An Assessment of Potential Mining Impacts

on Salmon Ecosystems of Bristol Bay, Alaska

Region 10, Seattle, WA

 www.epa.gov/bristolbay

EPA 910-R-14-001ES | January 2014

Executive Summary

The Bristol Bay watershed in southwestern Alaska supports the largest 

sockeye salmon fishery in the world, is home to 25 federally recognized 

tribal governments, and contains significant mineral resources. The potential 

for large-scale mining activities in the watershed has raised concerns 

about the impact of mining on the sustainability of Bristol Bay’s world-class 

commercial, recreational, and subsistence fisheries and the future of Alaska 

Native tribes in the watershed, who have maintained a salmon-based culture 

and subsistence-based way of life for at least 4,000 years.

The U.S. Environmental Protection Agency (USEPA) launched this assessment 

to determine the significance of Bristol Bay’s ecological resources and 

evaluate the potential impacts of large-scale mining on these resources. It 

uses the well-established methodology of an ecological risk assessment, 

which is a type of scientific investigation that provides technical information 

and analyses to foster public understanding and inform future decision 

making. As a scientific assessment, it does not discuss or recommend policy, 

legal, or regulatory decisions, nor does it outline or analyze options for 

future decisions.

Executive Summary

Tributary of Napotoli Creek, near the Humble claim Photo: Michael Wiedmer

1
This assessment characterizes the biological 

and mineral resources of the Bristol Bay 

watershed. It is intended to increase 

understanding of potential impacts of largescale mining on the region’s fish resources 

and serve as a technical resource for the 

public and for federal, state, and tribal 

governments as they consider how best to 

address the challenges posed by mining 

and ecological protection in the Bristol Bay 

watershed. It will inform ongoing discussions 

of the risks of mine development to the 

sustainability of the Bristol Bay salmon 

fisheries and thus will be of value to the many 

stakeholders in this debate.

The assessment also will inform the 

consideration of options for future 

government action, including, possibly, 

by USEPA, which has been petitioned by 

multiple groups to address mining activity 

in the Bristol Bay watershed using its 

authority under the Clean Water Act (CWA). 

Should specific mine projects reach the 

permitting stage, the assessment will enable 

state and federal permitting authorities to 

make informed decisions to grant, deny, or 

condition permits and/or conduct additional 

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Aleknagik

Dillingham

Portage Creek

Ekwok

Levelock

New Stuyahok

Koliganek

Igiugig

Kokhanok

Newhalen

Iliamna

Nondalton

Pedro Bay

Port Alsworth

Wood River

N ushagak River

Mulchatna River

Lake Clark

Iliamna Lake

Kvichak River

Stuyahok River

Klutuk Cree

Ko

k

kwok River

Nuyakuk River

Koktu li River

Nushagak River

Chulitna River

Napotoli Creek

Newhalen River Alagnak River Bristol Bay

Cook Inlet

NUSHAGAK

KVICHAK

Lake Clark

National Park

and Preserve

Wood-Tikchik

State Park

Katmai

National Park

and Preserve

Togiak

National

Wildlife

Refuge

The Nushagak River and Kvichak River Watersheds of Bristol Bay

Miles

Kilometers

0 25 50

0 25 50

Approximate Pebble Deposit Location

Towns and Villages

Parks, Refuges, or Preserves

Watershed Boundary

The Lower Nushagak River between Portage Creek and Ekwok Photo: Michael Wiedmer

2
research or assessment as a basis for 

such decisions. USEPA conducted this 

assessment consistent with its authority 

under the CWA Section 104(a) and (b).

Scope of the Assessment

This assessment reviews, analyzes, and 

synthesizes information relevant to 

potential impacts of large-scale mine 

development on Bristol Bay fisheries and 

consequent effects on wildlife and Alaska 

Native cultures in the region. Given 

the economic, ecological, and cultural 

importance of the region’s key salmonids 

(sockeye, Chinook, coho, chum, and 

pink salmon, as well as rainbow trout 

and Dolly Varden) and stakeholder 

and public concern that a mine could 

affect those species, the primary focus 

of the assessment is the abundance, 

productivity, and diversity of these 

fishes. Because wildlife in Bristol Bay are 

intimately connected to and dependent 

on these and other fishes, changes in 

these fisheries are expected to affect 

the abundance and health of wildlife 

populations. Alaska Native cultures have 

strong nutritional, cultural, social, and 

spiritual dependence on salmon, so 

changes in salmon fisheries are expected 

to affect the health and welfare of Alaska 

Native populations. Therefore, wildlife 

and Alaska Native cultures are also 

considered as assessment endpoints, but 

only as they are affected by changes in 

salmonid fisheries.

The assessment considers multiple 

geographic scales. The largest scale is the 

Bristol Bay watershed, which is a largely 

undisturbed region with outstanding 

natural, cultural, and mineral resources. 

Within the larger Bristol Bay watershed, 

the assessment focuses on the Nushagak 

and Kvichak River watersheds. These are 

the largest of the Bristol Bay watershed’s 

six major river basins, containing about 

50% of the total watershed area, and are 

identified as mineral development areas 

by the State of Alaska. Given its size and 

Nor t h Fork Koktuli River

Iliamna Lake

South Fork Koktuli River

Upper Talari k Creek

Koktuli R iv er

NORTH FORK KOKTULI

SOUTH FORKKOKTULI

UPPER TALARIK

NUSHAGAK

KVICHAK

Figure ES-2. Reported salmon (sockeye, Chinook, coho, pink, and chum combined) distribution in the South

and North Fork Koktuli Rivers and Upper Talarik Creek watersheds.

Designation of species spawning, rearing, and presence is based on the Anadromous Waters Catalog. Lifestage-specific reach designations are likely underestimates, given the challenges inherent in surveying all

streams that may support life-stage use throughout the year.

Iliamna Lake

Reported Salmon (Sockeye, Chinook, Coho, Pink, and Chum Combined) Distribution in the South and North Fork Koktuli River and Upper Talarik 

Creek Watersheds. Designation of species spawning, rearing, and presence is based on the Anadromous Waters Catalog (Johnson and Blanche 2012). Life-stagespecific reach designations are likely underestimates, given the challenges inherent in surveying all streams that may support life-stage use throughout the year.

Present

Spawning

Rearing

Pebble 6.5 Components

Mine Scenario Watersheds

Watershed Boundary

Miles

Kilometers

0 2.5 5

0 2.5 5

3
deposit. The final geographic scale is the 

combined area of the subwatersheds 

between the mine footprints and the 

Kvichak River watershed’s eastern 

boundary that would be crossed by a 

transportation corridor linking the mine 

site to Cook Inlet.

The assessment also addresses two 

periods for mine activities. The first is 

the development and operation phase, 

during which mine infrastructure would 

be built and the mine would be operated. 

This phase may last from 20 to 100 years 

or more. The second is the post-mining 

phase, during which the site would 

be monitored and maintained. Water 

treatment and other waste management 

activities would continue as necessary 

and any failures would be remediated. 

Because mine wastes would be persistent, 

this period could continue for centuries 

and potentially in perpetuity.

We began the assessment with a 

thorough review of what is known about 

the Bristol Bay watershed, its fisheries and 

wildlife populations, and its Alaska Native 

cultures. We also reviewed information 

about copper mining and publicly 

available information outlining proposed 

extent of characterization, the Pebble 

deposit is the most likely site for nearterm, large-scale mine development in 

the region. Because the Pebble deposit is 

located in the headwaters of tributaries 

to both the Nushagak and Kvichak Rivers, 

both of these watersheds are subject to 

potential risks from mining. The third 

geographic scale is the watersheds of 

the three tributaries that originate within 

the potential footprint of a mine on the 

Pebble deposit: the South Fork Koktuli 

River, which drains the Pebble deposit 

area and converges with the North 

Fork west of the Pebble deposit; the 

North Fork Koktuli River, located to the 

northwest of the Pebble deposit, which 

flows into the Nushagak River via the 

Koktuli and Mulchatna Rivers; and Upper 

Talarik Creek, which drains the eastern 

portion of the Pebble deposit and 

flows into the Kvichak River via Iliamna 

Lake, the largest undeveloped lake in 

the United States. The mine footprints 

in the three realistic mine scenarios 

evaluated in the assessment make up 

the fourth geographic scale. These 

scenarios—Pebble 0.25, Pebble 2.0, 

and Pebble 6.5—define three potential 

mine sizes, representing different stages 

in the potential mining of the Pebble 

Sockeye salmon near Gibraltar Lake Photo: Thomas Quinn (University of Washington)

4
mine operations for the Pebble deposit. 

The Pebble deposit has been the focus of 

much exploratory study and has received 

significant attention from groups in 

and outside of Alaska. With the help of 

regional stakeholders, we developed 

a set of conceptual models to show 

potential associations between salmon 

populations and the environmental 

stressors that might reasonably result 

from large-scale mining. Then, following 

the USEPA’s ecological risk assessment 

framework, we analyzed the sources and 

exposures that would occur and potential 

responses to those exposures. Finally, we 

characterized the risks to fish habitats, 

salmon, and other fish populations, as 

well as the implications of those risks for 

the wildlife and Alaska Native cultures 

that use them.

This is not an in-depth assessment of a 

specific mine, but rather an examination 

of potential impacts of reasonably 

foreseeable mining activities in the 

Bristol Bay region, given the nature of 

Proportion of Total Sockeye Salmon Run Sizes by (A) Region and (B) Watershed in the Bristol Bay Region. Values are averages from (A) 1956 to 2005 

from Ruggerone et al. 2010 and (B) 1956 to 2010 from Baker pers. comm.

the watershed’s mineral deposits and 

the requirements for successful mine 

development. The assessment analyzes 

mine scenarios that reflect the expected 

characteristics of mine operation at the 

Pebble deposit. It is intended to provide 

a baseline for understanding potential 

impacts of mine development, not just at 

the Pebble deposit but throughout the 

Nushagak and Kvichak River watersheds. 

The mining of other existing porphyry 

copper deposits in the region would be 

expected to include the same types of 

activities and facilities evaluated in this 

assessment for the Pebble deposit (open 

pit mining and the creation of waste 

rock piles and tailings storage facilities 

[TSFs]), and therefore would present 

potential risks similar to those outlined 

in this assessment. However, because the 

region’s other ore bodies are believed to 

be much smaller than the Pebble deposit, 

those mines would likely be most similar 

to the smallest mine scenario analyzed in 

this assessment (Pebble 0.25).

Bristol Bay

Russia Mainland & Islands

West Kamchatka

East Kamchatka

Western Alaska (excluding Bristol Bay)

South Alaska Peninsula

Kodiak

Cook Inlet

Prince William Sound

Southeast Alaska

North British Columbia

South British Columbia, Washington & Oregon

A. B.

Togiak

Nushagak

Kvichak

Naknek

Egegik

Ugashik

5
Ecological Resources

The Bristol Bay watershed provides 

habitat for numerous animal species, 

including at least 29 fish species, more 

than 40 terrestrial mammal species, 

and more than 190 bird species. Many 

of these species are essential to the 

structure and function of the region’s 

ecosystems and current economies. The 

Bristol Bay watershed supports several 

wilderness compatible and sustainable 

economic sectors, such as commercial, 

sport, and subsistence fishing; sport 

and subsistence hunting; and nonconsumptive recreation. Considering all 

these sectors, the Bristol Bay watershed’s 

ecological resources generated nearly 

$480 million in direct economic 

expenditures and sales in 2009 and 

provided employment for over 14,000 

full- and part-time workers. 

Chief among these ecological resources 

are world-class commercial and sport 

fisheries for Pacific salmon and other 

salmonids. The region’s commercial 

salmon fishery generates the largest 

component of economic activity. The 

watershed supports production of all 

five species of Pacific salmon found in 

North America: sockeye (Oncorhynchus 

This assessment considers many but 

not all potential impacts associated 

with future large-scale mining in the 

Bristol Bay watershed. Although the 

mine scenarios assume development 

of a deep-water port on Cook Inlet to 

ship product concentrate elsewhere 

for smelting and refining, impacts 

of port development and operation 

are not assessed. The assessment 

does not evaluate impacts of the one 

or more large-capacity electricitygenerating power plants that would 

be required to power the mine and 

the port. We recognize that large-scale 

mine development would induce the 

development of additional support 

services for mine employees and their 

families, vacation homes and other 

recreational facilities, and transportation 

infrastructure beyond the main 

corridor (i.e., airports, docks, and roads). 

The assessment describes but does 

not evaluate the effects of induced 

development resulting from large-scale 

mining in the region. Direct effects of 

mining on Alaska Natives and wildlife are 

not assessed. The assessment also does 

not include a cost-benefit analysis and does 

not compare mining to other ongoing 

activities such as commercial fishing.

Sockeye salmon near Pedro Bay, Iliamna Lake Photo: Thomas Quinn (University of Washington)

6
nerka), coho (O. kisutch), Chinook (O. 

tshawytscha), chum (O. keta), and 

pink (O. gorbuscha). These fishes are 

anadromous, meaning that they hatch 

and rear in freshwater systems, migrate 

to sea to grow to adult size, and return 

to freshwater systems to spawn and die. 

Because no hatchery fish are raised or 

released in the watershed, Bristol Bay’s 

salmon populations are entirely wild.

The most abundant salmon species in the 

Bristol Bay watershed is sockeye salmon. 

The watershed supports the largest 

sockeye salmon fishery in the world, with 

approximately 46% of the average global 

abundance of wild sockeye salmon. 

Between 1990 and 2009, the annual 

average inshore run of sockeye salmon in 

Bristol Bay was approximately 37.5 million 

fish. Annual commercial harvest of 

sockeye over this same period averaged 

25.7 million fish. Approximately half of 

Bristol Bay’s sockeye salmon production 

is from the Nushagak and Kvichak River 

watersheds, the main area of focus for 

this assessment.

Chinook salmon are also abundant in the 

region. Chinook returns to the Nushagak 

River are consistently greater than 

100,000 fish per year and have exceeded 

200,000 fish in 11 years between 1966 

and 2010, frequently placing Nushagak 

River Chinook runs at or near the world’s 

largest. This is noteworthy given the 

Nushagak River’s small watershed area 

compared to other Chinook-producing 

rivers such as the Yukon River, which spans 

Alaska and much of northwestern Canada, 

and the Kuskokwim River in southwestern 

Alaska, just north of Bristol Bay.

The Bristol Bay watershed also supports 

populations of non-salmon fishes that 

typically (but not always) remain in 

the watershed’s freshwater habitats 

throughout their life cycles. The region 

contains highly productive waters for 

sport and subsistence fish species, 

including rainbow trout (O. mykiss), Dolly 

Varden (Salvelinus malma), Arctic char (S. 

alpinus), lake trout (S. namaycush), Arctic 

grayling (Thymallus arcticus), northern 

pike (Esox lucius), and humpback 

whitefish (Coregonus pidschian). These 

fishes occupy a variety of habitats in the 

watershed, from headwater streams to 

wetlands to large rivers and lakes.

The Bristol Bay region is especially 

renowned for the size and abundance 

of its rainbow trout: between 2003 and 

2007, an estimated 183,000 rainbow 

trout were caught in the Bristol Bay 

Management Area.

Bald eagle Photo: Conrad Gowell (Oregon State University)

7
terrestrial habitats. The watershed 

continues to support large carnivores 

such as brown bears (Ursus arctos), bald 

eagles (Haliaeetus leucocephalus), and 

gray wolves (Canis lupus); ungulates 

such as moose (Alces alces gigas) and 

caribou (Rangifer tarandus granti); and 

numerous waterfowl and small mammal 

species. Brown bears are abundant in the 

Nushagak and Kvichak River watersheds. 

Moose also are abundant, particularly 

in the Nushagak River watershed where 

felt-leaf willow, a preferred forage 

species, is plentiful. The Nushagak and 

Kvichak River watersheds are used by 

caribou, primarily the Mulchatna caribou 

herd. This herd ranges widely through 

these watersheds, but also spends 

considerable time in other watersheds.

Alaska Native Cultures

The predominant Alaska Native cultures 

present in the Nushagak and Kvichak 

River watersheds—the Yup’ik and 

Dena’ina—are two of the last intact, 

sustainable, salmon-based cultures 

in the world. In contrast, other Pacific 

Northwest salmon-based cultures are 

severely threatened by development, 

degraded natural resources, and 

declining salmon resources.

The exceptional quality of the Bristol 

Bay watershed’s fish populations can be 

attributed to several factors, the most 

important of which is the watershed’s 

high-quality, diverse aquatic habitats 

unaltered by human-engineered 

structures and flow management 

controls. Surface and subsurface 

waters are highly connected, enabling 

hydrologic and biochemical connectivity 

between wetlands, ponds, streams, 

and rivers and thereby increasing the 

diversity and stability of habitats able to 

support fish. These factors all contribute 

to making the Bristol Bay watershed a 

highly productive system. High aquatic 

habitat diversity also supports the high 

genetic diversity of fish populations. 

This diversity in genetics, life history, 

and habitat acts to reduce year-toyear variability in total production and 

increase overall stability of the fishery.

The return of spawning salmon from 

the Pacific Ocean brings marine-derived 

nutrients into the watershed and fuels 

both aquatic and terrestrial foodwebs. 

Thus, the condition of Bristol Bay’s 

terrestrial ecosystems is intimately linked 

to the condition of salmon populations, 

as well as to almost totally undisturbed 

Salmon drying at Koliganek Photo: Alan Boraas (Kenai Peninsula College)

8
Salmon are integral to these cultures’ 

entire way of life via the provision of 

subsistence food and subsistencebased livelihoods, and are an important 

foundation for their language, spirituality, 

and social structure. The cultures have a 

strong connection to the landscape and 

its resources. In the Bristol Bay watershed, 

this connection has been maintained for 

at least 4,000 years and is in part both 

due to and responsible for the continued 

undisturbed condition of the region’s 

landscape and biological resources. The 

respect and importance given salmon 

and other wildlife, along with traditional 

knowledge of the environment, have 

produced a sustainable subsistencebased economy. This subsistence-based 

way of life is a key element of Alaska 

Native identity and serves a wide range of 

economic, social, and cultural functions in 

Yup’ik and Dena’ina societies.

There are 31 Alaska Native villages in 

the wider Bristol Bay region, 25 of which 

are located in the Bristol Bay watershed. 

Fourteen of these communities are 

within the Nushagak and Kvichak River 

watersheds, with a total population 

of 4,337 in 2010. Thirteen of these 14 

communities have federally recognized 

tribal governments and a majority Alaska 

Native population. Many of the nonAlaska Native residents in the watersheds 

have developed cultural ties to the 

region and they also practice subsistence. 

Virtually every household in the 

watersheds uses subsistence resources. In 

the Bristol Bay region, salmon constitute 

approximately 52% of the subsistence 

harvest; for some communities this 

proportion is substantially higher.

The subsistence-based way of life in many 

Alaska Native villages is augmented with 

activities that support cash economy 

transactions, including commercial fishing. 

Alaska Native villages, in partnership 

with Alaska Native corporations and 

other business interests, are considering 

a variety of economic development 

opportunities. Some Alaska Native 

villages have decided that large-scale 

mining is not the course they would 

like to pursue, whereas a few others are 

seriously considering this opportunity. 

All are concerned with the long-term 

sustainability of their communities.

Geological Resources

In addition to significant and valuable 

ecological resources, the Nushagak 

and Kvichak River watersheds contain 

considerable mineral resources. 

Fishing boats at Naknek Photo: USEPA

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The potential for large-scale mine 

development in the region is greatest for 

copper deposits and, to a lesser extent, 

for intrusion-related gold deposits. 

Because these deposits are low-grade—

meaning that they contain relatively 

small amounts of metals relative to the 

amount of ore—mining will be economic 

only if conducted over large areas and 

will necessarily produce large amounts of 

waste material.

The largest known and most explored 

deposit is the Pebble deposit. If fully 

mined, the claim holder estimates that 

the Pebble deposit would produce 

more than 11 billion tons of ore, which 

would make it the largest mine of its 

type in North America. A mine at the 

Pebble deposit could ultimately generate 

revenues between $300 billion to $500 

billion over the life of the mine, as well 

as provide more than 2,000 jobs during 

mine construction and more than 1,000 

jobs during mine operation. 

Although the Pebble deposit represents 

the most imminent site of mine 

development, other mineral deposits 

with potentially significant resources 

exist in the Nushagak and Kvichak River 

watersheds. Ten specific claims with 

more than minimal recent exploration 

(in addition to the Pebble deposit claim) 

have been filed for copper deposits. 

Most of these claims are near the 

Pebble deposit. The potential impacts 

of large-scale mining considered in this 

assessment are generally applicable to 

these other sites. 

Mine Scenarios

Like all risk assessments, this assessment 

is based on scenarios that define a set of 

possible future activities and outcomes. 

To assess mining-related stressors that 

would affect ecological resources in 

the watershed, we developed realistic 

mine scenarios that include a range of 

mine sizes and operating conditions. 

These mine scenarios are based on the 

Pebble deposit because it is the bestcharacterized mineral resource and 

the most likely to be developed in the 

near term. The mine scenarios draw on 

preliminary plans developed for Northern 

Dynasty Minerals, consultation with 

experts, and baseline data collected 

by the Pebble Limited Partnership to 

characterize the mine site, mine activities, 

and the surrounding environment.

The exact details of any future mine 

plan for the Pebble deposit or for other 

deposits in the watershed will differ 

from our mine scenarios. However, 

our scenarios reflect the general 

characteristics of mineral deposits in 

Mine pit at Fort Knox, Alaska Photo: Phil North (USEPA)

10
the watershed, modern conventional mining 

technologies and practices, the scale of mining 

activity required for economic development of 

the resource, and the infrastructure needed to 

support large-scale mining. Therefore, the mine 

scenarios evaluated in this assessment realistically 

represent the type of development plan that would 

be anticipated for a porphyry copper deposit in 

the Bristol Bay watershed. Uncertainties associated 

with the mine scenarios are discussed later in this 

executive summary.

The three mine scenarios evaluated in the 

assessment represent different stages of mining 

at the Pebble deposit, based on the amount of ore 

processed: Pebble 0.25 (approximately 0.25 billion 

tons [0.23 billion metric tons] of ore over 20 years), 

Pebble 2.0 (approximately 2.0 billion tons [1.8 billion 

metric tons] of ore over 25 years), and Pebble 6.5 

(approximately 6.5 billion tons [5.9 billion metric 

tons] of ore over 78 years). The major components 

of each mine would be an open mine pit, waste 

rock piles, and one or more TSFs. Other significant 

features include plant and ancillary facilities (e.g., 

a water collection and treatment system, an oreprocessing facility, and other facilities associated 

with mine operations) and the groundwater 

drawdown zone (the area over which the water 

table is lowered due to dewatering of the mine pit). 

An underground extension of the mine, which could 

increase the size of the mine to 11 billion tons of ore, 

is not included in this assessment.

Mine Scenario Parameters

Mine Scenario

Parameter Pebble 0.25 Pebble 2.0 Pebble 6.5

Amount of ore mined (billion metric tons) 0.23 1.8 5.9

Approximate duration of mining (years) 20 25 78

Ore processing rate (metric tons/day) 31,100 198,000 208,000 

Mine Pit

Surface area (km2

) 1.5 5.5 17.8

Depth (km) 0.30 0.76 1.24

Waste Rock Pile

Surface area (km2

) 2.3 13.0 22.6

PAG waste rock (million metric tons) 86 580 4,700

NAG waste rock (million metric tons) 320 2,200 11,000

TSF 1a

Capacity, dry weight (billion metric tons) 0.25 1.97 1.97

Surface area, exterior (km2

) 6.8 16.1 16.1

Maximum dam height (m) 92 209 209

TSF 2a

Capacity, dry weight (billion metric tons) NA NA 3.69

Surface area, exterior (km2

) NA NA 22.7

TSF 3a

Capacity, dry weight (billion metric tons) NA NA 0.96

Surface area, exterior (km2

) NA NA 9.82

Total TSF surface area, exterior (km2

) 6.8 16.1 48.6

Notes: a Final value, when TSF is full; PAG = potentially acid-generating; NAG = non-acid-generating; TSF = tailings storage facility; NA = not applicable.

11
Each of these mine scenarios includes a 

138-km (86-mile) transportation corridor; 

113 km (70 miles) of the corridor would fall 

within the Kvichak River watershed. This 

corridor would include a gravel-surfaced 

road and four pipelines (one each for 

product concentrate, return water, diesel 

fuel, and natural gas).

The assessment considers risks from 

routine operation of a mine designed 

using modern conventional design, 

practices, and mitigation technologies, 

assuming no significant human or 

engineering failures. The assessment also 

considers various types of failures that 

have occurred during the operation of 

other mines and that could occur in this 

case, including failures of a wastewater 

treatment plant, a tailings dam, pipelines, 

and culverts.

Risks to Salmon and

Other Fishes

Based on the mine scenarios, the 

assessment defines mining-related 

stressors that would affect the Bristol Bay 

watershed’s fish and consequently affect 

wildlife and human welfare. The scenarios 

include both routine operations and 

several potential failure scenarios. 

South Fork Koktuli River

Upper T a larik Creek

Nor th Fork Koktuli River

Major Mine Components for the Three Scenarios Evaluated in the Assessment.

Pebble 0.25 represents 0.25 billion tons of ore; Pebble 2.0 represents 2.0 billion tons of 

ore; Pebble 6.5 represents 6.5 billion tons of ore. Each mine footprint includes the mine 

components shown here, as well as the drawdown zone and the area covered by plant and 

ancillary facilities. Light blue areas indicate streams and rivers from the National Hydrography 

Dataset (USGS 2012) and lakes and ponds from the National Wetlands Inventory (USFWS 

2012); dark blue areas indicate wetlands from the National Wetlands Inventory (USFWS 2012).

Pebble deposit area Photo: Lorraine Edmond (USEPA)

Pebble 0.25 Components

Pebble 2.0 Components

Pebble 6.5 Components Miles

Kilometers

0 1 2

0 1 2

12
Mine Footprint

Effects on fish resulting from habitat 

loss and modification would occur 

directly in the area of mine activity and 

indirectly downstream because of habitat 

destruction. These habitat loss estimates 

are believed to be low due to incomplete 

delineation of streams, wetlands, and 

salmon distribution across the region. 

However, it is possible that careful siting of 

mine facilities could reduce habitat losses 

to some degree.

Due to the mine footprint (the area covered 

by the mine pit, waste rock piles, TSFs, 

groundwater drawdown zone, and plant 

and ancillary facilities), 38, 89, and 151 km 

(24, 55, and 94 miles) of streams would 

be lost—that is, eliminated, blocked, or 

dewatered—in the Pebble 0.25, 2.0, and 

6.5 scenarios, respectively. This translates 

to losses of 8, 22, and 36 km (5, 14, and 

22 miles) of streams known to provide 

spawning or rearing habitats for coho 

salmon, sockeye salmon, Chinook salmon, 

and Dolly Varden.

Altered streamflow due to retention 

and discharge of water used in mine 

operations, ore processing and transport, 

and other mine activities would reduce 

the amount and quality of fish habitat. 

Streamflow alterations exceeding 20% 

would adversely affect habitat in an 

additional 15, 27, and 53 km (9.3, 17, 

and 33 miles) of streams in the

Pebble 0.25, 2.0, and 6.5 scenarios, 

respectively, reducing production of 

sockeye salmon, coho salmon, Chinook 

salmon, rainbow trout, and Dolly 

Varden. Reduced streamflows would 

also result in the loss or alteration of an 

unquantifiable area of riparian floodplain 

wetland habitat due to loss of hydrologic 

connectivity with streams.

Off-channel habitats for salmon and 

other fishes would be reduced due to 

losses of 4.5, 12, and 18 km2

 (1,200, 

3,000 and 4,900 acres) of wetlands 

and 0.41, 0.93, and 1.8 km2

 (100, 230, 

and 450 acres) of ponds and lakes to 

the mine footprints in the Pebble 0.25, 

2.0, and 6.5 scenarios, respectively.

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Upper Talarik Creek

North Fork Koktuli River

Pile River

Cook Inlet

Newhalen River

Iliamna Lake

Knutson Bay

Pile Bay Pedro

Bay

Iliamna

Bay

Lake Clark

South Fork Koktuli River

Knutson Creek

Iliamn a River

Canyon Cr eek

Chek ok Creek

Chinkelyes Creek

Iliamna

Newhalen

Nondalton

Pedro Bay

Williamsport

Kokhanok

^ Approximate Pebble Deposit Location

Transportation Corridor (Outside Assessment Area)

Transportation Corridor

Existing Roads

Transportation Corridor Area

Subwatersheds within Area

±

0 5 10

Miles

The Transportation Corridor Area, Comprising 32 Subwatersheds in the Kvichak River Watershed that Drain to Iliamna Lake. Subwatersheds are 

defined by 12-digit hydrologic unit codes according to the National Hydrography Dataset (USGS 2012).

Approximate Pebble Deposit Location

Transportation Corridor (Outside Assessment Area)

Transportation Corridor

Existing Roads

Transportation Corridor Area

Miles Subwatersheds within Area

Kilometers

0 5 10

0 5 10

13
These losses would reduce availability of 

and access to hydraulically and thermally 

diverse habitats that provide enhanced 

foraging opportunities and important 

rearing habitats for juvenile salmon.

Indirect effects of stream and wetland 

losses would include reductions in 

the quality of downstream habitat for 

coho salmon, sockeye salmon, Chinook 

salmon, rainbow trout, and Dolly Varden. 

Although these indirect effects cannot be 

quantified, such effects would be expected 

to diminish fish production downstream of 

the mine site because fish depend on these 

habitats. Indirect effects would be caused 

by the following alterations.

� Reduced food resources would result 

from the loss of organic material and 

drifting invertebrates from streams

and streamside wetlands lost to the 

mine footprint.

� The balance of surface water and 

groundwater inputs to downstream 

reaches would shift, potentially 

reducing winter fish habitat and 

making streams less suitable for 

spawning and rearing.

South Fork Koktuli River

Upper T a larik Creek

Nor th Fork Koktuli River

TSF1

TSF2

TSF3

Waste Rock

Mine Pit

Waste Rock

Streams and Wetlands Lost (Eliminated, Blocked, or Dewatered) in the 

Pebble 6.5 Scenario. Light blue areas indicate streams and rivers from the 

National Hydrography Dataset (USGS 2012) and lakes and ponds from the National 

Wetlands Inventory (USFWS 2012); dark blue areas indicate wetlands from the 

National Wetlands Inventory (USFWS 2012).

Miles

Kilometers

0 1 2

0 1 2

Pebble 6.5 Components

Groundwater Drawdown Zone

Eliminated, Blocked, or Dewatered 

Streams, Lakes, and Ponds

Eliminated, Blocked, or Dewatered 

Wetlands

Sockeye salmon near Pedro Bay, Iliamna Lake Photo: Thomas Quinn (University of Washington)

14
� Seasonal temperatures could be altered by 

water treatment and reduced groundwater 

flowpaths, making streams less suitable

for salmonids. 

Water Quality

Leakage during Routine Operations

Water from the mine site would enter streams 

through wastewater treatment plant discharges 

and in uncollected runoff and leakage of 

leachates from the waste rock piles and TSFs. 

Wastewater treatment is assumed to meet 

all state standards and national criteria, or 

equivalent benchmarks for chemicals that have 

no criteria. However, water quality would be 

diminished by uncollected leakage of tailings 

and waste rock leachates from the containment 

system, which would occur during routine 

operations. Test leachates from the tailings and 

non-acid-generating waste rocks are mildly 

toxic. They would require an approximately 

two-fold dilution to achieve water quality criteria 

for copper, but are not estimated to be toxic 

to salmonids. Waste rocks associated with the 

ore body are acid-forming with high copper 

concentrations in test leachates, and would 

require 2,900- to 52,000-fold dilution to achieve 

water quality criteria. Several metals could be 

sufficiently elevated to contribute to toxicity, but 

copper is the dominant toxicant.

Uncollected leachates from waste rock 

piles and TSFs would elevate instream 

copper levels and cause direct effects 

on salmonids ranging from aversion and 

avoidance of the contaminated habitat 

to rapidly induced death of many or all 

fish. Avoidance of streams by salmonids 

would occur in 24 and 34 to 57 km (15 

and 21 to 35 miles) of streams in the 

Pebble 2.0 and Pebble 6.5 scenarios, 

respectively. Rapidly induced death of 

many or all fish would occur in 12 km 

(7.4 miles) of streams in the Pebble 6.5 

scenario. Copper would cause death 

or reduced reproduction of aquatic 

invertebrates in 21, 40 to 62, and 60 to 

82 km (13, 25 to 38, and 37 to 51 miles) 

of streams in the Pebble 0.25, 2.0, 

and 6.5 scenarios, respectively. These 

invertebrates are the primary food source 

for juvenile salmon and all life stages of 

other salmonids, so reduced invertebrate 

productivity would be expected to 

reduce fish productivity. These results 

are sensitive to the assumed efficiency 

of the leachate capture system, and a 

more efficient system could be devised. 

However, greater than 99% capture 

efficiency would be required to prevent 

exceedance of the copper criteria for 

Summary of Estimated Wetland, Pond, and Lake Area Potentially Affected in the Three Mine Scenarios, Assuming Routine Operations

Wetland, Pond, and Lake Area Affected (km2)

Effect Pebble 0.25 Pebble 2.0 Pebble 6.5

Lost to the mine footprint 4.9 13 20

Lost to reduced streamflow below mine footprint Unquantified

Filled by roadbed 0.11

Influenced by the road (within 200 m) 4.7

Notes: a Stream reaches with streamflow alterations partially overlap those with toxicity.

Summary of Estimated Stream Lengths Potentially Affected in the Three Mine Scenarios, Assuming Routine Operations

Stream Length Affected (km)

Effect Pebble 0.25 Pebble 2.0 Pebble 6.5

Eliminated, blocked, or dewatered 38 89 151

Eliminated, blocked, or dewatered—anadromous 8 22 36

>20% streamflow alterationa 15 27 53

Direct toxicity to fisha 0 24 34–57

Direct toxicity to invertebratesa 21 40–62 60–82

Downstream of transportation corridor 272

15
100 km (48 to 62 miles) of streams in all 

three scenarios. In the Pebble 2.0 and 

6.5 scenarios, a fish kill would occur 

rapidly in 3.8 and 31 km (2.4 and 19 

miles) of streams, respectively, following 

treatment failure.

Spillway Release

In the event of TSF overfilling, 

supernatant water would be released via 

a spillway. If the water was equivalent to 

the test tailings supernatant, 2.6 km (1.6 

miles) of streams would be avoided by 

fish and 3.4 to 23 km (2.1 to 14 miles) of 

streams would be toxic to invertebrates, 

independent of other sources.

Transportation Corridor

Construction and Routine Operation

In the Kvichak River watershed, the 

transportation corridor would cross 

approximately 64 streams and rivers. Of 

those, 55 are known or likely to support 

migrating and resident salmonids, 

including 20 streams designated as 

anadromous waters at the location of 

the crossing. The corridor would run 

near Iliamna Lake and cross multiple 

tributary streams near their confluences 

with the lake. These habitats are 

important spawning areas for sockeye 

salmon, putting sockeye particularly at 

the South Fork Koktuli River in the 

Pebble 6.5 scenario, which would require 

technologies beyond those specified in 

our scenarios or identified in the most 

recent preliminary mine plan.

Wastewater Treatment Plant Failure

Based on a review of historical and 

currently operating mines, some failure of 

water collection and treatment systems 

would be expected to occur during 

operation or post-closure periods. A 

variety of water collection and treatment 

failures are possible, ranging from 

operational failures that result in shortterm releases of untreated or partially 

treated leachates to long-term failures to 

operate water collection and treatment 

systems in perpetuity. A reasonable but 

severe failure scenario would involve a 

complete loss of water treatment and 

release of average untreated wastewater 

flows into average dilution flows. In that 

failure scenario, copper concentrations 

would be sufficient to cause direct 

effects on salmonids in 27, 64 to 87, and 

74 to 97 km (17, 40 to 54, and 46 to 60 

miles) of streams in the Pebble 0.25, 

2.0, and 6.5 scenarios, respectively. 

Aquatic invertebrates would be killed 

or their reproduction reduced in 78 to 

Rainbow trout Photo: USEPA

16
Probabilities and Consequences of Potential Failures in the Mine Scenarios

Failure Probabilitya Consequences

Tailings dam 4 x 10-4 to 4 x 10-6 per dam-year = recurrence frequency 

of 2,500 to 250,000 yearsb

More than 29 km of salmonid stream would be destroyed or would 

have greatly degraded habitat for decades.

Product concentrate pipeline 10-3 per km-year = 95% chance per pipeline in 25 years Most failures would occur between stream or wetland crossings and 

might have little effect on fish.

 Concentrate spill into a stream 1.5 x 10-2 per year = 1 stream-contaminating spill in 78 

years

Fish and invertebrates would experience acute exposure to toxic water 

and chronic exposure to toxic sediment in a stream and potentially 

extending to Iliamna Lake.

Concentrate spill into a wetland 2.6 x 10-2 per year = 2 wetland-contaminating spills in 

78 years

Invertebrates and potentially fish would experience acute exposure to 

toxic water and chronic exposure to toxic sediment in a pond or other 

wetland.

Return water pipeline spill Same as product concentrate pipeline Fish and invertebrates would experience acute exposure to toxic water 

if return water spilled to a stream or wetland.

Diesel pipeline spill Same as product concentrate pipeline Acute toxicity would reduce the abundance and diversity of 

invertebrates and possibly cause a fish kill if diesel spilled to a stream 

or wetland.

Culvert, operation Low Frequent inspections and regular maintenance would result in few 

impassable culverts, but for those few, blockage of migration could 

persist for a migration period, particularly for juvenile fish.

Culvert, post-operation 3 x 10-1 to ~6 x 10-1 per culvert; instantaneous = 11 to 

22 culverts

In surveys of road culverts, 30 to 61% are impassable to fish at any one 

time. This would result in 11 to 22 salmonid streams blocked at any one 

time. In 10 to 19 of the 32 culverted streams with restricted upstream 

habitat, salmon spawning may fail or be reduced and the streams would 

likely not be able to support long-term populations of resident species.

Truck accidents 1.9 x 10-7 spills per mile of travel = 4 accidents in 25 

years and 2 near-stream spills in 78 years

Accidents that spill processing chemicals into a stream or wetland could 

cause a fish kill. A spill of molybdenum concentrate may also be toxic.

Water collection and treatment, 

operation

0.93 = proportion of recent U.S. porphyry copper mines 

with reportable water collection and treatment failures.

Water collection and treatment failures could result in exceedance 

of standards potentially including death of fish and invertebrates. 

However, these failures would not necessarily be as severe or extensive 

as estimated in the failure scenario, which would result in toxic effects 

from copper in more than 60 km of stream habitat.

Tailings storage facility spillway 

release

No data, but spills are known to occur and are sufficiently 

frequent to justify routine spillway construction

Spilled supernatant from the tailings storage facility could result in 

toxicity to invertebrates and fish avoidance for the duration of the 

event.

Water collection and treatment, 

managed post-closure

Somewhat higher than operation Post-closure collection and treatment failures are very likely to result 

in release of untreated or incompletely treated leachates for days 

to months, but the water would be less toxic due to elimination of 

potentially acid-generating waste rock.

Water collection and treatment, 

after site abandonment

Certain, by definition When water is no longer managed, untreated leachates would flow to 

the streams. However, the water may be less toxic.

a

 Because of differences in derivation, the probabilities are not directly comparable .

b

 Based on expected state safety requirements. Observed failure rates for earthen dams are higher (about 5 x 10-4 per year or a recurrence frequency of 2,000 years).

17
Culverts can also fail to convey water due 

to landslides or, more commonly, floods 

that wash out undersized or improperly 

installed culverts. In such failures, the 

stream would be temporarily impassible 

to fish until the culvert is repaired or until 

erosion re-establishes the channel. If the 

failure occurs during a critical period in 

salmon migration, effects would be the 

same as with a debris blockage (i.e., a lost 

or diminished year-class).

Culvert failures also would result in the 

downstream transport and deposition of 

silt, which could cause returning salmon 

to avoid a stream if they arrived during 

or immediately following the failure. 

Deposition of silt would smother salmon 

eggs and alevins if they were present, 

and would degrade downstream habitat 

for salmonids and the invertebrates that 

they eat.

Blockages of culverts could persist 

for as long as the intervals between 

culvert inspections. We assume that 

the transportation corridor would be 

inspected daily and maintained during 

risk from the road. Diminished habitat 

quality in streams and wetlands below 

road crossings would result primarily 

from altered streamflow, runoff of road 

salts, and siltation of habitat for salmon 

spawning and rearing and invertebrate 

prey production.

Culvert Failure

Culverts commonly fail to allow free 

passage of fish. They can become 

blocked by debris or ice that may not 

stop water flow but that create a barrier 

to fish movement. Fish passage also may 

be blocked or inhibited by erosion below 

a culvert that “perches” the culvert and 

creates a waterfall, by shallow water 

caused by a wide culvert and periodic 

low streamflows, or by excessively high 

gradients. If blockages occurred during 

adult salmon immigration or juvenile 

salmon emigration and were not cleared 

for several days, production of a yearclass (i.e., fish spawned in the same year) 

would be lost from or diminished in the 

stream above the culvert.

^

Upper Talarik Creek

orth Fork Koktuli River

Pile River

Cook Inlet

Newhalen River

Iliamna Lake

Knutson Bay

Pile Bay Pedro

Bay

Iliamna

Bay

Lake Clark

South Fork Koktuli River

Knutson Creek

Iliamn a River

Canyon Cr eek

Chek ok Creek

Chinkelyes Creek

Note: Sampling intensity is greatly reduced away from the Pebble deposit area.

Streams without data points may not have been surveyed; thus, it is unknown ± whether or not they contain these species..

0 5 10

Miles

0 5 10

Kilometers

^ Approximate Location of Pebble Deposit

Transportation Corridor (Outside Assessment Area)

#* Dolly Varden (AFFI)

!( Rainbow Trout (AFFI)

Transportation Corridor

Transportation Corridor Area

Subwatersheds within Area

Salmon (AWC)

Reported Salmon, Dolly Varden, and Rainbow Trout Distribution Along the Transportation Corridor. Salmon presence data are from the 

Anadromous Waters Catalog (Johnson and Blanche 2012); Dolly Varden and rainbow trout presence data are from the Alaska Freshwater Fish Inventory (ADF&G 

2012). Note that rainbow trout have also been documented in the Iliamna River and Chinkelyes Creek, although these points are not indicated on this map.

Approximate Pebble Deposit Location

Transportation Corridor (Outside Assessment Area)

Transportation Corridor

Transportation Corridor Area

Subwatersheds within Area

Salmon

Dolly Varden

Rainbow Trout

Miles

Kilometers

0 5 10

0 5 10

Note: Sampling intensity is greatly reduced away from the Pebble 

deposit area. Streams without data points may not have been 

surveyed; thus, it is unknown whether or not they contain these species.

18
salmonids. Hence, 11 to 22 streams would 

be expected to have impeded passage 

of salmon, rainbow trout, or Dolly Varden 

for an indefinite period of time, and some 

proportion of those streams would have 

degraded downstream habitat resulting 

from sedimentation following washout of 

the road.

Truck Accidents

Trucks would carry ore processing 

chemicals to the mine site and 

molybdenum product concentrate to the 

port. Truck accident records indicate that 

truck accidents near streams are likely 

over the long period of mine operation. 

These accidents could release sodium ethyl 

xanthate, cyanide, other process chemicals, 

or molybdenum product concentrate 

to streams or wetlands, resulting in toxic 

effects on invertebrates and fish. However, 

the risk of spills could be mitigated by using 

impact-resistant containers.

Tailings Dam Failure

Tailings are the waste materials 

produced during ore processing. In 

our scenarios, these wastes would be 

stored in TSFs consisting of tailings dams 

and impoundments. The probability 

of a tailings dam failure increases with 

the number of dams. The Pebble 0.25 

mine operation. The level of surveillance 

along the corridor can be expected to 

affect the frequency of culvert failure 

detection. Driving inspections would 

likely identify a single erosional failure 

of a culvert that damaged the road, or 

a debris blockage sufficient to cause 

water to pool above the road. However, 

long-term fixes may not be possible 

until conditions are suitable for culvert 

replacement, and these fixes may not 

fully address fish passage, which may be 

reduced or blocked for longer periods. 

Extended blockage of migration would 

be less likely if daily road inspections 

included stops to inspect each end of 

each culvert. 

After mine operations cease, the road 

would likely be maintained less carefully 

by the operator or may be transferred 

to a government entity that would be 

expected to employ a more conventional 

inspection and maintenance schedule. In 

either case, the proportion of impassable 

culverts at any one time would be 

expected to revert to levels found in 

published surveys of public roads (mean 

of 48% [range of 30 to 61%] of culverts 

that had failed and not been repaired 

when surveyed). Of the approximately 45 

culverts that would be required, 36 would 

be on streams that are believed to support 

Washed out culvert on Kenai Peninsula Photo: Robert Ruffner (Kenai Watershed Forum)

19
tailings dam failure is to use the historical 

failure rate of similar dams. Three reviews 

of tailings dam failures produced an 

average rate of approximately 1 failure 

per 2,000 dam-years, or 5 x 10-4 failures 

per dam-year. Strictly speaking, these 

frequencies are properties that apply to 

a group of dams. However, by extension, 

if there is one dam and it is typical of 

the population, it would be expected 

to fail, on average, within a 2,000-year 

period. This does not mean it is expected 

to fail 2,000 years after it is built. Rather, 

it indicates that, after 2,000 years have 

passed, it is more likely than not that 

the dam would have failed and that 

expected failure could occur any year in 

that 2,000-year window with an average 

annual probability of 0.0005. 

The argument against this method is 

that the record of past failures does not 

fully reflect current engineering practice. 

Some studies suggest that improved 

design, construction, and monitoring 

practices can reduce the failure rate by 

an order of magnitude or more, resulting 

in an estimated failure probability 

within the range assumed here. The 

State of Alaska’s guidelines suggest that 

an applicant follow accepted industry 

design practices such as those provided 

scenario would include one TSF with 

a single dam, the Pebble 2.0 scenario 

would include one TSF with three dams, 

and the Pebble 6.5 scenario would 

include three TSFs with a total of eight 

dams. Because their removal is not 

feasible, the TSFs and their component 

dams would be in place for hundreds to 

thousands of years, long beyond the life 

of the mine. Available reports from the 

Pebble Limited Partnership suggest a 

tailings dam as high as 209-m (685 feet) 

at TSF 1. We evaluated two potential dam 

failures at TSF 1 in this assessment: one 

at a volume approximating the complete 

Pebble 0.25 scenario (92-m dam height) 

and one at a volume approximating the 

complete Pebble 2.0 scenario (209-m 

dam height). In both cases we assumed 

20% of the tailings would be released, a 

conservative estimate that is well within 

the range of historical tailings dam 

failures. Failures of the TSF 2 and TSF 

3 tailings dams were not analyzed but 

would be expected to be similar in terms 

of types of effects.

The range of estimated dam failure 

probabilities is wide, reflecting the great 

uncertainty concerning such failures. 

The most straightforward method of 

estimating the annual probability of a 

Height of the Dam at TSF 1 in the Pebble 2.0 and Pebble 6.5 Scenarios, Relative to U.S. Landmarks

Tailings Reservoir

Tailings Dam

Transamerica Building – 260 Meters

Tailings Dam TSF 1 – 209 Meters

Washington Monument – 169 Meters

200 Gateway Arch – 192 Meters

180

160

140

Meters

120

100

80

60

40

20

0

220

240

260

20
by the U.S. Army Corps of Engineers 

(USACE), the Federal Energy Regulatory 

Commission (FERC), and other agencies. 

Based on safety factors in USACE and 

FERC guidance, we estimate that the 

probability of failure for all causes 

requires a minimum factor of safety 

of 1.5 against slope instability for the 

loading condition corresponding to 

steady seepage from the filled storage 

facility. An assessment of the correlation 

of dam failure probabilities with slope 

instability safety factors suggests an 

annual probability of failure of 1 in 

250,000 per year for facilities designed, 

built, and operated with state-ofthe-practice engineering (Category 

I facilities) and 1 in 2,500 per year for 

facilities designed, built, and operated 

using standard engineering practice 

(Category II facilities). The advantage of 

this approach is that it addresses current 

regulatory guidelines and engineering 

practices. The disadvantage is that we do 

not know whether standard practice or 

state-of-the practice dams will perform as 

expected, particularly given the potential 

dam heights and subarctic conditions in 

these scenarios.

Failure of the dam at TSF 1 (the TSF 

included in all three mine scenarios) 

would result in the release of a flood of 

tailings slurry into the North Fork Koktuli 

River. This flood would scour the valley 

and deposit many meters of tailings fines 

in a sediment wedge across the entire 

valley near the TSF dam, with lesser 

quantities of fines deposited as far as the 

North Fork’s confluence with the South 

Fork Koktuli River. The North Fork Koktuli 

River currently supports spawning and 

rearing populations of sockeye, coho, and 

Chinook salmon; spawning populations 

of chum salmon; and rearing populations 

of Dolly Varden and rainbow trout. The 

tailings slurry flood would continue 

down the mainstem Koktuli River with 

similar effects, the extent of which 

cannot be estimated at this time due to 

model and data limitations.

The tailings dam failures evaluated in

the assessment would be expected to 

have the following severe direct and 

indirect effects on aquatic resources, 

particularly salmonids.

It is expected that the North Fork 

Koktuli River below the TSF 1 dam and 

much of the mainstem Koktuli River 

would not support salmonids in the 

short term (less than 10 years).

Landscape near the Pebble deposit location Photo: Joe Ebersole (USEPA)

21
The affected streams would provide 

low-quality spawning and rearing 

habitat for a period of decades.

� Recovery of suitable substrates via 

mobilization and transport of tailings 

would take years to decades, and 

would affect much of the watershed 

downstream of the failed dam.

� Ultimately, spring floods and 

stormflows would carry some of the 

tailings into the Nushagak River.

� For some years, periods of high 

flow would be expected to suspend 

sufficient concentrations of tailings to 

cause avoidance, reduced growth and 

fecundity, and even death of fish.

Near-complete loss of North Fork 

Koktuli River fish populations 

downstream of the TSF and additional 

fish population losses in the mainstem 

Koktuli, Nushagak, and Mulchatna 

Rivers would be expected to result 

from these habitat losses.

� The Koktuli River watershed is an 

important producer of Chinook salmon. 

The Nushagak River watershed, of which 

the Koktuli River watershed is a part, is 

the largest producer of Chinook salmon 

� In the tailings dam failure scenarios, 

spilled tailings would bury salmon 

habitat under meters of fines along 

nearly the entire length of the North 

Fork Koktuli River valley downstream

of the dam (over 29 km or 18 miles in 

the Pebble 0.25 dam failure scenario), 

and beyond (in the Pebble 2.0 dam 

failure scenario). 

� Deposited tailings would degrade 

habitat quality for both fish and the 

invertebrates they eat. Based largely 

on their copper content, deposited 

tailings would be toxic to benthic 

macroinvertebrates, but existing data 

concerning toxicity to fish are less clear.

� Deposited tailings would continue to 

erode from the North Fork Koktuli River 

and mainstem Koktuli River valleys.

� Suspension and redeposition of tailings 

would be expected to cause serious 

habitat degradation in the mainstem 

Koktuli River and downstream into

the Mulchatna River; however, the 

extent of these effects cannot be 

estimated at this time due to model 

and data limitations.

Nushagak River at Koliganek Photo: Alan Boraas

22
in the Bristol Bay region, with annual 

runs averaging over 190,000 fish.

� A tailings spill could eliminate 29% or 

more of the Chinook salmon run in 

the Nushagak River due to loss of the 

Koktuli River watershed population. An 

additional 10 to 20% could be lost due 

to tailings deposited in the Mulchatna 

River and its tributaries.

� Sockeye are the most abundant 

salmon returning to the Nushagak 

River watershed, with annual runs 

averaging more than 1.9 million fish. 

The proportion of sockeye and other 

salmon species of Koktuli-Mulchatna 

origin is unknown.

� Similarly, the North Fork Koktuli River 

populations of rainbow trout and 

Dolly Varden would be lost for years to 

decades if they could not successfully 

be maintained entirely in headwater 

networks upstream of the affected zone. 

Quantitative estimates of these losses are 

not possible given available information.

Effects would be qualitatively similar

for both the Pebble 0.25 and Pebble 2.0 

dam failures, although effects from the 

Pebble 2.0 dam failure would extend 

farther and last longer. Failure of dams 

at the two additional TSFs in the Pebble 

6.5 scenario (TSF 2 and TSF 3) were not 

modeled, but would have similar types 

of effects in the South Fork Koktuli River 

and downstream rivers.

Pipeline Failure

In the mine scenarios, the primary mine 

product would be a sand-like copper 

concentrate with traces of other metals, 

which would be pumped via pipeline to 

a port on Cook Inlet. Water that carried 

the concentrate would be returned 

to the mine site in a second pipeline. 

Based on the general record of pipelines 

and further supported by the record of 

metal concentrate pipelines at existing 

mines, one near-stream failure and two 

near-wetland failures of each of these 

pipelines would be expected to occur 

over the life of the Pebble 6.5 scenario 

(approximately 78 years). 

Failure of either the product or the 

return water pipeline would release 

water that is expected to be highly toxic 

due to dissolved copper and possibly 

processing chemicals. Invertebrates and 

potentially early fish life stages would 

be killed in the affected stream over a 

relatively brief period. If concentrate 

The Koktuli River Photo: Jeff Frithsen (USEPA)

23
likely fish as well. Remediation would be 

difficult but recovery would be expected 

to occur within 3 years. Failure of the 

natural gas pipeline would also be 

expected, but significant effects on fish 

would not be expected.

Spills into wetlands that support fish 

would be expected to have greater toxic 

effects because contaminants would 

be washed out slowly, if at all. However, 

retention of contaminants within the 

wetland would make remediation by 

removal more practical.

Common Mode Failures

Multiple, simultaneous failures could 

occur due to a common event, such as 

a severe storm with heavy precipitation 

(particularly precipitation that fell on 

spring snow cover) or a major earthquake. 

Over the long period that tailings 

impoundments, a mine pit, and waste 

rock piles would be in place, the likelihood 

of multiple extreme precipitation events, 

earthquakes, or combinations of these 

events becomes much greater. Multiple 

events further increase the chances that 

facilities remaining in place will weaken and 

eventually fail.

spilled into a stream, it would settle 

and form highly toxic bed sediment 

based on its high copper content and 

acid generation. The mean velocities of 

many streams crossed by the pipelines 

are sufficient to carry the concentrate 

downstream to Iliamna Lake, but some 

would collect in low-velocity areas of 

the receiving stream. If the spill occurred 

during low streamflows, dredging could 

recover some concentrate but would 

cause physical damage to the stream. 

Concentrations in Iliamna Lake could not 

be predicted, but near the pipeline route 

Iliamna Lake contains important sockeye 

salmon beach spawning areas that would 

be exposed to a spill. Sockeye also spawn 

in the lower reaches of streams that 

could be directly contaminated by a spill.

Based on petroleum pipeline failure 

rates, the diesel fuel pipeline also would 

be expected to spill near a stream over 

the life of the Pebble 6.5 mine. Evidence

from modeling the dissolved and 

dispersed oil concentrations in 

streams, laboratory tests of diesel 

toxicity, and studies of actual spills in 

streams indicates that a diesel spill at 

a stream crossing would be expected 

to immediately kill invertebrates and 

Knutson Creek draining into the Knutson Bay area of Iliamna Lake Photo: Keith Denton

24
Such an event could cause multiple tailings 

dam failures that would spill tailings 

slurry into both the South and North Fork 

Koktuli Rivers; road culvert washouts that 

would send sediments downstream and 

potentially block fish passage; and pipeline 

failures that would release product slurry, 

return water, or diesel fuel. The effects 

of each of these accidents individually 

would be the same as discussed 

previously, but their co-occurrence would 

cause cumulative effects on salmonid 

populations and make any remedial 

responses more difficult.

Fish-Mediated Risks

to Wildlife

Although the effects of salmonid 

reductions on wildlife—that is, fishmediated risks to wildlife—cannot be 

quantified given available data, some 

reduction in wildlife would be expected 

in the mine scenarios. Changes in the 

occurrence and abundance of salmon 

have the potential to change animal 

behavior and reduce wildlife population 

abundances. The mine footprints would 

be expected to have local effects on 

brown bears, wolves, bald eagles, and 

other wildlife that consume salmon, 

due to reduced salmon abundance 

from habitat loss and degradation in or 

immediately downstream of the mine 

footprint. Any of the accidents or failures 

evaluated would increase effects on 

salmon, which would further reduce the 

abundance of their predators.

The abundance and production of wildlife 

also is enhanced by the marine-derived 

nutrients that salmon carry upstream on 

their spawning migration. These nutrients 

are released into streams when the 

salmon die, enhancing the production of 

other aquatic species that feed wildlife. 

Salmon predators deposit these nutrients 

on the landscape, thereby fertilizing 

terrestrial vegetation that, in turn, 

provides food for moose, caribou, and 

other wildlife. The loss of these nutrients 

due to a reduction in salmon would be 

expected to reduce the production of 

riparian and upland species.

Fish-Mediated Risks to

Alaska Native Cultures

Under routine operations with no major 

accidents or failures, the predicted loss 

and degradation of salmonid habitat 

in the South and North Fork Koktuli 

Rivers and Upper Talarik Creek would be 

expected to have some impact on Alaska 

Brown bear in the Kvichak River watershed Photo: USEPA

25
subsistence is integral to Alaska Native 

cultures, the effects of salmon losses 

go beyond the loss of food resources. If 

salmon quality or quantity was (or was 

perceived to be) adversely affected, the 

nutritional, social, and spiritual health of 

Alaska Natives would decline.

Cumulative Risks of

Multiple Mines

This assessment has focused on the 

effects that a single large mine at the 

Pebble deposit would have on salmon 

and other resources in the Nushagak and 

Kvichak River watersheds, including the 

cumulative effects of multiple stressors 

associated with that mine. However, 

multiple mines and their associated 

infrastructure may be developed in these 

watersheds. Each mine would pose risks 

similar to those identified in the mine 

scenarios. Estimates of the stream and 

wetland habitats lost would differ across 

different deposits, based on the size 

and location of mine operations within 

the watersheds. Individually, each mine 

footprint would eliminate some amount 

of fish-supporting habitat and, should 

operator or engineering failures occur, 

affect fish habitats well beyond the

mine footprint.

Native cultures of the Nushagak and 

Kvichak River watersheds. Fishing and 

hunting practices would be expected 

to change in direct response to the 

stream, wetland, and terrestrial habitats 

lost to the mine footprints and the 

transportation corridor. It is also possible 

that subsistence use of salmon resources 

would decline based on perceptions of 

reduced fish or water quality resulting 

from mining.

The potential for significant effects on 

Alaska Native cultures is much greater 

from mine failures that reduced or 

eliminated fish populations in affected 

areas, including areas significant 

distances downstream from the 

mine. In the case of the tailings dam 

failures described in the assessment, 

the significant loss of Chinook salmon 

populations would have severe 

consequences, especially for villages in 

the Nushagak River watershed.

Any loss of fish production from these 

failures would reduce the availability 

of these subsistence resources to local 

Alaska Native villages, and the reduction 

of this highly nutritious food supply 

could have negative consequences on 

human health. Because salmon-based 

Kvichak River below Iliamna Lake and Igiugig Photo: Joe Ebersole (USEPA)

26
We considered development of mines 

at the Pebble South/PEB, Big Chunk 

South, Big Chunk North, Groundhog, 

AUDN/Iliamna, and Humble claims 

in the Nushagak and Kvichak River 

watersheds. These sites were chosen 

because all contain copper deposits that 

have generated exploratory interest. If 

all six mine sites were developed, the 

cumulative area covered by these six 

mine footprints could be 37 to 57 km2 

(9,100 to 14,000 acres). Stream habitats 

eliminated or blocked could be 43 to 70 

km (27 to 43 miles). Cumulative wetland 

losses could be 7.9 to 27 km2 (2,000 to 

6,700 acres).

These are conservative estimates 

of habitat loss, because we did not 

estimate the hydrologic drawdown 

zones around each mine pit as was done 

for the Pebble scenarios. Inclusion of 

the drawdown area in the Pebble 0.25 

scenario increased the area of stream and 

wetlands losses by roughly 50%. A similar 

increase might be expected at the other 

mine sites, depending on local geology. 

These mines also would be expected to 

modify streamflows and diminish water 

quality to approximately the same extent 

as the Pebble 0.25 scenario. Waters on 

these claim blocks include the Chulitna 

River and Rock, Jensen, Yellow, Napotoli, 

Klutuk, and Kenakuchuk Creeks, as well 

as over 250 unnamed tributaries and over 

50 unnamed lakes and ponds. Although 

not all support salmon, many do. Loss of 

substantial habitat across the watersheds 

could contribute to diminishing the 

genetic diversity of salmon stocks and 

thereby increasing annual variability in 

salmon returns.

Mitigation and Remediation

The mine scenarios assessed here 

include modern conventional mitigation 

practices as reflected in Northern 

Dynasty Mineral’s published plan for 

the Pebble deposit, plus practices 

suggested in the mining literature 

and consultations with experts. These 

practices include, but are not limited 

to, processing all potentially acidgenerating waste rock before closure, 

managing effluent water temperatures, 

inspecting and maintaining roads daily, 

and providing automatic monitoring 

and remote shut-off for the pipelines. 

However, we recognize that risks could 

be further reduced by unconventional 

or even novel mitigation measures, such 

as dry stack tailings disposal or the use 

of armored containers on the trucks 

carrying process chemicals to the site. 

These practices may be unconventional 

because they are expensive, unproven, 

or impractical. However, these obstacles 

to implementation might be overcome 

and justified by the large mineral resource 

and the highly valued natural and cultural 

resources of the Bristol Bay watershed.

Although remediation would be 

considered if spills contaminated 

streams, features of the Pebble deposit 

area would make remediation difficult. 

Spilled tailings from a dam failure would 

flow into streams, rivers, and floodplains 

that are in roadless areas and that are not 

large enough to float a barge-mounted 

dredge. Recovery, transport, and disposal 

of hundreds of millions of metric tons of 

tailings under those conditions would 

be extremely difficult and would result 

in additional environmental damage. 

Compensatory mitigation measures 

could offset some of the stream and 

wetland losses, although there are 

substantial challenges regarding the 

efficacy of these measures to offset 

adverse impacts. Pipeline crossings of 

streams would be near Iliamna Lake, 

so the time available to block or collect 

spilled material before it reached the 

lake would be short. Spilled return 

water and the aqueous phase of the 

product concentrate slurry would be 

unrecoverable. The product concentrate 

itself would resemble fine sand, and 

mean velocities in many receiving 

streams would be sufficient to suspend 

and transport it. Hence, concentrate 

27
spilled or washed into streams could 

be recovered only where it collected in 

low-velocity locations. Diesel spills would 

dissolve, vaporize, and flow as a slick to 

Iliamna Lake. Booms and absorbents are 

not very effective in moderate- to highvelocity streams. 

Summary of Uncertainties in 

Mine Design and Operation

This assessment considers realistic mine 

scenarios that are based on specific 

characteristics of the Pebble deposit and 

preliminary plans proposed by Northern 

Dynasty Minerals. These scenarios 

are generally applicable to copper 

deposits in the Bristol Bay watershed. 

If the Pebble deposit is mined, actual 

events will undoubtedly deviate from 

these scenarios. This is not a source of 

uncertainty, but rather an inherent aspect 

of a predictive assessment. Even an 

environmental assessment of a specific 

plan proposed for permitting by a mining 

company would be an assessment of a 

scenario that undoubtedly would differ 

from actual mine development.

Multiple uncertainties are inherent 

in planning, designing, constructing, 

operating, and closing a mine. These 

uncertainties, summarized below, are 

inherent in any complex enterprise, 

particularly when that enterprise involves 

an incompletely characterized natural 

system. However, the large spatial scales 

and long durations required to mine 

the Pebble deposit make these inherent 

uncertainties more prominent.

� Mines are complex systems requiring 

skilled engineering, design, and 

operation. The uncertainties facing 

mining and geotechnical engineers 

include unknown geological features, 

uncertain values of geological 

properties, limited knowledge 

of mechanisms and processes, 

and human error in design and 

construction. Models used to predict 

the behavior of engineered systems 

represent idealized processes and 

by necessity contain simplifications 

and approximations that potentially 

introduce errors.

� Accidents are unplanned and 

inherently unpredictable. Although 

systems can be put into place to 

protect against system failures, 

seemingly logical decisions about how 

to respond to a given situation can 

have unexpected consequences due 

Upper Talarik Creek floodplain Photo: Joe Ebersole (USEPA)

28
to human error (e.g., the January 2012 

overflow of the tailings dam at the 

Nixon Fork Mine near McGrath, Alaska). 

Further, unforeseen events or events 

that are more extreme than anticipated 

can negate apparently reasonable 

operation and mitigation plans. 

Climate change will likely exacerbate 

this uncertainty. In the Bristol Bay 

region, climate change is expected 

to lead to changes in snowpack and 

the timing of snowmelt, an increased 

chance of rain-on-snow precipitation, 

and increased flooding. All of these 

changes are likely to affect multiple 

aspects of any large-scale mining in 

the area, including mine infrastructure, 

the transportation corridor, water 

treatment and discharge, and postclosure management, in unknown and 

potentially unpredictable ways.

� The ore deposit would be mined for 

decades and wastes would require 

management for centuries or even in 

perpetuity. Engineered mine waste 

storage systems have been in existence 

for only about 50 years, and their 

long-term behavior is not known. The 

response of current technology in 

tailings dam construction is untested 

and unknown in the face of centuries of 

unpredictable events such as extreme 

weather and earthquakes.

� Over the long time span (centuries) 

of mining and post-mining care, 

generations of mine operators must 

exercise due diligence. Priorities 

could change in the face of financial 

circumstances, changing markets for 

metals, new information about the 

resource, political priorities, or any 

number of currently unforeseeable 

changes in circumstance.

Summary of Uncertainties and 

Limitations in the Assessment

The most important uncertainties 

concerning estimated effects of the mine 

scenarios, as judged by the assessment 

authors, are identified below.

� Consequences of habitat loss and 

degradation for fish populations 

could not be quantified because of 

the lack of quantitative information 

concerning salmonid populations in 

freshwater habitats. The occurrence 

of salmonid species in rivers and 

major streams is known, but detailed 

and comprehensive information 

on abundances, productivities, 

Homes in Nondalton Photo: Alan Boraas (Kenai Peninsula College)

29
and limiting factors in each of the 

watersheds is not available. Estimating 

fish population changes would 

require population modeling, which 

requires knowledge of life-stagespecific survival and production 

and limiting factors and processes. 

Further, it requires knowledge of how 

temperature, habitat structure, prey 

availability, density dependence, and 

sublethal toxicity influence life-stagespecific survival and production. 

Obtaining this information would 

require more detailed monitoring and 

experimentation. Salmon populations 

naturally vary in size due to many 

factors that vary among locations and 

years. At present, data are insufficient 

to establish reliable salmon population 

estimates, and obtaining such data 

would take many years. Estimated 

effects of mining on fish habitat thus 

become the best available surrogate for 

estimated effects on fish populations.

� Standard leaching test data are 

available for test tailings and waste 

rocks from the Pebble deposit, but 

these results are uncertain predictors 

of the actual composition of leachates 

from waste rock piles, tailings 

impoundments, or tailings deposited in 

streams and on their floodplains.

� Leachate capture efficiencies are 

uncertain. We assume 50% capture for 

waste rock leachates outside of the 

mine pit drawdown zone. In the Pebble 

2.0 scenario, for example, this would 

result in capture of 84% of the leachate 

by the pit drawdown zone and the 

wells combined. To avoid exceeding 

water quality criteria for copper, more 

than 99% capture would be required.

� The quantitative effects of tailings 

and product concentrate deposited 

in spawning and rearing habitat are 

uncertain. It is clear that they would 

have harmful physical and toxicological 

effects on salmonid larvae or sheltering 

juveniles, but the concentration in 

spawning gravels required to reduce 

salmonid reproductive success

is unknown.

� The estimated annual probability of 

tailings dam failure is uncertain because 

it is based on design goals. Historical 

experience is presumed to provide 

an upper bound of failure probability. 

Features that should reduce failure 

frequencies have not been tested for 

the thousands of years that they must 

function properly. Hence, actual failure 

rates could be higher or lower than the 

estimated probability.

Salmon art on a building in Dillingham Photo: Alan Boraas (Kenai Peninsula College)

30
� The proportion of tailings that would 

spill in the event of a dam failure 

could be larger than the largest value 

modeled (20%).

� The long-term fate of spilled tailings in 

the event of a dam failure could not be 

quantified. It is expected that tailings 

would erode from areas of initial 

deposition and move downstream 

over more than a decade. However, the 

data needed to model that process and 

the resources needed to develop that 

model are not available.

� The actual response of Alaska Native 

cultures to any impacts of the mine 

scenarios is uncertain. Interviews with 

village Elders and culture bearers and 

other evidence suggest that responses 

would involve more than the need 

to compensate for lost food, and 

would be expected to include some 

degree of cultural disruption. It is not 

possible to predict specific changes in 

demographics, cultural practices, or 

physical and mental health.

� Because we mention but do not 

evaluate potential direct effects 

of mining on wildlife or on Alaska 

Natives, this assessment represents a 

conservative estimate of how these 

endpoints would be affected by mine 

development and operation.

Uses of the Assessment

This assessment is a scientific 

investigation. It does not reflect any 

conclusions or judgments about 

the need for or scope of potential 

government action, nor does it offer 

or analyze options for future decisions. 

Rather, it is intended to provide a 

characterization of the biological and 

mineral resources of the Bristol Bay 

watershed, increase understanding of 

the risks from large-scale mining to the 

region’s fish resources, and inform future 

government decisions. The assessment 

will also better inform dialogues among 

interested stakeholders concerning the 

resources in the Bristol Bay watershed 

and the potential impacts of large-scale 

mining on those resources.

Frying Pan Lake Photo: R. Halford

31
References

ADF&G (Alaska Department of Fish and Game). 2012. Alaska Freshwater Fish Inventory Database.

Available: http://www.adfg.alaska.gov/index.cfm?adfg=ffinventory.main.

Baker, T. Area Fishery Research Biologist, Bristol Bay Salmon Program. Alaska Department of Fish and Game, 

Anchorage, AK. July 8, 2011—email to Rebecca Shaftel.

Johnson, J., and P. Blanche. 2012. Catalog of Waters Important for Spawning, Rearing, or Migration of Anadromous 

Fishes—Southwestern Region, Effective June 1, 2012. Special Publication No. 12-08. Anchorage, AK: Alaska 

Department of Fish and Game.

Ruggerone, G. T., R. M. Peterman, and B. Dorner. 2010. Magnitude and trends in abundance of hatchery and wild pink 

salmon, chum salmon, and sockeye salmon in the North Pacific Ocean. Marine and Coastal Fisheries: Dynamics, 

Management, and Ecosystem Science 2:306–328.

USFWS (U.S. Fish and Wildlife Service). 2012. National Wetlands Inventory.

Available: http://www.fws.gov/wetlands/Wetlands-Mapper.html. Accessed: December 11, 2012.

USGS (U.S. Geological Survey). 2012. National Hydrography Dataset, High Resolution, Alaska.

Available: ftp://nhdftp.usgs.gov/DataSets/Staged/States/FileGDB/HighResolution. Accessed: October 16, 2012.

Photo Credits

Front Cover, Main Photo ........... Upper Talarik Creek, Joe Ebersole (USEPA)

Front Cover, Thumbnail 1 ........ Brown bear, Steve Hillebrand (USFWS)

Front Cover, Thumbnail 2 ........ Fishing boats at Naknek, USEPA

Front Cover, Thumbnail 3 ........ Iliamna Lake, Lorraine Edmond (USEPA)

Front Cover, Thumbnail 4 ........ Sockeye salmon in the Wood River, Thomas Quinn (University of Washington)

Inside Front Cover....................... Sockeye salmon in the Wood River, Thomas Quinn (University of Washington)

Inside Back Cover ........................ Sockeye salmon in the Wood River, Thomas Quinn (University of Washington)

Back Cover...................................... Tributary of Napotoli Creek, near the Humble claim, Michael Wiedmer

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