchery 

and wild steelhead generally stayed more than 4.6 m away from the upstream face of the 24.4-mdeep forebay curtain, and tagged fish in the upper 4.6 m of the water column generally stayed 

upstream of the 1.2-m-deep trash shear boom. In 2002, 80% of the tagged wild steelhead that 

passed within 70 m, 80% of tagged hatchery steelhead that passed within 40 m, and 80% of the 

tagged hatchery Chinook salmon that passed within 20 m of the RSW passed through the surface 

weir (Cash et al. 2003). 

Discussion 

Surface bypass performance is a function of forebay collection, safe passage through the 

surface bypass entrance conveyance, and safe and rapid tailrace egress. Testing of prototype 

surface bypass systems during the past decade focused primarily on measuring how many fish 

were guided out of the forebay and into the bypass, an issue considered the greatest challenge at 

most sites. Results indicate that bypass guidance efficiency varied with location, and where 

77
successful, no single feature or operation was responsible. Rather, a combination of factors 

appear to influence both forebay hydraulic conditions and related fish behavioral responses. 

These include: 

1) selecting a location where fish accumulate (naturally or artificially through guidance 

structures); 

2) developing surface-oriented entrance(s) with unimpeded surface drawdown, and open, 

natural lighting beyond the weir crest; 

3) having a gradual, increasing velocity as flow approaches the weir crest; 

4) using a floating trash boom or guidance curtain, which may improve forebay collection 

performance; 

5) creating larger bypass flows, which increase passage efficiency but do not overcome 

other deficiencies or compromises; and 

6) increasing discharge through turbines and the spillway, which reduces passage through 

surface bypass systems. 

Deep-slot entrances at Wells Dam were highly effective, passing approximately 90% of 

yearling and subyearling migrants. Also, surface-oriented entrances with weir-type flow have 

historically passed a large number of fish with small quantities of flow. Surface bypass facilities 

that have proven successful are the Bonneville Dam Second Powerhouse sluice chute and the 

Lower Granite Dam RSW. The weir entrance appears to collect surface-oriented fish, but also 

some deeper fish move vertically to pass into the entrance (Cash et al. 2003). Furthermore, there 

are no signs of fish holding for extended periods upstream of the RSW as compared to 

observations in front of deep-slot entrances. These observations suggest that volitional passage 

of juvenile salmonids is more attainable over weirs than through deep-slot entrances. However, 

fish respond differently to hydraulic conditions in the weir entrance and approach to the weir, 

and favor gradual rather than sharp velocity increases (Haro et al. 1998). 

In addition, data from most sites suggest that deep-slot entrances do not perform as well 

as surface-oriented weirs. Fish appeared to delay and avoid entering multiple, deep-slot 

entrances with lower velocities that ranged from 0.3 to 1.2 ms-1 at Lower Granite Dam and the 

Bonneville Dam First Powerhouse, and many radio-tagged fish moved back upstream. Multibeam hydroacoustic studies also identified fish holding immediately upstream from the deep-slot 

entrances, suggesting a lack of volitional movement into this type of entrance. Also, many radiotagged fish moved laterally rather than directly into, or under, the prototype surface collector 

entrances at turbine units 3–6 at the First Powerhouse in 1998. Fish behavioral investigations 

have not been conducted upstream of the surface bypass entrances at Wells Dam, and therefore it 

is not known whether delay is associated with this type of deep-slot entrance. 

Surface bypass system performance is related to the proportion of flow through the 

system. Johnson et al. (in prep.) suggest that surface bypass flow needs to be greater than 7% of 

project flow to establish a large enough flow net in the forebay to be effective, based on studies 

at Wells Dam. However, the configuration of the Wells Dam surface bypass system is unlike 

any other dam. Therefore, this criterion should be used as a guide and adjusted to meet unique 

conditions associated with each potential surface bypass location. Performance also appears 

78
related to total project flows. For example, at Lower Granite Dam when average project 

discharge was 85,000 cfs, approximately 60% of the tagged juvenile fish passed through the 

RSW or spillway, and this dropped to 40% when average project discharge was 110,000 cfs. 

While the forebay guidance curtain at Lower Granite Dam appeared to influence 

horizontal distribution of fish at the powerhouse in 1998, the potential benefit appears dependent 

on total project discharge and project operations, and it remains to be determined as to when and 

under what conditions guidance curtains perform best. For example, the RSW performed better 

with the guidance curtain removed in 2003 than when deployed in 2002, and both shallow 

(1.2-m-deep trash shear boom) and deep (24.4-m-deep curtain) devices have been shown to alter 

fish behavior. 

During the mid- and late-1990s, surface bypass designs and implementation decisions 

were made based on the best possible synthesis of biological and hydraulic observations. 

Hydraulic observations were largely from physical hydraulic models and fish behavioral 

information was primarily from hydroacoustic and radiotelemetry studies. Radiotelemetry 

provided an indication of fish location in two dimensions and to the nearest 10 m. Recently, 

however, an emphasis has been placed on developing the ability to track fish in three dimensions 

at finer spatial scales to gain an improved understanding of fish behavior; computational fluid 

dynamics models (with visualization software) were developed that allowed coarse- and finegrid observation of forebay hydraulic conditions. Integrating acoustic tag and hydraulic model 

outputs was necessary to identify fish behaviors and optimize surface bypass system designs and 

performance. 

Thus three-dimensional evaluations at Lower Granite Dam using acoustically tagged fish 

were conducted to assess fish movement under various operations and forebay hydraulic 

conditions and identify any previously undetected fish responses to forebay hydraulic conditions 

(Cash et al. 2003). Fish behaviors near both the guidance curtain and trash shear boom were 

noted in 2002, where fish appeared to avoid zones immediately upstream of both the 24.4-mdeep guidance curtain (first observed in 1998) and the 1.2-m-deep trash shear boom. 

Investigation of hydraulic conditions in the zones immediately upstream of both devices suggests 

these are areas where strain, as defined as the derivative of the vertical velocity component, can 

be detected. Nestler and Goodwin (in prep.) developed a model to analyze potential future 

configurations and operations using fish behavioral responses to hydraulic conditions. 

Preliminary analyses suggest the model can be used at larger scales, such as designing the 

appropriate length, depth, and location of forebay guidance curtains. 

A successful surface bypass system also requires safe passage through the structure at the 

point where the surface bypass discharge jet enters the tailrace and during egress through the 

tailrace. Routing a large surface bypass discharge directly to the tailrace results in high impact 

velocities and potential losses associated with mechanical effects (shear, strike, abrasion) or 

predation. Northern pikeminnow and gulls (Larus spp.) may accumulate at sluiceway or bypass 

outfalls and shallow areas directly downstream (Hansel et al. 1993, Ward et al. 1995, Jones et al. 

1997, Snelling and Mattson 1998). 

In addition, northern pikeminnow can respond quickly in tailraces to changes in project 

operations (Faler et al. 1988), emphasizing the need to design good egress conditions in the 

79
immediate tailrace. Surface bypass system outfalls are generally located where bypass flow 

quickly passes downstream to minimize fish exposure to eddies and stagnation zones. For 

example, radio-tagged spring Chinook migrated past a transect 1.2 km downstream of Lower 

Granite Dam in 19–23 minutes when passage was through the RSW with training spill, 

compared to some fish taking up to five times that long when passing during normal spill 

conditions (Plumb et al. 2004). 

Development of surface bypass systems at collector dams required evaluating the 

feasibility of dewatering large flow volumes if surface-bypassed fish were to be transported. 

Mechanical screening systems of unprecedented size would be required to achieve uniform

through-screen velocity distributions, and reliable debris cleaning and removal capability would 

have to be included in the design. Therefore, a study of high-flow dewatering and high-flow 

outfall alternatives was conducted for the Bonneville Dam First Powerhouse (ENSR et al. 1998). 

Based on the options evaluated, NOAA Fisheries Service and other salmon management 

agencies recommended use of high-flow outfalls to convey flow to tailraces and that further 

high-flow dewatering investigations be curtailed.

The high-flow outfall alternative, however, required confirmation that discharges greater 

than 1,000 cfs and impact velocities greater than 7.6 ms

-1 (25 fts-1 NMFS criteria) would not 

result in elevated injury rates. Based on laboratory studies, impact velocities of up to 15.8 ms

-1

at the point of jet entry produced safe conditions for juvenile salmon. Similarly, field studies at 

the Bonneville Dam Second Powerhouse sluice chute indicated no mortality and injury rates of 

2% associated with tailrace impact velocities of 14.6 ms

-1 (Pacific Northwest National 

Laboratory et al. 2001). These data suggest that surface bypass system outfalls with discharges 

greater than 1,000 cfs and impact velocities of up to 15.2 ms

-1 can be safely designed. 

Passage Through Surface Bypass Systems Conclusions 

1. Results from studies of the Lower Granite Dam removable spillway weir and Wells Dam

deep-slot surface bypass systems demonstrate that surface bypass is a viable concept that can 

produce high rates of nonturbine fish passage with a relatively small percentage of project 

discharge. However, poor or marginal results from other studies indicate that satisfactory 

results are not always achieved with surface bypass systems, and site conditions may limit 

performance at some locations. Also, it can take several years of testing various 

configurations under different flow regimes to determine the best design and project 

operations. 

2. Optimum surface bypass performance is attained when the entrance is at a location known to 

attract large numbers of fish. Weir entrances have proven more successful at passing 

juvenile salmon and steelhead migrants than deep-slot entrances. Fish appear to pass weirtype entrances most readily if flow velocity into and through the weir increases gradually. 

Discharge capacity should be sufficient to create a strong upstream velocity flow-field, but 

high discharge alone will not overcome a marginal entrance location. 

3. Surface bypass systems operated 24 hours per day reduce forebay residence times of juvenile 

salmon and potential exposure to predators relative to the existing program of 12-hour spill at 

night. At Lower Granite Dam, forebay residence times of radio-tagged juvenile salmon were 

80
from two to four times lower with a removable spillway weir operating compared to spill-togas-cap operations, potentially reducing mortality due to predation in the forebay. 

4. A surface bypass entrance by itself may not be enough to achieve high rates of dam passage 

through nonturbine routes. Additional passage facilities, such as a floating forebay curtain or 

upper turbine intake occlusion device, have been successful in guiding fish. 

5. Successful surface bypass designs need to address guidance into the entrance, passage

through the collector, discharge into the tailrace, and egress through the immediate tailrace to 

achieve passage performance goals of high passage efficiency and survival. Recent data 

indicate that for discharges greater than 1,000 cfs, impact velocities up to 15.2 ms

-1 are safe 

for juvenile salmon. 

6. Tracking acoustically tagged fish in three dimensions and integrating these data with 

hydraulic conditions from computational fluid dynamics modeling improves the 

understanding of fish behavior upstream of large hydroelectric dams and may increase the 

potential for achieving fish passage objectives. 

81
Juvenile Salmon Passage Through Turbines 

Early studies of juvenile salmon passage at FCRPS dams reported high levels of 

mortality (8–19%) for fish passing through turbines (Holmes 1952, Schoeneman et al. 1961, 

Long et al. 1968). As a result, salmon managers have focused on providing safer, nonturbine 

passage routes for juvenile fish to improve survival. Nevertheless, substantial numbers of 

juvenile fish continue to pass through turbines, and reducing turbine-related mortality remains an 

important factor in improving the survival of fish migrating through the hydropower system. 

Salmon managers need to know current levels of mortality for different species, runs, and 

life history stages that NOAA Fisheries Service will use to evaluate hydropower system effects 

on listed stocks. Also, it is important to inform managers about whether survival through 

turbines can be increased by modifying current turbine operations, and whether new turbine 

designs hold promise for improving physical conditions within the turbine environment. In this 

section we discuss the methods available to estimate turbine passage survival and injury, provide 

estimates of fish survival through turbines made under various operations, and present results 

from studies conducted under COE and U.S. Department of Energy (DOE) programs to improve 

turbine designs and operations for fish passage. 

Methods 

Turbine Survival Estimates Based on Juvenile Versus Adult Returns 

Holmes (1952) conducted releases of juvenile fish at Bonneville Dam from 1939 to 1945 

and adult recoveries from 1940 to 1950 to test for differences in survival between spillway and 

turbine passage routes. He found a somewhat greater loss of fingerling Chinook salmon 

associated with turbine passage, and by assigning varying weights to individual observations, 

estimated that mortality was from 0.11 to 0.15 through the turbines and spillway mortality was 

from 0.03 to 0.04. However, the data were highly variable and he concluded that the observed 

differences between the turbines and spillway could also have been due to “chance variation in 

sampling where no real differences existed.” 

Schoeneman et al. (1961) used inclined plane traps at various locations from 32.2 to 

80.5 km downstream from McNary Dam to recapture tagged juveniles and evaluate mortality. 

Their results were similar to Holmes’, where estimated spillway mortality was 0.02 (95% CI, 0–

0.04). Estimated turbine mortality was 0.13 at 80% wicket gate opening and 0.08 at 75% of gate 

opening. Overall, based on studies conducted at both McNary Dam and Big Cliff on the Santiam

River, estimated mortality through turbines was 0.11 (95% CI, 0.09–0.13). 

From 1987 to 1992, NOAA Fisheries Service released 1.5–2.2 million differentially 

marked upriver bright and tule stock juvenile fall Chinook salmon each year through various 

passage routes at Bonneville Dam to estimate short-term survival based on recovery of test fish 

82
in the estuary, and long-term survival based on salmon caught in fisheries and adult returns to 

hatcheries. Sample sizes were established to detect differences of 4–5% between release 

locations based on an estimated adult return rate of 0.5%. However, adult returns of fall 

Chinook salmon to the Columbia River were much lower than expected during the test period. 

Recoveries of test fish averaged 0.24% for fish released in 1987, and in all other years average 

recoveries ranged from 0.02% to 0.04% (Gilbreath et al. 1993). 

These poor return rates increased the range of detectable differences to 12–40% and 

eliminated our ability to make meaningful comparisons among treatments based on adult 

recovery data. Based on analyses of juvenile recapture data, Ledgerwood et al. (1990) found no 

significant difference between fish released into the turbine and immediate tailrace; however, 

survival of fish released through turbines was significantly different from, and approximately 

91% that of fish released 2.5 km downstream from the Second Powerhouse. 

These studies demonstrate the strengths and weaknesses of studies based on adult versus 

juvenile recapture data. Adult returns require long release and recapture periods to obtain results 

and include additional portions of the salmonid life cycle where biological and environmental 

effects can result in low returns, weakening the power to test for differences between specific 

treatments. To obtain tight confidence bounds about return rates for different treatments, they 

also require large juvenile releases because of low adult return rates. Studies where adult returns 

are used as the basis for comparison or adult data are used for trend analysis are presented in 

Williams et al. (2005). Studies based on juvenile recaptures can produce statistically valid 

results based on recaptures the same year, if recapture rates and sample sizes are sufficient. 

Results of studies based on juvenile recapture data (Schoeneman et al. 1961, Ledgerwood 

et al. 1990, Muir et al. 1998, Bickford and Skalski 2000, Muir et al. 2001, Absolon et al. 2003b) 

indicate that juvenile recoveries a significant distance downstream from the release site can be 

used to estimate turbine mortality, because indirect effects from turbine passage expressed 

downstream of the turbine to the point of recapture are incorporated into the estimate. When 

treatments through turbines, spillways, bypass systems, and sluiceways are combined into a 

single test, relative comparisons of survival can be made across the treatments (passage routes) 

using juvenile recapture data. 

Turbine Survival Estimates Based on Balloon-Tag, Radiotelemetry, 

and PIT-Tag Methodologies 

Mortality associated with turbine passage has two components: direct mortality, which is 

expressed immediately, and indirect mortality, which is expressed downstream of the 

powerhouse. 

In recent years, investigators have used three types of tagging methods to evaluate 

behavior, injury, and survival associated with turbine passage: 1) balloon tags, which are 

attached externally and inflate so the test fish can be recaptured by netting in the tailrace, 2) 

telemetry tags (acoustic or radio) that are surgically implanted into the body cavity or inserted in 

the esophagus, and 3) passive integrated transponder (PIT) tags, which are injected into the body 

cavity. Both the PIT and telemetry methods require subsequent detection downstream from the 

treatment area, but do not require subsequent handling once tagged. 

83
Each method has potential biases, such as whether fish selected for tagging represent the 

general population (species, size, condition, etc.) and whether fish are released such that their 

redistribution represents the general population. In addition, researchers need to address any bias 

related to a specific tagging method, such as effects on fish behavior and fate (e.g., vulnerability 

to predation). 

Looking at the relative range in turbine passage survival estimates at a variety of dams

based on PIT- and radio-tag methodologies and described below, we note that in general, PITand radio-tag based estimates are similar. For example, Absolon et al. (2002) estimated that 

relative survival of PIT-tagged yearling Chinook and coho salmon through turbines at The Dalles 

Dam was 0.790 and 0.830 in 2000 during the daytime and evening, respectively, and the 24-hour 

survival estimate was 0.810 (95% CI, 0.760–0.850). This compares with an estimated yearling 

Chinook salmon survival of 0.869 (95% CI, 0.804–0.933) in 2000, based on radiotelemetry 

(Counihan et al. 2002). There was no statistically significant difference between the estimates 

generated by the two methods. 

Comparisons of data from balloon- and radio-tag evaluations indicate differences in the 

survival estimates derived by these two methods may exist. At McNary Dam in 2002, NOAA 

Fisheries Service used radio-tagged yearling Chinook salmon to evaluate fish behavior and 

survival through two different turbine operations: a discharge of 11,200 cfs, which is at the upper 

end of the range in turbine efficiency defined as within 1% of peak efficiency, and a discharge of 

16,400 cfs, which is outside the 1% range and at the position where the turbine blades are at their 

maximum angle to the flow. We found no statistically significant differences in fish survival

between the two treatments. Estimated relative survival was 0.858 (SE = 0.034) and 0.871 (SE = 

0.016) for the lower discharge, and 0.814 (SE = 0.037) and 0.856 (SE = 0.011) for the higher 

discharge for two detection locations 46 and 15 km downstream, respectively. These estimates 

were based on radio tags and included both direct and indirect effects from passing the turbines 

to the point of detection downstream. 

Concurrent with our radiotelemetry study, Normandeau Associates et al. (2003) 

conducted a survival study using balloon-tagged yearling Chinook salmon. They also found no 

significant differences between the two operating conditions. For fish released during May, they 

estimated that turbine survival was 0.930 (90% CI, 0.900–0.970) and 0.946 (90% CI, 0.915–

0.981) at the 11,200 cfs and 16,400 cfs operations, respectively. Test fish were released into the 

turbine intake and recaptured in the immediate tailrace shortly thereafter. Survival estimates 

using balloon tags include direct effects from passing the turbines that are expressed immediately 

within the tailrace prior to recapture or in the first 24 or 48 hours of post-recapture holding. 

Comparison of the two methodologies was facilitated in this case because identical test 

conditions, time periods within the juvenile salmon migration, and release apparatus and 

locations were used in the studies, along with similar test fish (yearling Chinook salmon 

collected at McNary Dam). Comparing the two estimates provides insights into potential turbine 

effects expressed in the reach of river between the immediate tailrace where balloon-tagged fish 

were recaptured and 46 km downstream based on detections of radio-tagged fish. The balloontag estimates of survival (Normandeau Associates et al. 2003) were 6–7% higher than our radiotag estimates for the 11,200 cfs operation, and from 9% to 13% higher for the 16,400 cfs 

operation (Ferguson et al. in prep.). 

84
Bickford and Skalski (2000) analyzed 102 replicate releases of salmonid smolts from 53 

survival investigations conducted from 1971 to 1996 at Snake and Columbia River dams and 

found similar results. Average survival that incorporated both direct and indirect effects from

turbine passage was 0.873 through Kaplan turbines. Estimated turbine survival based on nine 

balloon-tag studies of direct mortality was 0.933. 

In contrast, Muir et al. (1996) compared balloon-tag and PIT-tag estimates of survival for 

yearling Chinook salmon released at identical locations within a turbine at Lower Granite Dam. 

Estimated survival was 0.927 (SE = 0.027) using PIT tags and 0.940 (SE = 0.023) based on 

balloon tags. Thus there was no difference between the estimates derived by the two methods. 

Single- and Paired-Release Protocols for Turbine Survival Estimates 

Two protocols have been developed to estimate smolt passage survival. One is the 

release of a single experimental group of uniquely identified fish that can be detected at 

downstream sites. Complete capture history permits survival to be estimated based on the 

assumption that upstream detections have no influence on detection downstream or survival 

probability. This method requires detections at a minimum of two sites downstream from the 

treatment area. The single-release protocol allows fish tagged from the general population to 

redistribute after release and approach the study site in a distribution that represents the general 

population. Both radio- and PIT-tagged fish can be released using this protocol. Because the 

passage route distribution of radio-tagged fish can be determined, estimates of turbine passage 

survival using this methodology reflect the general population. 

A second protocol is to use paired treatment and control releases, or upstream and 

downstream releases. Radio-, balloon-, and PIT-tagged fish all can be released using this 

protocol. For radio- and PIT-tagged fish, the protocol assumes the groups mix downstream and 

thereafter have equal survival and recapture probabilities. A release device (hose, pipe, or 

canister) is required for both the treatment and control releases, which forces passage distribution 

within the turbine to reflect the location of the release mechanism.

This is a positive aspect if specific portions of the turbine environment are being 

evaluated, and can be a negative aspect if the test release is designed to represent the distribution 

of the general population but the release mechanism is placed in a location that either does not 

represent the general population or the distribution of the general population is unknown. Also, 

careful attention should be paid to the design of the release pipe or hose. For example, the exit 

velocity should match the velocity of the receiving water, and test fish must be released into a 

hose or pipe without injury or stress, otherwise the estimate will be biased. Placement of control 

releases is another consideration so these releases are not preyed upon differentially by predators. 

Typically, control fish are released into the turbine boil “front roll” via a hose or in the 

immediate tailrace via a boat. 

Test fish released through hoses or pipes are surface acclimated. Abernathy et al. (2001) 

found that juvenile salmonids acclimated to surface pressure showed little direct mortality, but 

injury rates increased due to expansion of gas in the swim bladders when fish were acclimated to 

depth. Therefore, results from studies that used surface-acclimated test fish may not incorporate 

the full range of mortality effects associated with turbine passage. 

85
86

Results of Survival Estimates for Juvenile Fish Passing Turbines 

Turbine passage survival estimates from each study conducted at each dam are provided 

in Table 12. Survival estimates were categorized as being “direct” if test fish were recaptured 

immediately (balloon tags or turbine-outlet fyke nets), and “total” if test fish were allowed to 

proceed downstream before detection (PIT tags or radiotelemetry) such that any delayed effects 

from turbine passage up to the point of detection could be incorporated into the estimate. 

Operation of Existing Turbines 

All mainstem FCRPS turbines are six-bladed, vertical-axis Kaplans, except for 

Bonneville Dam turbines which have five blades due to lower head (difference between forebay 

and tailwater elevations) (Figure 6). Curves relating efficiency to unit performance (power 

output or flow) are developed for any given head. 

Turbine efficiency is thought to have a direct effect on fish passage survival. Inefficient 

turbine operation is a result of a poor blade-to-wicket gate relationship, where efficiency drops 

due to turbulence that results from the rotating machinery (hub and blades) being misaligned 

with the hydraulic flow field coming off the stationary but adjustable wicket gates. The 

relationship between survival of juvenile fish passing through Kaplan turbines is thought to be 

positively correlated with unit efficiency, based largely on a visual interpretation of the figures 

provided by Oligher and Donaldson (1966). Bell (1981) reviewed all the available data and 

recommended making every effort to operate turbines at peak efficiency during periods of peak 

fish passage to minimize fish mortality. 

NOAA Fisheries Service recognized the relationship between turbine efficiency and fish 

survival and adopted this recommendation as part of their FCRPS BiOps (NMFS 1995, 1998b, 

2000). Since 1992 turbines at COE projects have been operated within 1% of peak unit 

efficiency during the juvenile migration period (NMFS 1995). The COE provides the turbine 

operating points necessary to meet the BiOp requirement in their annual Fish Passage Plan, and 

any updates are coordinated with Bonneville Power Administration and salmon managers 

through the COE Fish Facility Operation and Maintenance Team. 

Juvenile salmon mortality associated with passing mainstem Columbia River turbines is 

potentially caused by several mechanisms, including strike, pressure, cavitation, and shear 

(Ferguson 1993, COE 1995b, Cada et al. 1997). The turbine environment through which 

juvenile salmon pass varies physically with load, head, runner speed, and fish location within the 

intake. It also varies over time with environmental conditions such as temperature and fish 

condition. While efforts are underway to explore the relationships between these various factors 

and fish survival (for example, Carlson and Duncan 2003a), turbines are currently operated 

within 1% of peak efficiency based on the assumption that fish survival and hydraulic efficiency 

are correlated. 
Table 12. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Passive Integrat

B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmon, YCS = ysalmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish 

Treatment release type/location 

Reference release type/location 

Turbine operation Direct surviva

Lower Granite Dam1988 Giorgi et al. 1988 PIT P YCS Point; turbine intake Downstream from 

Unit 3 turbine boil 

Normal loadresponse 

Not estimated 

1993 Iwamoto et al. 1994 PIT P YCS Point; turbine intake Lower tailrace midriver off

juvenile bypass outfall 

Normal loadresponse 

Not estimated 

1994 RMC Environmental

et al. 1994 

B P YCS Point; turbine intake 

elevation 623 ft msl 

Draft-tube exit Normal load

response 

0.946 (90% CI0.992) 1 hr sur

Point; turbine intake elevation 603 ft msl 

0.975 (90% CI0.992) 1 hr sur

Point; turbine intake elevation 603 ft msl 

0.975 (90% CI0.992) 1 hr sur

Point; turbine intake elevation 603 ft msl 

0.953 (90% CI0.973) 1 hr sur

Point; turbine intake elevation 603 ft msl 

18 kcfs Discharge 

0.972 (90% CI0.989) 1 hr sur

Point; turbine intake elevation 603 ft msl 

13.5 kcfsDischarge 

0.946 (90% CI0.965) 1 hr sur

Point; turbine intake elevation 603 ft msl 

19 kcfs Discharge 

0.949 (90% CI0.979) 1 hr sur

1995 Normandeau 

Associates et al. 1995 

B P YCS

Pooled 

Draft-tube exit 

0.961 (90% CI

0.969) 1 hr sur

1997 Muir et al. 2001 PIT P YCS Point; turbine intake Lower tailrace mid-river 

downstream of juvenile bypass outfall 

Normal Not estimated 

Little Goose Dam 1993 Iwamoto et al. 1994 PIT P YCS Point; turbine intake Lower tailrace mid-river off

juvenile bypass outfall 

Normal loadresponse 

Not estimated 

1997 Muir et al. 1998 PIT P SHT Point; turbine intake Lower tailrace mid-river 

downstream of juvenile bypass outfall 

Normal loadresponse 

Not estimated 

1997 Muir et al. 1998 PIT P YCS Point; turbine intake Lower tailrace mid-river off

juvenile bypass outfall 

Normal loadresponse 

Not estimated 

87
Table 12. Continued. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Pas

Transponder, B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmChinook salmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish 

Treatment release type/location 

Reference release type/location 

Turbine operation Direct survival

Lower Monumental Dam 1997 Muir et al. 2001 PIT P YCS Point; turbine intake Lower tailrace midriver downstream of 

juvenile bypass outfall 

Normal loadresponse 

Not estimated 

Ice Harbor Dam 1968 Long et al. 1968 -- P Coho Units 1–3 Frontroll Normal load

response 

0.810–0.900 

2003 Absolon et al.

2003a 

PIT P YCS Unit 1 Frontroll Normal load

response 

Not estimated 

2003 Absolon et al.

2003a 

PIT P YCS Unit 3 Frontroll Normal load

response 

Not estimated 

2003 Absolon et al.

2003a 

PIT P SYCS Unit 1 Frontroll Normal load

response 

Not estimated 

McNary Dam1955 Schoeneman

et al. 1961 

Tattoo P YCS Hose-unit not

specified 

Control-not specified 0.80% wicket 

gate opening 

Not estimated 

1956 Schoeneman

et al. 1961 

Tattoo P YCS Hose-unit not

specified 

Control-not specified 0.75% wicket 

gate opening 

Not estimated 

1999 Normandeau

Associates et al. 1999 

B P YCS Turbine intake

upstream of wicket gate 

Stay vane- mid runner blade 

12.0 kcfs 0.98 (90% CI, 0.95

1hr survival 

Stay vane-runner tip 0.98 (90% CI, 0.95

1 hr survival 

Stay vane-runner hub 0.978 (90% CI, 0.9

1.004), 1 hr surviv

2002 Normandeau

Assoc. et al. 2003 

B P YCS Point release all three

intake bays 

Draft-tube exit 8.0 kcfs 0.944 (90% CI, 0.9

0.977), 1 hr surviv

11.2 kcfs 0.955 (90% CI, 0.9

0.982), 1 hr surviv

0.930 (90% CI, 0.9

0.970), 1 hr surviv

0.944 (90% CI, 0.9

0.977), 1 hr surviv

16.4 kcfs 0.945 (90% CI, 0.9

0.964), 1 hr surviv

0.953 (90% CI, 0.9

0.994), 1 hr surviv

88
Table 12. Continued. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Pas

Transponder, B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmChinook salmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish 

Treatment release type/location 

Reference release type/location 

Turbine operation Direct survival survival 

Total 

McNary Dam (Continued)

11.2 kcfs To 15 km dow

To 46 km dow

2002 Absolon et al. 2002 R P YCS Unit 9 point release 

all three intake bays

Tailrace 2 km below dam; 

16.4 kcfs 

Not estimated 

To 15 km dowTo 46 km dow

2003 Peery and Bjornn

2003 

R P SYCS Unit 9 point release Frontroll Normal load

response 

Not estimated 0.816 (95% C

John Day Dam2002 Counihan et al. 

in prep. a 

R P SHT Rock Creek Tailrace 1 km 

downstream from dam 

Normal loadresponse 

Not estimated Powerhouse 0

Spill: 30 day/ 0.93 (95% CInight 

2002 Counihan et al. 

in prep. a 

R P SYCS Rock Creek Tailrace 1 km 

downstream from dam 

Normal load response 

Not estimated Powerhouse 0

Spill: 30 day/Powerhouse 0Spill: 0 day/60.778 (SE 0.0Spill: 0% day

2002 Counihan et al. 

in prep. a 

R P YCS Point; turbine intake 

U15 

Tailrace 1 km downstream from dam 

Normal loadresponse 

Not estimated 

0.832 (SE 0.0Spill: 30% da0.820 (SE 0.0Spill: 0% day

YCS 

0.764 (SE 0.0Spill: day/45% 0.719 (SE 0.0Spill: 0% day

2003 Counihan et al. 

in prep. b 

R P

SYCS 

Point; turbine intake U4 and U15 

Tailrace 1 km downstream from dam 

Normal loadresponse 

Not estimated 

0.722 (SE 0.0Spill: 30% da

89
Table 12. Continued. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Pas

Transponder, B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmChinook salmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish 

Treatment release type/location 

Reference release type/location 

Turbine operation 

Directsurvival 

The Dalles Dam 2000 Counihan et al. 

2002 

R P YCS Point; several

turbine intakes 

Downstream of dam atproposed bypass outfall 

Normal loadresponse 

Not estimated 

YCS and Coho 

2000 Absolon et al.

2002 

PIT P

SYCS 

Point; severalturbine intakes 

Downstream of dam atproposed bypass outfall 

Normal load response 

Not Estimated 

90
Table 12. Continued. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Pas

Transponder, B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmChinook salmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish 

Treatment release type/location 

Reference release type/location Turbine operation Direct sur

Bonneville Dam First Powerhouse 1939- 1945 Holmes 1952 Fin Clip P SYCS Various, upstream OR, WA, spillway, turbine Various tailrace locations Normal load response Not estima

Stay vane-blade tip 0.947 (SE Stay vane-mid-blade 0.964 (SE Stay vane-blade hub 

Draft-tube exit Original

Kaplan 6.2 kcfs 0.986 (SE 

Stay vane-blade tip 0.933 (SE Stay vane-mid-blade 0.959 (SE Stay vane-blade hub 

Draft-tube exit Original 

Kaplan 7.0 kcfs 1.009 (SE 

Stay vane-blade tip 0.963 (SE Stay vane-mid-blade 0.986 (SE Stay vane-blade hub 

Draft-tube exit Original 

Kaplan 10.5 kcfs 0.968 (SE 

Stay vane-blade tip 0.909 (SE Stay vane-mid-blade 0.968 (SE Stay vane-blade hub 

Draft-tube exit Original 

Kaplan 12.0 kcfs 1.004 (SE 

Stay vane-blade tip 0.955 (SE Stay vane-mid-blade 0.981 (SE Stay vane-blade hub 

Draft-tube exit MGR 

Kaplan 6.2 kcfs 0.986 (SE 

Stay vane-blade tip 0.949 (SE Stay vane-mid-blade 0.963 (SE Stay vane-blade hub 

Draft-tube exit MGR 

Kaplan 7.0 kcfs 0.974 (SE 

1999- 2000 

Normandeau Associateset al. 2000 

B P YCS

Stay vane-blade tip Draft-tube exit MGR Kaplan 

10.5 kcfs 

0.977 (SE0

Stay vane-mid-blade 0.977 (SE 

Stay vane-blade hub 0.986 (SE Stay vane-blade tip 0.947 (SE Stay vane-mid-blade 0.977 (SE Stay vane-blade hub 

Draft-tube exit MGR 

Kaplan 12.0 kcfs 0.980 (SE 

Tailrace downstream of turbine discharge frontroll 

Not estima

2002 Counihan et al. 

2003 

R P YCS Point; turbine intake 

MGR unit 

Tailrace downstream of PH2 JBS outfall 

Normal loadresponse 

Not estima

91
92

Table 12. Continued. Turbine passage survival estimates for Snake and Columbia River dams. Abbreviations: PIT = Pas

Transponder, B = Balloon, C = Coded-wire tag, R = Radio, SHT = steelhead, SYCS = subyearling Chinook salmChinook salmon, S = Single release, P = Paired release. 

Year Report

Tag type 

Survival model 

Test fish Treatment release type/location

Reference release type/location 

Turbine operation Direct s

Bonneville Dam Second Powerhouse

Upper turbine — 1 m below gate slot Lower turbine — 1 m below tip of STS 

Tailrace 2.5 km downstream 

response 

Upper turbine — 1 m below gate slot 

1988, 1989 

Ledgerwood et al. 1990 

CWT and Cold Brand 

P SYCS

Lower turbine — 1 m below tip of STS 

Frontroll 

Normal load 

Not esti
Figure 6. Cross-sectional view of a Columbia River dam showing turbine passageway components 

(adapted from Ploskey and Carlson 2004). 

Results from some recent studies do not match this basic assumption. Mathur et al. 

(1996, 2000) used balloon-tagged yearling Chinook salmon to evaluate turbine survival at Lower 

Granite Dam. They found no significant differences (P > 0.05) between survival estimates for 

fish passing through a turbine operated at the low end of the 1% efficiency range, near peak 

efficiency, and at a high-flow level outside the 1% range that caused cavitation. Maximum 

survival occurred at the low end of the 1% range. While we report the findings from several 

balloon-tag studies here, we have also noted above that balloon tags may not capture the indirect 

mortality that may occur in a tailrace and reservoir after turbine passage.

Normandeau Associates et al. (2000) evaluated direct smolt survival through Bonneville 

Dam First Powerhouse turbines under four discharge levels and at three different release 

locations within the turbine (tip, mid-blade, and hub) using balloon tags. Direct turbine survival 

varied more with fish distribution (release location) than discharge, and maximum survival did 

93
not occur at peak efficiency. Normandeau Associates and Skalski (1996) evaluated direct smolt 

survival through Rocky Reach Dam turbines under three discharge levels and at two different 

release locations within the turbine (3.0 and 9.1 m below the intake ceiling). Peak survival was 

observed to occur within the 1% range, but did not occur at peak efficiency. In contrast, in a 

similar study conducted at Wanapum Dam, Normandeau Associates et al. (1996b) concluded that 

maximum survival occurred outside of the 1% range in efficiency. 

In 2002 NOAA Fisheries Service conducted an evaluation of fish survival through 

McNary Dam turbines. We found no difference in fish survival between two treatments. 

Estimated relative survival to detection locations 46 and 15 km downstream was 0.858 

(SE=0.034) and 0.871 (SE=0.016) for the lower discharge (11,200 cfs, which was within the 1% 

range), and 0.814 (SE=0.037) and 0.856 (SE=0.011) for the higher discharge (which was 16,400 

cfs and outside the 1% range), respectively (Ferguson et al. in prep.). 

Normandeau Associates et al. (2003) conducted a series of balloon-tag tests in April 2002 

to evaluate direct turbine survival of Chinook salmon under four different operating levels of a 

single unit at McNary Dam. This evaluation provided direct survival estimates (90% CI) of 

0.944 (0.914–0.977), 0.955 (0.931–0.982), 0.983 (0.957–1.000), and 0.945 (0.925–0.964) for 

operating levels of 8,000, 11,200, 14,000, and 16,400 cfs, respectively. None of the estimates 

were significantly different. Of the four operating levels, 8,000 and 16,400 cfs were outside the 

1% range, while 11,200 and 14,000 cfs were within 1% of peak turbine efficiency. 

Skalski et al. (2002) conducted a retrospective analysis of recent balloon-tag studies at 

four Snake and Columbia River dams and concluded that peak passage survival did not occur at 

peak turbine efficiency. They found as much as a 3.2% difference between the maximum

survival and survival at peak turbine efficiency. Furthermore, a meta-analysis using results from

11 balloon-tag survival studies conducted at different sites found no association between survival 

and efficiency (r2

 = 0.0311, P = 0.2640). 

Operating near peak efficiency offers project operators guidelines that are convenient for 

management purposes, but as Skalski et al. (2002) pointed out, a statistical relationship between 

turbine efficiency and fish survival has not been observed. They recommended that turbines be 

operated where fish passage survival is maximum, and not solely on peak operating efficiency. 

While this recommendation is valid based on their analyses, currently, operations that correspond 

with maximum fish survival for turbines at COE dams on the Columbia River have not been 

determined. 

Thus given the complex and variable nature of the relationship between fish survival and 

turbine operations, guidelines for project operators are needed that provide the highest level of 

fish survival possible given the current information. Skalski et al. (2002) noted that operation of 

units within 1% of peak efficiency most often encompasses the turbine operation that

corresponds with maximum turbine survival, which was the case at McNary Dam in 2002 

(Absolon et al. 2003b). 

94
Studies Related to New Turbine Designs 

Since 1997 the COE has evaluated juvenile fish passage through Kaplan turbines with an 

emphasis on identifying turbine structures and operations responsible for injuring fish (COE 

2003). Their Turbine Survival Program (TSP) has focused on hydraulic model studies, 

engineering evaluations, and biological investigations to relate survival and injury to physical

conditions encountered during turbine passage. 

Under the TSP, Carlson et al. (2002) compared three-dimensional tracks of juvenile 

steelhead and Chinook salmon with neutrally buoyant drogues passing through a turbine at 

McNary Dam under two different operations. They found a significant difference in the 

trajectories and time to pass the turbine intakes between the juvenile salmon and drogues and 

recommended that particles placed in physical models cannot be used to simulate the elevation 

component of the trajectories of smolts passing through turbines. 

Carlson and Duncan (2003a) released sensor fish through turbines at McNary Dam in 

2002 in conjunction with Absolon et al. (2003b) and Normandeau Associates et al. (2003). They 

observed several differences in the physical conditions fish were exposed to under the two 

different operations. Exposure to draft tube turbulence was lower in both duration and 

magnitude and less dependent on intake (bay) location at a turbine discharge of 16,400 cfs 

compared to 8,000 cfs. These observations agreed with physical model data. When compared to 

similar exposure metrics for spill at Bonneville and The Dalles Dams, draft tube turbulence at 

McNary Dam under low turbine discharge may not be a major source of injury. Acceleration 

impulses believed to be associated with the stay-vane-wicket-gate cascade were more frequent at 

the lower discharge, possibly indicating increased potential for strike, scraping, and shear. 

A minimum gap runner (MGR) is a Kaplan turbine design that minimizes the clear 

openings between the rotating blade tip and stationary discharge ring, and between the base of 

the blade and the hub. This reduces the opportunity for fish to be entrained in potentially 

harmful areas of high velocity discharge through narrow gaps (Eicher Associates Inc. 1987, COE 

1995b). This is accomplished by incorporating a spherical hub, a specially shaped runner cone 

that allows water passing the blades to exit with a constant velocity and without excessive 

turbulence, and a spherical-shaped discharge ring to close the gap at the blade tips. Minimum

gap runners are being installed in all 10 units of the Bonneville Dam First Powerhouse. The 

efficiency curves of an MGR design are steeper and have more of a “peak” than standard Kaplan 

designs. Operating within 1% of peak efficiency for fish passage using this steeper curve 

reduces the turbine operating range. 

Fish survival through a MGR turbine at Bonneville Dam First Powerhouse was tested 

during the winter of 1999–2000 using balloon tags (Normandeau Associates et al. 2000). The 

MGR (Unit 6) was compared to a standard unit (Unit 5). The blade-to-wicket-gate relationships 

for the standard unit were adjusted or “tuned” prior to the test. Survival of juvenile Chinook 

salmon through the MGR turbines was evaluated for three different release locations (tip, midblade, and hub) under four different discharge levels: 6,200, 7,000, 10,500, and 12,000 cfs. 

Survival varied more with fish distribution (release location) than discharge. For both types of 

turbines, survival was higher for fish released near the hub and mid-blade locations than for fish 

released at blade tips (Table 13). 

95
Table 13. Ranges in estimated survival for three turbine release locations; each range represents the four 

turbine loads tested (Normandeau Associates et al. 2000). 

Release location MGR Unit 5 

Hub releases 0.974–0.986 0.968–1.009

Mid-blade releases 0.963–0.981 0.959–0.986 

Tip releases 0.947–0.977 0.909–0.963

Maximum survival occurred at peak unit efficiency for the blade-tip and mid-blade 

releases. Maximum survival for the hub releases did not occur at peak efficiency, and in fact, the 

lowest survival was at peak efficiency. Skalski et al. (2002) conducted additional analyses of the 

Bonneville Dam MGR data and found no significant relationship between salmonid survival and 

turbine efficiency at any of the three release locations, nor between survival and average head, 

blade angle, discharge, or power generated. Survival and injury comparisons between the MGR 

unit and the standard unit indicated that MGR survival was equal to or better than the standard 

unit. Absolute survival for fish passing near the MGR blade tip was up to 3% higher than for the 

same location in the standard unit. Overall, survival was not significantly different between the 

units; however, the incidence of fish injury was lower for fish passing through the MGR unit 

(0.014) compared to the standard unit (0.025). 

Since 1994 the DOE has funded studies under the Advanced Hydropower Turbine 

Systems Program (AHTS) to develop environmentally friendly turbines, with the goal of

improving hydropower’s environmental performance. Ploskey and Carlson (2004) modeled the 

probability of blade strike for the Bonneville Dam First Powerhouse turbines and compared these 

predictions to empirical data collected in 1999 and 2000 by Normandeau Associates et al. 

(2000). Blade-strike models predicted that the probability for injury from strike increases with 

decreasing unit discharge and with increasing distance from the center of the runner where the 

fish passes the blade. However, the predicted injury from strike was from two to five times 

higher than that observed in the empirical data and there was not a strong relationship between 

injury and discharge in the empirical data. They pointed out that the researcher’s inability to 

assign causal mechanisms to observed injuries hinders relating injury or mortality to discharge. 

They concluded that efforts to empirically determine fish distributions at the turbine intake 

trashracks, wicket gates, and runner blades are needed before models can be used more 

effectively to identify turbine operations and designs that reduce injury and mortality rates. 

To investigate the relative importance of pressure as a source of turbine passage mortality 

and injury, Abernathy et al. (2001) examined the responses of fish both acclimated to and not 

acclimated to gas supersaturation to rapid pressure changes. They concluded that if gas 

saturation is not a problem, exposure of juvenile salmonids to areas of low pressure associated 

with passing a turbine runner causes little direct mortality. Under high saturation levels, passage 

through areas of low pressure could produce injury and mortality if fish responded to 

supersaturation by seeking greater depths, because if fish are acclimated to depth, injury rates 

increase due to expansion of gas in the swim bladder. 

Coutant and Whitney (2000) conducted a literature-based evaluation to develop 

biological information regarding normal behavioral patterns of fish for use in engineering 

designs to improve the survival of fish passing through turbines. They suggested that models of 

96
fish trajectories should not assume neutral buoyancy for the period fish are passing through a 

turbine. This was supported by empirical results from Carlson et al. (2002), discussed at the 

beginning of this subsection. 

Discussion 

The primary question that salmon managers and the action agencies need answered is 

whether existing operations provide the best protection for juvenile salmon migrating 

downstream, or whether alternative operations are warranted based on the available data. The 

data in Table 12 reflect the variability associated with survival estimates through turbines 

relative to specific operating loads, conditions, and methods used to make the estimates. The 

data and analyses reported in the literature support two points: 1) operating within 1% of peak 

unit efficiency is a reasonable surrogate operation to maximize fish survival until additional, 

detailed information on the relationship between fish survival and turbine unit operations is 

developed, and 2) additional, rigorous biological testing will be required to determine the 

optimal turbine operations for each species and life history type of salmon and each set of 

turbines. 

The need for rigorous evaluations of fish survival associated with each set of turbines 

presents a dilemma to salmon managers and hydro operators. The sample sizes required to 

distinguish small differences between test conditions can only be achieved at a high cost in terms 

of the number of fish required. For example, Perry et al. (2003) estimated that 244,000 

subyearling Chinook salmon would be required to detect a difference between treatments of 1% 

using a 2-tailed test when "=0.05 and $=0.80. This number is reduced by use of a 1-tailed test, 

and is significantly reduced by use of a significant t-test and by adjusting ", $, or the difference 

between treatments, but still remains high. For example, when "=0.05 and $=0.80 and using a 

1-tailed test, more than 21,000 subyearling Chinook salmon would be required to detect a 

difference of 2% between treatments with a significant t-test. Based on comparisons of balloontag and radiotelemetry data from studies conducted at McNary Dam in 2002 (Ferguson et al. in 

prep.) and by Bickford and Skalski (2000), indirect mortality can be a significant component of 

total estimated turbine mortality and should be incorporated into any new study designs. 

Releases of treatment groups made upstream of the dam for these studies represent the 

distribution of the general population, but also incorporate forebay and reservoir mortality into 

the treatment unless these are partitioned out of the estimate. Point releases made into turbine 

intakes allow for specific areas of the passage environment to be evaluated, and can represent the 

overall population passing through the turbine if an assumption is made that the point release 

represents the overall turbine environment. Data presented in Table 12 present few opportunities 

to directly compare estimates made within the same year using both release strategies. Use of 

the paired-release protocol requires that reference groups be released downstream of the test 

turbine, typically either in the front roll of the turbine boil or 1–2.5 km downstream from the 

immediate tailrace. 

Although few studies made both daytime and nighttime releases, the available 

information suggests that tailrace predation may be a component of turbine mortality if the 

tailrace is included in the treatment. We base this statement on trends observed in a study of 

survival at The Dalles Dam in 2000. Absolon et al. (2002) estimated that relative survival of 

97
PIT-tagged yearling Chinook and coho salmon through turbines (turbine releases compared to 

tailrace releases made approximately 2 km downstream from the project) was 0.790 (95% CI, 

0.748–0.834) for daytime releases and 0.830 (95% CI, 0.785–0.879) for nighttime releases. 

Estimated relative survival of subyearling Chinook salmon passing through turbines during the 

summer was 0.791 (95% CI, 0.703–0.890) and 0.889 (95% CI, 0.790–1.000) during the day and 

night, respectively. Although nighttime survival was higher during both seasons, these 

differences were not statistically significant. 

An important finding from the COE TSP is that draft tubes were designed for specific 

turbine loadings, and hydraulic performance of the draft tube increases with discharge. This is 

part of the reason that turbulence through turbines and passage times of neutrally buoyant beads 

are higher at lower discharges. This observation may be of considerable importance and turbine 

investigators will likely focus on this in the future. In addition, the most severe hydraulic 

conditions occur at the trailing edges of wicket gates and runner blades, within the hub “rope,” 

and near the leading edge of draft tube splitter walls. 

Potential negative effects from fish diversion screens installed in turbine intakes on 

turbine passage survival is an aspect of passage through turbines that should be addressed. The 

survival data presented in Table 12 include screens installed in turbine intakes in most cases, the 

exceptions being the early studies before screens were installed starting in the 1970s and The 

Dalles Dam. Therefore, most recent studies incorporated effects from screens into the reported 

survival estimates. Because the potential effect from screens has not been partitioned from the 

overall estimates, little can be discussed regarding this issue. However, we note that engineering

studies have shown that intake screens reduce turbine unit efficiency, which may affect juvenile 

salmonid survival. 

Passage Through Turbines Conclusions 

1. Estimates of survival through turbines from studies conducted during the period of early 

FCRPS development, from 1939 through 1968, ranged from 0.81 to 0.92 (Holmes 1952, 

Schoeneman et al. 1961, Long et al. 1968) (Table 12). Studies conducted under contemporary conditions using radiotelemetry and PIT-tag methodologies suggest mean survival 

through turbines generally remains in this range, but means as low as 0.72 have been 

reported. Therefore, turbines continue to represent a significant source of mortality for 

juvenile salmon migrating through the FCRPS. 

2. Balloon-, PIT-, and radio-tag methodologies all have valid applications, depending on the 

study objectives. Radio or PIT tags should be used when the goal of the study is to 

incorporate both direct and indirect mortality into the survival estimate. The balloon-tag 

methodology is a useful diagnostic tool to examine test fish for injuries and correlate injury 

types with physical parameters experienced during passage, but it is not a good estimator of 

overall survival rates in all cases. All studies require that careful consideration be given to 

the use of release mechanisms, selection of representative test fish, location of treatment and 

control releases, and verification of test assumptions. 

3. We do not recommend obtaining estimates of turbine survival based on adult returns due to 

large sample sizes required to detect small differences between treatments. The action

agencies and salmon managers should rely on juvenile recapture data to judge turbine 

98
passage effects and differences between treatments, as no apparent differences exist in 

measurements of turbine survival between studies based on juvenile recaptures or adult 

returns. 

4. Indirect mortality can be a significant component of total estimated turbine mortality. Future 

efforts to improve survival through turbines have to address this component if improvements 

are to be made to the turbine passage route. 

5. A statistically significant relationship between juvenile salmonid survival and Kaplan turbine 

peak efficiency for Snake and Columbia River dams has not been demonstrated. This is 

because it is difficult to detect small differences between test conditions under highly

variable conditions. Operation of units within 1% of peak efficiency most often 

encompassed the operation that produced the maximum estimated survival. Thus continued 

operation within 1% of peak turbine efficiency is a reasonable operating guideline to follow 

until more detailed information on the relationship between unit operations and maximum

fish survival for each family of turbines at FCRPS dams is developed. To develop more 

refined guidelines, additional rigorous biological testing for each set of turbines will be

required. These tests appear warranted because maximum survival can be as much as 3.2% 

higher than survival measured at peak efficiency. This potential improvement in survival is 

large compared to other potential FCRPS configuration modifications to improve survival. 

6. Since 1997 a significant amount of effort has gone into understanding the causal mechanisms

of mortality associated with fish passage through turbines. However, we have not seen a 

significant reduction in turbine passage mortality associated with any new turbine designs 

tested to date. Additional studies will be needed and new designs developed before turbine 

passage survival is improved to levels similar to alternative passage routes. There are no

easy solutions to improving fish passage survival through this challenging route, and efforts 

to do so should be viewed as a long-term process. 

99
Adult Passage 

Background 

All eight Columbia and Snake River mainstem dams, from Bonneville through Lower 

Granite, provide upstream passage for adult salmon and steelhead through one or more fishways. 

The fishway systems are operated according to criteria developed by the COE, NOAA Fisheries 

Service, and state and tribal fishery co-managers. Each criterion is based on results of biological 

testing to determine the hydraulic conditions that maximize fish attraction and minimize delay. 

The COE annually coordinates with salmon managers and publishes the Fish Passage Plan 

(FPP). The FPP provides detailed operating procedures and criteria for adult fish passage 

facilities and special operations to accommodate research. This includes criteria on water depth 

and head on the entrance gates, powerhouse collection channels, floating orifices, ladder flow, 

counting windows, and fishway exits. 

There is a significant backlog of unfunded maintenance and repair projects on entrance 

gates and gate lifting machinery at Snake and Columbia River dams. Also, at some Snake River 

project entrance gates, the auxiliary water supply (AWS) systems cannot supply sufficient 

attraction water to meet the required minimum of 0.3 m head differential between the adult 

collection channel and tailrace. This situation occurs during low tailrace conditions, when river 

flows are low and the downstream project is operated within the lower foot of the operating 

range of the reservoir. 

At Lower Granite, Little Goose, and Lower Monumental Dams on the Snake River, all 

fishway entrances are served by a single AWS pump station. Fishway entrances located on the 

opposite side of the spillway from the pump station cannot satisfy the established flow criteria. 

During high river flow conditions, submergence and head differential criteria for south entrances 

at Lower Monumental and the north entrances at Little Goose and Lower Granite Dams are not 

met as pumps cannot get enough water into the AWSs. During low river flow, and with 

reservoirs at minimum operating pool, submergence criteria for south entrances at Lower 

Monumental Dam and the north entrances at Little Goose and Lower Granite Dams are not met. 

The NMFS 2000 BiOp requires the COE to provide an emergency source of water at 

each project to satisfy fishway criteria if the main AWS system fails. Engineering studies have 

been completed for all projects except Little Goose and Lower Granite Dams. The Walla Walla 

District has selected a contractor to complete the design studies for these projects. John Day and 

McNary Dams were able to meet the BiOp requirement with existing facilities. Engineering 

alternatives for meeting emergency AWS capability at The Dalles Dam and Bonneville Dam

Second Powerhouse are being evaluated. 

100
Research Results and Discussion 

Dam Passage 

Migration behavior of adult salmonids in the Columbia and Snake River drainages has 

been documented using uniquely coded radio transmitters. The transmitters are implanted in the 

fish’s esophagus or surgically inserted into the abdominal cavity and are detected using an 

extensive array of fixed-site antennas. Matter and Sandford (2003) compared the upstream travel 

times of PIT-tagged and radio-tagged spring/summer Chinook salmon from Bonneville to Lower 

Granite Dams in 2000. They found no evidence that radio tagging negatively affected Chinook 

salmon behavior. 

Salmonids moving up through the Columbia and Snake rivers tend to stay near the shore. 

This seems to hold true for all species and runs. This trend is documented by tracking radiotagged fish, and data from individual fish tracks indicate crossing between sides of the river or 

reservoir occurs at what appear to be random locations. Natural migration behavior is a 

consideration when designing fishway entrances and exits, and Keefer et al. (2003a) showed that 

existing fishway entrances located near shorelines had high net entrance rates (number of fish 

entering orifices minus number of fish leaving), and high fishway entrance efficiency (entrances 

that produce passages). 

For upstream migrating adult salmonids, the first approach to a dam changes as the 

proportion of river discharge changes from powerhouse to spillway. With no spill, most fish 

approach the dam on the shoreline adjacent to the powerhouse, and as spill begins a portion of 

those fish move to the shoreline adjacent to the spillway. At moderate spillway discharge, more 

fish approach the dam at the junction of the powerhouse and the spillway. Under rare high-flow 

and high-spill events, when turbidity is also usually high, upstream movement of adults can slow 

considerably for a few days until the event subsides. Radiotelemetry studies at lower Snake 

River dams have indicated passage delays associated with high spill ($40 kcfs) and highly 

fluctuating spill (Turner et al. 1983, 1984). 

Once at the dam, fish search across the downstream face looking for passage routes. 

Entrance preferences are for deep/wide openings with significant attraction flow. Shallower/ 

smaller openings tend to have low numbers of entrances and negative net entrance rates. Median 

times for Chinook salmon to first enter fishways ranged from 1.9 to 2.6 hours at Ice Harbor and 

Lower Granite Dams in 1993 and 1994 (98.7% and 98.9% entered in less than 5 days). At 

Lower Monumental and Little Goose Dams, 98.5% and 98.2% entered in less than 5 days, and 

median times to first entries were 4.6 and 3.9 hours, respectively (Bjornn et al. 1998d). 

Radiotelemetry studies in 1993 and 1994 have shown that adult fish both enter and exit 

the floating orifices in the powerhouse collection channels of Snake River dams. The net entry 

rate indicated that for fish entering the collection channel via the orifices, more fish left via the 

orifices than stayed in the collection channel (Bjornn et al. 1998d). The COE Fish Passage 

Operations and Maintenance (FPOM) committee concluded that closing the floating orifices at 

Snake River dams would improve the operation of the adult fishways because more water would 

be available for the main fishway entrances, and maintenance would be improved because the 

bulkhead could be sealed and the collection channels dewatered on a more frequent basis. 

101
First entry times in the lower Columbia River are comparable to those at the three lower 

Snake River dams. At Bonneville Dam, Chinook salmon median first entry times were 2.0 and 

2.2 hours in 1996 and 1997, respectively; steelhead median first entry times were 1.9 and 

0.3 hours in 1996 and 1997, respectively. First entry times ranged from 0 (when the first at-dam

record was in the collection channel) to 20 days for Chinook salmon and 17.8 days for steelhead. 

Bjornn et al. (1998b) concluded that when powerhouses are not at full load, changing the 

end of the powerhouse where turbines were operating had little if any influence on the time for 

steelhead to approach, enter, or pass fishways at Snake River dams. They saw a slight change in 

first-approach sites, but not first-entrance locations. Bjornn et al. (1998f) did not detect any 

differences in Chinook and sockeye salmon or steelhead entrance locations or passage times 

when turbine unit 1 (and its associated discharge located near the south shore adult entrance) at 

John Day Dam was operated at 150 and 100 megawatts. 

Fishway fences installed adjacent to the north powerhouse entrances at Little Goose and 

Lower Granite Dams in 1991 to reduce fallouts as fish moved upstream in the collection channel 

were not effective (Bjornn et al. 1995). Funneling the down-channel moving fish away from the 

entrances at the downstream end of the collection channels at Little Goose Dam in 1994 

improved the net entrance rate at those entrances (Bjornn et al. 1998d). 

A high rejection rate of the transition area between the collection channels and the fish 

ladders has been documented for spring/summer Chinook salmon and steelhead (Stuehrenberg 

et al. 1995, Bjornn et al. 1998d). For steelhead, the rejection rates range from 46% to 71% for 

their first approach to the transition area at Snake River dams. Similar rates were found for 

Chinook salmon at both Columbia and Snake River dams in 1996 (Keefer et al. 2003a). 

In 1994 the University of Idaho Cooperative Fish and Wildlife Research Unit installed 

radiotelemetry antennas in the transition pools of the four lower Snake River projects and tracked 

the progress of 220–246 steelhead at each project. From 36% to 61% of the fish turned around 

in the transition pools, moved downstream, and exited the fishway at least once, and an 

additional 8–27% moved downstream in the fishway but did not exit (Bjornn et al. 1998a). 

From 4% to 35% of spring/summer Chinook salmon turned around in the transition pool 

and headed downstream at McNary Dam and Ice Harbor Dam. At Bonneville Dam and Lower 

Granite Dam, 53–55% turned around in the transition pool (Keefer et al. 2003a). Similarly, fall 

Chinook entering fishways at lower Columbia River dams turn downstream in the transition pool 

more so than any other fishway segment.7

Several hypotheses have been offered to explain this transition and junction pool 

behavior: 1) velocities are too low and unsteady, 2) flow rates and velocity are inadequate to 

attract fish to the submerged section of the ladders and through the orifices at the base of the 

submerged ladder weirs, 3) seasonal and intermittent temperature gradients exist between the 

ladder flow and the diffuser flow, 4) high flow rates through large floor diffuser areas obscure 

 

7

 B. Burke, NWFSC, Seattle. Pers. commun., October 2003. 

102
attraction flow at the base of the ladders, and 5) fish may be wary and reluctant to move into 

confined ladder pools. 

Jumping within and from fishways is another adult passage issue. This behavior is 

typically associated with the transition from overflow weirs to vertical slot weirs, and with 

diffuser flow at the top of fishways (Dresser and Stansell 1998). This occurs primarily with 

steelhead at both John Day Dam fishways. However, at Lower Granite Dam faster passage times 

and more direct passage routes were observed for both Chinook salmon and steelhead when weir 

panels were in the down position (Naughton and Peery 2003). 

Migration Rates 

The earliest study of migration rates in the Snake River prior to impoundment was 

conducted from 1954 to 1957 (Oregon Fish Commission 1960). Chinook salmon migration rates 

averaged 17.7–24.1 km/day. Steelhead migration rates varied with the season; spring rates 

averaged 11.3–16.0 km/day, whereas fall rates averaged 8.0–9.7 km/day. Steelhead tagged at 

McNary Dam in January, February, and April migrated at average rates of 3.2, 3.9, and 

12.2 km/day, respectively. Sockeye averaged 19.3 km/day to a weir at Redfish Lake in Idaho. 

The effect of season and water temperature on steelhead migration rates was also seen in 

studies conducted from 1969 to 1971, where steelhead had summer migration rates of 10.7–

16.7 km/day, and late fall migration rates as low as 0.5 km/day. Bjornn and Peery (1992) 

concluded that in the Snake River prior to impoundment, salmon migrated from 18 to 24 km/day, 

and steelhead migration rates varied with season and water temperature and ranged from 11 to 

17 km/day during the spring and summer to as low as 0.5 km/day during the late fall. 

Spring/summer Chinook salmon migration rates through the Snake River reservoirs in 

1991–1993 ranged from 31 to 65 km/day, while migration rates through free-flowing river 

sections above Lower Granite Dam ranged from 10 to 30 km/day (Bjornn 1998c). Travel 

through the Columbia River hydrosystem was slower, with median rates of 13–33 km/day for 

spring Chinook in 1996–2000. Migration times from Bonneville Dam to McNary Dam ranged 

from 6 to 20 days for individual fish. For all groups, migration rate variation between years was 

correlated to mean flow and temperature; the date of migration was the most influential predictor 

of migration rates, followed by river discharge (Keefer et al. in press). 

The median steelhead migration rate through Snake River reservoirs in 1993 was 

30 km/day, while migration rates through free-flowing river sections were generally less than 

11 km/day (Bjornn 1998c). Based on the passage times at the dam and faster passage in the 

reservoirs than in free-flowing rivers, Bjornn et al. (1999) estimated that median time for salmon 

to pass the four dams and reservoirs in the lower Snake River in 1993 was the same or less with 

dams as without the dams. 

In 1998, 38 hatchery steelhead of known Snake River origin (based on PIT tags) were 

detected at both Bonneville and Lower Granite Dams (through 31 December 1998). Their 

median time for passage between the two dams was 32 days, or 14.4 km/day (ranged from 5.7 to 

40.7 km/day for the 460 km reach). Also in 1998, 22 spring/summer Chinook salmon of known 

Snake River origin (based on PIT tags) were detected at both Bonneville and Lower Granite 

103
Dams. The median time for passage between the two dams (a total of 460 km) was 16.4 days, or 

28.0 km/day (ranged from 12.4 to 47.9 km/day). 

Also in 1998, 38 fall Chinook salmon of known Snake River origin (based on PIT tags) 

were detected at both Bonneville and Lower Granite Dams. Of these fish (1 wild and 37 

hatchery fish), the median time for passage between the two dams was 12.0 days, or 38.3 km/day 

(ranged from 13.9 to 51.1 km/day).8

 This suggests that the 1998 migration rate of PIT-tagged 

steelhead through the FCRPS falls within the range observed in the Snake River prior to 

construction of dams, and the migration rate of PIT-tagged spring/summer Chinook salmon 

slightly exceeds the range reported by Bjornn and Peery (1992) for Chinook salmon (run-type 

was not specified). 

Matter and Sandford (2003) compared the upstream travel time of PIT- and radio-tagged 

adult spring/summer Chinook salmon from Bonneville Dam to Lower Granite Dam in 2000. 

Travel time for adult Chinook salmon outfitted with PIT tags (15.9 days) was significantly 

longer than passage time for fish outfitted with radio transmitters (14.1 days). The authors 

concluded that this difference was probably related to differences in study design or data 

complications. 

Raymond (1964) compared the median migration timing of sockeye and Chinook salmon 

past Bonneville and Rock Island Dams between 1938 and 1950 when no other dams existed in 

the hydropower system corridor. The mean passage time between Bonneville and Rock Island 

Dams for sockeye salmon was 16.5 days (range 7–27 days). NOAA Fisheries Service computed 

the same statistic for the period between 1985 and 1999 and found a mean difference in passage 

time of 15 days (range 11–19 days). Quinn et al. (1997) also studied migration rates of 

Columbia River sockeye salmon. They found that travel time decreased from 1954 to 1994 

between Bonneville and McNary Dams, but was unchanged between McNary and Rock Island 

Dams. 

Fallback 

A substantial percentage of adult salmon and steelhead passing dams have been observed 

to fall back through spillways and turbines at certain dams under certain conditions (Bjornn and 

Peery 1992). High fallback rates are usually associated with high river flows and spill, as well as 

the location of fishway exits relative to the spillways. For example, the south fishway at 

Bonneville Dam exits into the forebay at Bradford Island in close proximity to the spillway and 

has been associated with the highest fallback rates on the Columbia River. Bjornn et al. (1998e) 

found that fallback rates were 2.5–3.7 times higher for spring/summer Chinook salmon that 

passed via the Bradford Island fishway than the Washington shore fishway, and 16–20 times 

higher for sockeye salmon. When only fallbacks within 24 hours of passage are considered, 92–

97% of all fish that fell back at Bonneville Dam between 1996 and 1998 had used the Bradford 

Island fishway (Bjornn et al. 1998e). In 1996 almost all steelhead that fell back passed via the 

Bradford Island fishway. 

 

8

 L. Stuehrenberg, NWFSC, Seattle. Pers. commun., September 1999. 

104
Bjornn and Peery (1992) presented fallback rates from over 50 radiotelemetry studies of 

Chinook and sockeye salmon and steelhead at various dams from 1966 through 1985. Keefer 

and Bjornn (1999) estimated (based on radiotelemetry) that adult counts at Bonneville Dam were 

overcounted by 13.5–19.3% for spring/summer Chinook salmon from 1996 through 1998, by 

4.7–8.2% for steelhead from 1996 through 1997, and by 12.6% for sockeye in 1997 when 

fallback and reascension were taken into account. Similarly, from 1996 to 2001, passage 

overestimation at lower Columbia and Snake River dams ranged from 1% to 16% for 

spring/summer Chinook salmon, 1% to 38% for fall Chinook salmon, and 1% to 12% for 

steelhead (Boggs et al. 2004). 

Between 1996 and 2001, fallback rates for spring/summer Chinook salmon in the lower 

Columbia River ranged from 2% to 20% and from 1% to 14% at Snake River dams. Fallback 

rates for steelhead during that same time period ranged from 5% to 13% at lower Columbia 

River dams and from 2% to 9% at Snake River dams. From 1998 to 2001, fallback rates for fall 

Chinook ranged from 2% to 12% at lower Columbia River dams and from 3% to 58% at Snake 

River dams (Boggs et al. 2004). 

In addition to fish that fall back within 24 hours of exiting the fishway, a large number of 

fallbacks occur after fish have migrated significant distances upstream. Dam operating 

procedures or environmental conditions at individual dams are unlikely to affect fallback 

associated with this behavior. The cause of this wandering behavior has not been determined. 

Potential causes include difficulty locating natal tributaries, natural survival adaptation, the use 

of hatchery broodstock not native to the drainage, and some level of homing impairment from

having been transported. Mendel and Milks (1996) attributed much of the fall Chinook salmon 

fallback they observed to poor entrance conditions at the Lyons Ferry Hatchery and extensive 

wandering up and down the Snake River and into tributaries. They estimated that the fallback at 

Lower Granite Dam was 16–39% in 1993 and 30–41% in 1992. They noted that over 80% of the 

radio-tagged fall Chinook salmon that entered Lyons Ferry Hatchery each year had reached 

Little Goose Dam before descending downstream to the hatchery. 

Keefer and Bjornn (1999) reported that for all seven dams studied in 1996, the median 

passage time at a dam was higher for spring/summer Chinook salmon that fell back one or more 

times. The impacts of increased passage times due to fallback on adult delayed mortality and 

reproductive success are unknown. 

Survival 

Losses can be broken down into two segments: 1) losses between dams, defined as the 

difference between fish counted at consecutive dams minus those accounted for between those 

dams (harvest and tributary turn off), and 2) losses between the upper dam and the spawning 

grounds or hatchery. 

Losses between dams that are based on dam counts are susceptible to counting errors and 

assumptions. For example, the largest portion of overcounts at dams is likely due to multiple 

counts of fish that have fallen back over the dam and reascended the ladder. Because fallback 

rates vary with each dam, environmental conditions, and project operations, bias or error 

associated with dam counts is variable. Radiotelemetry provides a more accurate assessment of 

105
actual passage rates and behavior than dam counts, although use of radiotelemetry assumes that 

tagged fish represent the general population and that tagging and release do not affect the 

behavior of the fish. Furthermore, estimates of survival based on radiotelemetry may be 

conservative because they include both actual mortality and unaccounted-for losses, such as tag 

regurgitation, unreported harvest, and tributary turnoff that is missed or not monitored. Survival 

estimates based on detection of PIT-tagged adults is also accurate and increasingly possible as 

detectors are installed at more dams. 

With respect to losses between the upper dam and the spawning grounds or hatchery, 

little early information is available. Survival-to-spawning for spring/summer Chinook salmon 

was estimated at 55% for fish counted at Ice Harbor Dam from 1962 to 1968 when only one 

Snake River dam was present, and 46% from 1975 to 1988 when all four dams were in place, 

based on relating counts at Ice Harbor Dam with redd counts in Snake River tributaries (Bjornn 

et al. 1998g). 

More recently, with all four Snake River dams in place, radiotelemetry has been used to 

estimate the survival of spring/summer Chinook salmon from Ice Harbor Dam to spawning 

grounds or hatcheries. Bjornn et al. (1995) estimated survival through this reach was 77% in 

1993, 73% in 1992, and 54% in 1991 for fish that were radio tagged at John Day Dam in 1993, 

and Ice Harbor Dam in 1992 and 1991. Caution should be used in comparing survival between 

the pre-development and post-development periods because of differences in technique and 

methodology. 

For spring/summer Chinook salmon radio tagged at Ice Harbor Dam, survival from Ice 

Harbor Dam to Lower Granite Dam was 80.8% and 74.4% in 1991 and 1992, respectively. For 

spring/summer Chinook salmon radio tagged at John Day Dam in 1993, survival from Ice 

Harbor Dam to Lower Granite Dam was 85.9% (Bjornn et al. 1999). 

For spring/summer Chinook radio tagged at Bonneville Dam in 1996, 1997, and 1998, 

survival from Ice Harbor Dam to Lower Granite Dam was estimated two ways: first, for only 

those fish that passed the top of Ice Harbor Dam and moved upstream, and second, for all fish 

that approached Ice Harbor Dam (whether or not they passed the dam). Survival from Ice 

Harbor Dam to Lower Granite Dam for fish that passed the top of Ice Harbor Dam was 96.4%, 

97.7%, and 98.4% in 1996, 1997, and 1998, respectively. Survival from Ice Harbor Dam to 

Lower Granite Dam for fish that approached Ice Harbor Dam was 92.8%, 95.2%, and 98.4% in 

1996, 1997, and 1998, respectively (in 1998 both survival estimates were 98.4% because all fish 

that approached Ice Harbor either passed the top of the dam or were harvested or detected 

downstream. 

Estimates of survival through the Snake River based on tagging conducted at John Day 

and Bonneville Dams probably reflect a more accurate estimate of survival for fish passing 

Snake River dams as they do not include effects from trapping, tagging, and release because 

these likely occurred prior to entering the lower Snake River.

Bjornn et al. (1998g) found that the survival of spring/summer Chinook salmon from Ice 

Harbor Dam to over Lower Granite Dam from 1991 to 1993 was similar between fish marked at 

Ice Harbor Dam with radio transmitters (83–87%) and unmarked fish (70–92%), based on 

106
comparing radiotelemetry data with dam counts. This suggests that there was no effect on 

survival estimates from trapping fish at Ice Harbor Dam.

The installation of a complete PIT-tag detection system at Bonneville Dam in 2001 

combined with additional detection capabilities upriver has allowed adult survival from 

Bonneville Dam to Lower Granite Dam to be estimated based on PIT tags. Data from

transportation studies conducted by NOAA Fisheries Service shows that for spring/summer 

Chinook salmon adults, survival varied between 80% and 85%, with no effects of transportation 

as juveniles. Steelhead survival for this section was 70% for adults that were transported as 

juveniles and 80% for nontransported fish. 

Assigning mortality associated with fallback to dam operations or behavior is difficult 

because some fish may have “overshot” and then returned to lower river tributaries. Bjornn 

(1998a) observed fallback mortality of 8% for sockeye salmon at Bonneville Dam (a species 

with no spawning below Bonneville Dam). Mendel and Milks (1996) estimated fall Chinook 

fallback mortality at 26% and 14% in 1993 and 1994, respectively, for fish that fell back through 

one or more of the four lower Snake River dams. This higher mortality for fall Chinook 

occurred during periods of no spill, when the fallback was assumed to have been through 

turbines. Keefer and Bjornn (1999) used radio-tagged fish known to have passed Bonneville 

Dam in 1996 to estimate survival to tributaries or the top of Priest Rapids Dam. Steelhead and 

spring/summer Chinook salmon that did not fall back over Bonneville Dam had survival rates 

that were 3.8–5.2% higher than fish that did fall back. Fish that did not fall back over any dam

had survival rates from 3.0% to 5.4% higher than fish that did fall back.

Marine Mammal Predation 

When considering all causes of salmonid decline in the Columbia River Basin, predation 

by marine mammals, while not considered a dominant regional cause, can be a significant local 

factor, especially when salmonid runs are low (NRC 1996). Marine mammals prey on salmonids 

near man-made structures such as dams or fish passage facilities where fish congregate, and the 

presence of marine mammals in the lower Columbia River and estuary during adult salmonid 

migrations raises serious concern for already depressed populations. However, the current 

precision of estimates of the percentage of salmonid spawner escapement consumed by marine 

mammals in the Columbia River Basin is poor. This is due in part to high temporal variability 

(interannual, seasonal, weekly, and day versus night foraging) and spatial variability in salmonid 

consumption, and the difficulties in sampling sufficiently to capture all major sources of 

variability. 

Three species of pinnipeds—harbor seals (Phoca vitulina), California sea lions (Zalophus 

californianus), and Stellar’s sea lions (Eumetopias jubatus)—occur in the Columbia River. 

Harbor seals are the most abundant pinniped species in the lower Columbia River, with peak 

haul-out counts exceeding 2,000 at Desdemona Sands, a tidal sandbar adjacent to Astoria, 

Oregon, which is the largest seal haul-out. However, harbor seals in the Columbia River can 

forage many kilometers away from haul-outs, including upriver areas and coastal regions outside 

the river. 

107
Markings (including apparent tooth and claw abrasions) commonly observed on returning 

adult salmon are indicative of marine mammal predation. Harmon et al. (1994) estimated annual 

injury rates of 14–19.2% for spring and summer Chinook salmon and 5.4–14.2% for steelhead 

that arrived at Lower Granite Dam from 1990 to 1993. For the years 1994 through 2001, the 

injury rate for adult spring and summer Chinook salmon at Lower Granite Dam ranged from

15.7% to 28.6%.9

 Based on the size of some of the teeth marks, it is believed they were made by 

harbor seals. A NOAA Fisheries Service report to Congress (NMFS 1999b) under the Marine 

Mammals Protection Act concluded that pinniped populations are now abundant, increasing, and 

widely distributed along the West Coast, where there is a high potential for impacts on salmonid 

populations. 

Zero Flow Operations 

Powerhouse discharge can be reduced to near zero during hours of darkness to preserve 

water for periods of higher power demand. Although the proportions of adult fish recovered 

were small, Bjornn et al. (1998c) found no statistically significant evidence that reducing flows 

to near zero at night affected adult steelhead migration rate, the proportion of fish passing dams, 

the proportion of fish captured in the fishery, or the proportion of fish returning to hatcheries in 

1991, 1992, and 1993. A consistent pattern of slower steelhead migration in the early and late 

portions of the runs was found, which was associated with warm and cold water temperatures, 

respectively. Adult fishways were operated continuously throughout the tests. McMaster et al. 

(1977) also found no difference in the migration rates of Chinook salmon and steelhead when 

discharge was reduced to near zero at night. 

Water Temperature 

Bell (1991) described the preferred temperature range and upper and lower lethal 

temperature limits for most salmonid species. For example, for Chinook salmon the preferred 

range is 7–14.5o

C (45–58oF), with a lower and upper lethal limit of 0°C and 25oC (32°F and 

77o

F). Summer water temperatures in the Snake and Columbia rivers often exceed 21oC (70EF), 

and water temperatures in adult fishways can be even higher than ambient river temperatures. 

River water temperatures greater than 20EC increase travel times between dams for 

migrating salmonids (Peery et al. 2003). Affected fish may abort their migrations or seek cooler 

water that may not be in the direct migration route to their spawning site. Snake River fall 

Chinook salmon and steelhead often slow their migration through the Columbia River and delay 

entering the Snake River when water temperatures are high (Stuehrenberg et al. 1978). These 

fish seek temporary refuges in lower Columbia River tributaries en route to their final 

destinations at upriver locations or hold in cooler Columbia River water off the mouth of the 

Snake River until Snake River water temperatures cool. 

Of 125 Chinook salmon recorded at Ice Harbor Dam in 1996, 20% had been recorded in 

lower Columbia River tributaries (Bjornn et al. 2000). For steelhead, 65% of fish passing Lower 

 

9

 J. Harmon, NWFSC, Lower Granite Research Station, Clarkston, WA, and D. Marsh, NWFSC, Seattle. Pers. 

commun., December 2001. 

108
Granite Dam were observed in one or more lower tributaries (Keefer et al. 2002). In both cases, 

peak tributary use coincided with peak mean daily temperatures at dams (approximately 22EC), 

while recorded temperatures in the tributaries were less than 20EC. Delays associated with high 

water temperatures can subsequently affect reproductive success. Of 71 radio-tagged sockeye 

salmon that held in the Columbia River until the Okanogan River cooled in 1992, only 24 

(33.8%) survived to the spawning grounds (Swan 1994). Delay caused by high water 

temperatures could also impact the reproductive success of fall Chinook salmon, which spawn in 

the fall. The effect of delays on steelhead, which spawn in the spring, are unknown. 

The NMFS 2000 BiOp requires the COE to provide water temperature control in adult 

fishways. The University of Idaho Cooperative Fish and Wildlife Research Unit began 

monitoring temperatures in the forebays and adult fishways at Ice Harbor and Lower Granite 

Dams in 1995. Temperature changes along the length of the fish ladders were found to be slight: 

less than 0.5EC with differential increases to 2EC found on a few occasions (Keniry and Bjornn 

1998). Temperature differences upstream and downstream from diffusers were also minimal. 

However, the use of warm surface water from reservoirs affects temperature in the entire 

fishway, and a discontinuity may occur at the transition pools at the base of fish ladders. Here 

cooler water is added to augment flow at fishway entrances. At Lower Granite Dam, this has 

resulted in temperatures up to 4EC warmer in transition pools than at fishway entrances (Peery 

et al. 2003). The COE also monitors water temperatures in the fishways at John Day Dam,

which exhibits similarly steep thermal gradients near the bottom of its south fish ladder. 

Salmonid passage times at John Day Dam are longer as a result, and more fish exit fishways 

(interrupting passage) as temperatures increase, which further contributes to extended passage 

times (Keefer et al. 2003b). 

Since 1992 Dworshak Dam has been operated to provide cold water for lower Snake 

River temperature control to benefit outmigrating juvenile fall Chinook salmon. Up to

1.2 million acre feet of water at temperatures from 5.6°C to 13.3EC (typically 8.9°C to 11.1EC) 

have been used to augment flows and reduce high water temperatures. These releases, typically 

made during July and August, are managed by regional salmon and reservoir managers to 

improve migration conditions for migrating smolts and adults. 

Karr et al. (1998) estimated that because of water releases from Dworshak Dam in 1994–

1996, water temperatures were 3.1–5.3o

C cooler for 18–32 days at Lower Granite Dam and 2.4–

3.1o

C cooler for 14–43 days at Ice Harbor Dam. A literature review by McCullough et al. (2001) 

of the effect of temperature on salmonids found that laboratory and field studies show warmer 

water temperatures (>13–15.6°C) cause a detrimental effect on the size, number, or fertility of 

eggs. Warm water temperatures during the summer, therefore, have the potential to reduce 

reproductive success of salmonids. 

Dauble and Mueller (1993) suggest that reducing mainstem water temperatures to below 

21EC could reduce risk to populations of migrating adult salmon. It appears likely that operation 

of the Hells Canyon Complex has decreased late winter/spring temperatures and increased fall 

109
temperatures in the Snake River (Ebel and Koski 1968, Graves10). Bjornn11 found a similar trend 

when he plotted mean water temperatures at Ice Harbor Dam from August through October, and 

compared two periods: 1962–1968 and 1975–1989. The 1975–89 period was cooler during 

August, but warmer in September and October than the 1962–68 period. 

Recently, comparisons have been made of the migration timing of steelhead and fall 

Chinook salmon from Bonneville Dam to Ice Harbor Dam, based on radiotelemetry studies 

conducted from 1996 to 1998 and dam counts from 1991 to 1998. Plots were made comparing 

date, cumulative proportional count at Ice Harbor Dam, and river temperature (turbine scroll 

case). 

The plots indicated several general trends. First, steelhead passage at Ice Harbor Dam is 

slightly later than fall Chinook salmon. Second, a high proportion of fall Chinook salmon move 

into the Snake River when water temperatures exceed 20°C. This was particularly apparent in 

1998 when over 80% of the fall Chinook salmon crossed Ice Harbor Dam before water 

temperatures dropped below 20°C. Third, fish appeared to move rapidly upstream through the 

system relative to spring/summer Chinook (also noted in Matter and Sandford 2003). If fall 

Chinook salmon migrations over Ice Harbor Dam are later than normal, it appears their 

migration dates are delayed from Bonneville Dam upstream, rather than being delayed by 

specific warm water conditions at the mouth of the Snake River.12

Dissolved Gas Supersaturation 

Mortality of adult salmon from GBD has occurred in the Columbia River intermittently 

since the first dams were constructed. However, effects of GBD on adult salmonids are less 

understood than are the effects on juveniles. Relationships between TDGS exposure, GBD 

signs, and mortality are not defined. Depth distributions of upstream migrants are poorly 

documented; thus, the mitigative effects of hydrostatic compensation at ambient TDGS is 

unknown. Also, insufficient data exists for determining a threshold TDGS level below which 

successful spawning is known to occur (NMFS 1997). 

Following implementation of voluntary spill to enhance dam passage for juvenile 

salmonids, monitoring was initiated to examine upstream migrating adult salmonids for GBD 

signs. Beginning in 1994, adult fish were examined annually at Bonneville and Lower Granite 

Dams and intermittently at Ice Harbor and Priest Rapids Dams. Under voluntary spill conditions 

where reservoir and tailrace TDGS is limited to 115% and 120%, respectively, signs of GBD 

were not seen and effects were assumed to be benign. Monitoring was also done at Three Mile 

Dam on the Umatilla River, Oregon. However, facilities to capture and examine adult salmonids 

at this site are limited. Thus the ability to obtain representative samples from locations where the 

greatest impacts may occur, for example downstream from McNary Dam, is limited. Also, 

spawning success in relation to TDGS has not been monitored. 

 

10 R. Graves, NOAA Fisheries Service, Portland, OR. Pers. commun., January 2000. 11 T. Bjornn, Idaho Cooperative Fish and Wildlife Research Unit, Moscow, ID. Pers. commun., January 2000. 12 See footnote 11. 

110
The only period in the 1990s when GBD signs were readily observed on adult salmonids 

was during the spring freshet in 1997 (the highest runoff volume year in recent history) (NMFS 

1998a). However, GBD signs were documented at only a few sites and no estimates of impact 

could be made. During that period, TDGS downstream from Bonneville Dam exceeded 135% 

for 16 days and 130% for 24 days. TDGS exceeded 125% for extended periods at other river 

reaches. 

At Bonneville Dam Second Powerhouse, daily prevalence of GBD was high for sockeye 

salmon (14–100% for more than 3 weeks) and steelhead (6–50% for 2 weeks). Chinook salmon 

during the same period showed relatively few signs, with 0–6.5% prevalence. No samples were 

collected from fish traversing the Bonneville Dam spillway tailrace, where TDGS was highest. 

No GBD signs were observed at Lower Granite Dam; however, fish were not examined until 

they had spent several hours in the low TDGS conditions of the fishway; GBD signs may have 

disappeared during fishway passage. At Priest Rapids Dam, sampling took place after TDGS 

decreased to moderate levels of 113–124%. In 1996, prior to spillway deflectors being installed, 

average TDGS downstream from Ice Harbor Dam was very high for almost one month, generally 

exceeding 135%. Adults examined at Lower Granite Dam showed no signs of GBD. However, 

this site is 172 km upstream from Ice Harbor Dam. Furthermore, TDGS in the Little Goose 

reservoir (59 km long) was less than 125% for all but 3 days. 

Head burn (exfoliation of the skin and underlying connective tissue) on the top or sides of 

the heads of adult salmonids has been observed in the Snake River during periods of high spill 

and was thought to be a sign of GBD at one time. Head burn was considered by the Gas Bubble 

Disease expert panel held by NOAA Fisheries Service in 1995, but given a low research priority 

ranking (NMFS 1996). Efforts to address the cause of head burn were transferred to the COE 

Fish Passage Operations and Maintenance Coordination Team, which has discussed the potential 

causes of head burn. Elston (1996) conducted clinical evaluations of fish with typical head burns 

from Lower Granite Dam and suggested that head burns were caused by mechanical abrasion 

and laceration, rather than necrosis associated with subcutaneous emphysema from GBD. 

Monitoring for head burn has been conducted at Lower Granite Dam by NOAA Fisheries 

Service since the early 1990s. From 1993 through 1999, the percentage of adult Chinook salmon 

with head burn ranged from 0% to 9.8%. Monitoring for head burn has been conducted at 

Bonneville Dam by the Columbia River Inter-Tribal Fish Commission since the mid-1990s. 

From 1997 through 1999, the percentage of adult Chinook salmon with head burn was less than 

1%.13 This indicates that the incidence of head burn likely is occurring during passage through 

the hydropower system. The exact cause of head burn remains unknown. Monitoring will 

continue to document the rate of injury. Research on the migration behavior of fish with head 

burn will be needed to relate the injury to survival. For example, Bjornn et al. (1995) found that 

of 66 radio-tagged Chinook salmon that were noted as having head scrapes or injuries, 38% did 

not migrate to known spawning areas and were classified as possible prespawning moralities.

 

13 L. Basham, Fish Passage Center, Portland, OR. Pers. commun., February 2000. 

111
Kelts 

Post-spawning, downstream migrations of steelhead (kelts) have been documented 

throughout the existence of the FCRPS (Long and Griffin 1937, Whitt 1954, Evans and Beaty 

2000, Evans and Beaty 2001, Wertheimer et al. 2001, Wertheimer et al. 2002) and the potentially 

negative effects of the hydropower system on iteroparity rates have been recognized (NPPC 

1986, ISG 1996). However, effects of the hydropower system on steelhead kelt survival and 

reproductive success are poorly understood. Only recently have studies been directed at 

conclusively identifying kelts (vs. prespawners or fallbacks) and documenting their downstream

timing and project-specific passage. (Recent information for upper Snake River Basin areas 

indicates that repeat spawning rates in the Snake River probably average less than 2% [Evans 

and Beaty 2000].) 

In 2000 the COE initiated a program to study the downstream passage behavior of kelts 

at Bonneville Dam. Overall, 39.5% of radio-tagged kelts passed through the spillway, 39.5% 

through First Powerhouse, and 21% through Second Powerhouse. At First Powerhouse, 83% 

(43/52) kelts passed through turbine units, 8% (4/52) passed through the prototype surface 

collector into the ice and trash sluiceway, and 10% were guided by traveling screens into 

gatewells and the juvenile bypass system. Median forebay residence times were longest at First 

Powerhouse (8 hours, 39 minutes), followed by Second Powerhouse (4 hours, 34 minutes), and 

were significantly shorter at the spillway (15 minutes) (Wertheimer et al. 2001). 

In 2001 low-flow conditions provided an opportunity to evaluate kelt passage under lowor zero-spill conditions. Sluiceway passage efficiencies for kelts of 69% and 89% were 

estimated at The Dalles and Bonneville Dams, respectively (Wertheimer et al. 2002), and only 

3% (7/212) of kelts released at Lower Granite Dam were detected below Bonneville Dam.

Studies in 2002 focused on evaluating the abundance of kelts at McNary and John Day 

Dams (Wertheimer et al. 2003). An estimated 14,057 kelts were present in McNary reservoir 

and an estimated 13,081 kelts passed John Day Dam during a 9-week period. Passage through 

John Day Dam under 24-hour spill (30%) and nighttime spill (0% day / 54% night) were 

evaluated (Table 14). At The Dalles Dam, 30%-spill kelt-passage efficiency was 99% and 

sluiceway efficiency increased from 64% to 89%. 

In spring 2003, Boggs and Peery (2003) examined 1,838 steelhead (using ultrasound) 

collected from the Lower Granite Dam juvenile bypass separator and reported 93% were kelts, of 

which 17% were males and 83% females; 50% of the kelts were wild. They also reported that 

return rates for kelts PIT tagged and released in 2002 were 2.7% for kelts transported from

Lower Granite Dam to the estuary, and 0.8% for kelts released in the Lower Granite Dam

tailrace to migrate in river. 

In 2003 mean migration rates of radio-tagged kelts were slower in Snake River reaches 

(32.4 km/d) than in lower Columbia River reaches (55.2 km/d) (Boggs and Peery 2003). 

Migration rates were positively correlated with river flow and generally increased in 

progressively downstream reaches (P < 0.0001, R2 = 0.63). Of the 112 kelts released in the 

Lower Granite Dam tailrace, 55% were detected in the tailrace of Ice Harbor Dam and 34% were 

detected in the tailrace of Bonneville Dam (with no difference in the migration rates or

112
hydrosystem survival between hatchery and wild kelts). This compares with detections below 

Bonneville of 4.1% and 17% in 2001 and 2002, respectively.

Table 14. Project kelt passage (PE), guidance (GE), sluiceway (SLE), and spillway (SPE) efficiencies, 

sluiceway (SLF) and spillway effectiveness (SPF), at John Day (JDD), The Dalles (TDA), and 

Bonneville (BON) dams, 2001–2002 (Wertheimer et al. 2003). 

Project Year N*

Spill 

(%) 

PE 

(%) 

GE 

(%) 

SLE 

(%) SLF

SPE 

(%) SPF

JDD 2002 58 0:54 90 50 NA NA 79 1.5:1

JDD 2002 97 30:30 95 58 NA NA 88 2.9:1

JDD 2002 209 NA 93 46 NA NA 87 NA

TDA 2001 28 0 64 NA 64 16.8:1 NA NA

TDA 2001 70 30 99 NA 89 19.3:1 87 2.9:1

TDA 2002 207 37 95 NA 52 20.0:1 90 2.4:1

BON 2001 73 2 58 53 (B2) 87 (B1) 124.3:1 NA NA 

BON 2001 68 37 88 55 100 349.9:1 54 1.6:1

BON 2002 207 45 90 58 100 250.0:1 67 1.5:1

* N = number of kelts. 

However, migration conditions in 2001 consisted of low flows and little spill, conditions 

in 2002 included BiOp spills and below-average flows with a sustained late runoff, and 

conditions in 2003 included BiOp spills, below-average flows, and a large freshet in late 

May/early June when kelts were in the lower Snake and Columbia rivers. Because migrating 

kelts predominately choose spill and sluiceway routes of passage when available and migrate 

faster with higher flows, spill, sluiceway operation, and flow augmentation appear to

substantially improve kelt survival to below Bonneville Dam.

Other Adult Passage Issues 

The use of individually coded radiotelemetry tags for adult fish has greatly increased the 

precision associated with studies of migration behavior and survival at dams and through the 

mainstem corridor. Individual fish have been uniquely tagged, their approach behaviors and 

passage over dams and through reservoirs monitored, and their run histories reconstructed. A 

large amount of information has been obtained and reported, but a number of uncertainties 

associated with adult passage at dams and through the hydropower system remain. 

Adult passage through the lower river and passage efficiency at Bonneville Dam have not 

been fully investigated. Flow control has altered the lower river and estuarine hydrograph and 

potential adverse impacts include delay or increased mortality in the estuary. In addition, only 

fish captured in Bonneville Dam fishways are tagged and used to assess passage efficiency at 

this, the most downstream obstacle to passage. Studies are needed to confirm that passage 

efficiency estimates at Bonneville are not biased due to the exclusive use of “successful” fish 

(i.e., those that make it into the fishways where they are captured and tagged). This information 

is particularly critical if passage recorded at Bonneville Dam PIT-tag detection systems is used to 

compare relative SAR rates. 

113
Limited information is available regarding the effects on adult Pacific salmon from

passing through turbines. Bell et al. (1967, 1981) used mathematical formulas to estimate injury 

and mortality associated with turbine passage. In their formulas, the probability of strike 

increases with fish length. Thus we assume that mortality rates for adults passing through 

turbines will be higher than for juveniles. Several studies have shown mortality rates from 22% 

to 57% for summer steelhead passing through Kaplan turbines at Foster and Lower Monumental 

Dams (Wagner and Ingram 1973, Liscom et al. 1985, Buchanan and Moring 1986). The 22% 

and 41% mortalities observed in 1969 and 1970 by Wagner and Ingram (1973) represented only 

those fish with observable injuries, indicating direct strike; as such, these estimates are 

minimums. Theoretical strike calculations by Chet Scott, consulting engineer in the Rock Island 

Settlement hearing, showed 41% and 49% mortality for 0.64- and 0.76-m fish, respectively 

(NOAA Hydro Division memorandum, December 5, 198514). 

The COE and Battelle Pacific Northwest National Laboratory conducted a turbine 

passage survival workshop, which included a panel discussion of adult passage (Carlson 2001). 

Effects on adults associated with passage through turbines can affect upstream migrants of any 

species that “fall back” through turbines because they overshot their natal tributary or from

behavior associated with searching for that stream. It can also affect steelhead kelts that have 

spawned and are migrating downstream to return to the Pacific Ocean. The topic has two 

components: what percentage of adults fall back through turbines, and what are the injury and 

mortality rates associated with this route of passage?

Although the rate of adult fallback at dams has been documented, the proportion that can 

be attributed to dam operations versus migratory behavior is not clear. A high proportion of 

fallback behavior is associated with the Bonneville Dam Bradford Island exit during spill. Adult 

salmon are shoreline-oriented and follow the Bradford Island forebay shoreline around to the 

spillway where they are entrained by spill. However, a substantial number of fish fall back from

several miles upstream and this behavior appears unrelated to dam operations. Mobile tracking 

of radio-tagged fish has been conducted in the First Powerhouse forebay. This information has 

been modeled in conjunction with physical models of hydraulic conditions and dam operations to 

further understand the causes of fallback at the Bradford Island fish ladder exit. Fallback over 

Ice Harbor Dam of fish destined to the upper Columbia River appears more related to searching 

for the appropriate tributary than dam operations. Similarly, fallback of fall Chinook in the 

Snake River appears related to fish overshooting the entrance to Lyons Ferry Hatchery. 

Despite limited information regarding cause, fallback of adult spring/summer Chinook 

past dams (Bonneville, The Dalles, John Day and Ice Harbor) during spill significantly reduced 

escapement (Keefer and Peery 2004); fallback (at Bonneville and Ice Harbor Dams) of steelhead 

similarly reduced escapement. On average, escapement was lower for fallback fish by 0.065 for 

spring/summer Chinook salmon (P < 0.05), 0.195 for fall Chinook salmon (P < 0.005), and 0.133 

for steelhead (P < 0.005).15 The difference in escapement multiplied by system-wide fallback 

 

14 C. Ross, NMFS, Northwest Regional Office, Portland, OR. Pers. commun., August 2004. 15 M. Keefer and C. Peery, Idaho Cooperative Fish and Wildlife Research Unit, Moscow, ID. Pers. commun., June

2004. 

114
proportions showed reductions in overall run escapements of 1.30% (range: 0.46–2.27%), 2.26% 

(range: 1.32–2.91%), and 2.84% (range: 1.34–4.02%) for spring/summer Chinook salmon, fall 

Chinook salmon, and steelhead, respectively. 

Project-specific data can show high mortality from fallback. Mendel and Milks (1996) 

estimated fall Chinook salmon fallback mortality was 26% and 14% in 1993 and 1994, 

respectively, at Lower Snake River dams; these fallbacks occurred during periods of no spill. 

System-wide information shows no difference in escapement of spring/summer Chinook 

salmon and steelhead due to fallback during spill and no-spill periods in 2001.16 Escapement 

estimates for adult steelhead from Bonneville Dam to Lower Granite Dam in 2000, 2001, and 

2002, adjusted for harvest, were very similar (87.6%, 87.7%, and 88.8%) even though 2001 had 

very little spill at dams compared to 2000 and 2002. No differences (P < 0.05) in escapement 

were found for fallback of spring/summer and fall Chinook salmon with and without spill for all 

years (1996–2002) pooled (Keefer and Peery 2004). These similar conversion rates with and 

without spill may be due to so few fish falling back during nonspill periods. 

There may also be increased energetic costs associated with migration delays and reascending dams may impact reproductive success. For example, Brown et al. (2002) showed a 

62% increase in energetic demand for spring Chinook salmon in the tailrace of Bonneville Dam

when compared to the forebay and a 26% increase compared to fishways. 

Survival of spring and summer Chinook salmon from Ice Harbor Dam to spawning 

grounds or hatcheries varied between years, and was estimated to be as high as 77% and as low 

as 54% in 1993 and 1991, respectively. Further studies are required to resolve the variability of 

these preliminary observations and improve our understanding of losses that occur above Lower 

Granite Dam. 

The potential effects of migration through the hydropower system on adult reproductive 

success is unknown. Successful reproduction requires a migration of both sexes to the spawning 

grounds, appropriate lipid reserves to carry out the necessary reproductive behavior (nest 

building and defense, and spawning), high gamete quality, proper embryonic development, and 

survival of offspring for the downstream migration. Delays, excessive energy consumption, and 

exposure to higher water temperatures during the migration are factors that could lead to reduced 

reproductive function, disease, and low quality and quantity of gametes. A literature review of 

the effect of temperature on salmonids by McCullough et al. (2001) found that laboratory and 

field studies show warmer water temperatures (>13–15.6EC) cause a detrimental effect on the 

size, number, or fertility of eggs. 

Test fish for radiotelemetry studies in the past were selected from the entire population 

crossing a given dam, without knowing the source, origin, or evolutionary significant unit (ESU) 

of the fish tagged. This was problematic when using radio-tagged fish to determine inter-dam

losses, because the origin of the fish lost between release and upstream sites was unknown. In 

recent years, the number of PIT-tagged adults returning to the Columbia River has increased and 

 

16 See footnote 15. 

115
this, along with the development of adult PIT-tag detection systems at key locations, will enable 

more precise estimates of survival and inter-dam losses for known-source fish. 

Adult counting occurs at all mainstem dams to ensure, among other purposes, that fish 

passage facilities are operating properly. Partial hourly counts are expanded, and little counting 

occurs during hours of darkness. Counts include all adults passing each dam and are upwardly 

biased by any fish that fall back and reascend. The present counting schedule and systems meet 

this intended purpose. However, additional counts during winter months are now necessary for 

management of winter steelhead passage at Bonneville Dam and steelhead passage in the lower 

Snake River. The ISAB noted a number of problems associated with the precision and accuracy 

of adult counts at mainstem dams, especially if the data are used to make fisheries management 

decisions (Bisson et al. 1999). 

Straying of steelhead into the Deschutes River, Oregon, has been observed during recent 

radiotelemetry studies. The natal origin of these fish was unknown. The behavior was observed 

during periods when the water of the Columbia River was warmer than the Deschutes River. 

Possible causes for this behavior include fish seeking the cooler water, straying behavior 

associated with transportation, and an evolutionary adaptation that enhances survival. Additional 

studies of known-source fish will be required to better understand the cause of this observed 

behavior. 

The potential impacts adult fish passage systems have on nonsalmonid species is an 

uncertainty associated with salmonid passage. Concurrent with the decline of salmonid 

populations, Columbia River Pacific lamprey populations have also declined. Indigenous 

peoples from the Pacific coast to the interior Columbia River have harvested adult lamprey for 

subsistence, religious, and medicinal purposes for many generations (Close et al. 2002). In the 

Columbia River drainage, adult Pacific lamprey support fisheries that have recently experienced 

dramatic declines, and unprecedented regulation and concern about the status of Pacific lamprey 

has resulted in a recent petition to list this species under the Endangered Species Act (Moser and 

Close 2003). 

As an anadromous species, lamprey must pass through the same hydropower system as 

salmonids. However, they have different migration behaviors and lack the physical swimming 

capabilities of salmon. Passage success studies show that although approximately 90% of the 

lamprey released downstream from Bonneville Dam from 1997 to 2000 reapproached the dam,

less than 50% successfully passed and required a median passage time of 4–6 days (Moser et al. 

2002). Radiotelemetry studies also indicated that lamprey had the least success in areas with 

confusing flows or structural obstacles, primarily where gratings exist on the floor of the fishway 

system, and at the serpentine weirs near the tops of ladders. Additional studies are needed to 

address whether the hydropower system is impacting lamprey recruitment, homing, and life 

histories. 

White sturgeon are an important recreational, commercial, and cultural resource in the 

Columbia River Basin. However, white sturgeon populations vary considerably in abundance 

and age structure throughout the basin (Beamesderfer et al. 1995). The abundance and density of 

fish is greatest in the unimpounded river downstream from Bonneville Dam and this area 

supports one of the largest and most productive sturgeon populations in the world. However, the 

116
white sturgeon population from the Kootenai River in northern Idaho has been listed as 

endangered since 1994, and populations in the Snake River downstream from Hells Canyon Dam

appear to be persisting, but at a lower abundance than prior to impoundment (De Vore et al. 

1995). 

This variation in population status can be attributed to a number of factors including 

differences in exploitation rates and recruitment success, access to marine food resources, and 

suitability of hydrologic conditions and available habitats (Beamesderfer et al. 1995, DeVore 

et al. 1995). In particular, construction and operation of hydroelectric dams on the Columbia and 

Snake rivers has directly affected white sturgeon populations in several ways. Spawning habitats 

are reduced (Parsley and Beckman 1994), upstream and downstream passage is limited, and 

substantial numbers of juvenile and adult fish can be entrained and killed during dam

maintenance activities. White sturgeon seldom ascend the existing fishways at the hydroelectric 

projects, presumably because the fish passage facilities for upstream migrating fish at Columbia 

River Basin dams were designed primarily for anadromous salmonids (Warren and Beckman 

1993). 

Little is known about sturgeon downstream passage through dams. Fisheries sampling 

done by the Oregon and Washington Departments of Fish and Wildlife and angler returns of 

tagged fish have resulted in several recaptured fish that had moved downstream past one or more 

dams. North et al. (1995) reports that since 1987, the Oregon Department of Fish and Wildlife 

has recovered 4 tagged white sturgeon that passed upstream and 74 fish that passed downstream

through one or more dams. Thus construction of hydroelectric dams on the Columbia and Snake 

rivers may restrict movements of white sturgeon to impounded reaches, effectively resulting in a 

series of individual landlocked populations with potentially reduced genetic diversity (North et 

al. 1993). Prior to dam construction in the Columbia River Basin, white sturgeon likely 

responded to seasonal changes in food and habitat availability by ranging extensively between 

freshwater, estuarine, and marine environments. Because the physiochemical and biotic 

characteristics of reservoirs vary greatly within the basin, individual impoundments may not 

contain optimal conditions for all life stages of white sturgeon (Parsley and Beckman 1994, 

Beamesderfer et al. 1995). Identifying the specific factors currently influencing or limiting white 

sturgeon movement at dams and improving passage could enhance the productivity of white 

sturgeon populations. 

American shad are not indigenous to the Columbia River Basin, but this species has 

successfully exploited the reservoir habitat currently available in the hydropower system. An 

estimated total of 4.6 million shad passed Bonneville Dam in 2003. A fishery on shad at The 

Dalles Dam resulted in an increased passage time for Chinook salmon as well as an increased 

rate of fallback (Jepson et al. 2003). Potential biological interactions between shad and 

salmonids exist but have not been studied. In particular, adult salmonid behavior in fishways 

may be altered when shad passage peaks occur during late spring and physical crowding in the 

ladders occurs due to high shad densities. Examinations of fine-scale salmonid behaviors in and 

around the fishways during these periods are needed to assess potential interspecific interactions. 

117
Adult Passage Conclusions 

1. Recent radiotelemetry estimates of adult salmonid survival from Ice Harbor to Lower Granite 

Dam on the Snake River have been generally well over 90%. Recent PIT-tag estimates of 

survival from Bonneville Dam on the lower Columbia River to Lower Granite Dam have 

been over 80% for spring/summer Chinook salmon and 70–80% for steelhead. For adult 

salmonids that do not fall back at dams, estimates of survival range from 3% to 5% higher 

than for those that do fall back. 

2. Delays in dam passage for upstream migrating adult salmonids are associated with high

levels of spill (≥40 kcfs), highly fluctuating spill, and high turbidity. Fishway entrance 

preferences are for deep/wide openings with significant attraction flow. The transition areas 

between collection channels and ladders are the fishway segments where adult Chinook 

salmon and steelhead are most likely to turn around and exit the fishway. 

3. Overall travel times and migration rates of radio- and PIT-tagged adult salmonids from 

Bonneville Dam to upriver Columbia or Snake River sites vary by species/run, time of year, 

and environmental conditions. While delay of adult salmonid passage at Columbia and 

Snake River dams has been documented, recent estimates of travel times and migration rates 

are similar to times and rates observed during the early development of the FCRPS when 

there were fewer dams and the Snake River was unimpounded. 

4. Fallback rates vary by species/run, environmental conditions, and dam. Higher fallback rates 

are associated with high river flows and spill, and with adult fishway exit locations in close 

proximity to spillways. Fallbacks result in an overestimation of adult counts at fish ladder 

count windows. 

5. Powerhouse zero-flow operations at night do not appear to affect migration rates and passage 

of adult salmonids. 

6. For adult salmonids, elevated river water temperatures (>20EC) increase travel times between 

dams and passage through fishways at dams. Warm water temperatures during the summer, 

which often exceed 20EC in the Columbia and Snake rivers, also may reduce reproductive 

success of salmonids. 

7. Although monitoring of adult salmonids for signs of GBD has occurred at Columbia and 

Snake River dams since 1994 (including periods of very high TDGS), very few signs have 

been observed. The relationships between TDGS exposure, GBD signs, mortality, and 

spawning success are not well understood. 

8. Estimates of repeat spawning rates for steelhead kelts that migrate downstream through the 

Snake and Columbia rivers are low (≈2%). While potentially negative effects of the 

hydrosystem on iteroparity rates have been recognized, the magnitude of the effects on 

survival and reproductive success are not clearly understood. Higher flows and spill 

significantly reduce travel and passage times for downstream-migrating kelts. 

9. Adult survival above the hydrosystem to hatcheries or spawning grounds is variable and 

estimates from Ice Harbor Dam using radiotelemetry range from 54% to 77%. The effects of 

passage through the hydrosystem on reproductive success are unknown. 

118
10. The impacts of predation by marine mammals on Columbia River Basin salmon populations 

are not well understood. In general, very little is known regarding how the existence and 

operation of the FCRPS may have altered the historical habitat and salmonid consumption 

rates of marine mammals. 

11. Relative to salmonids, upstream passage at Columbia and Snake River dams is poor for adult 

Pacific lamprey and white sturgeon. American shad have thrived in the hydrosystem habitat, 

but the effects of the biological interactions between adult shad and salmonids (in particular 

with regard to extremely high fish densities in the fishways during late spring) have not been 

studied. 

119
120
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